tenance of monogamy in Tree Swallows (Tachycineta bicolor). Mated, male Tree Swallows were removed from territories to simulate the lack of male parental ...
The Auk 109(3):488-499, 1992
EFFECTS
OF FOOD
REPRODUCTIVE
ABUNDANCE
SUCCESS
AND
AND
MALE
PARENTAL
MONOGAMY
CARE
ON
IN TREE SWALLOWS
PETERO. DUNN • AND SUSAN J. HANNON Department of Zoology,Universityof Alberta,Edmonton, AlbertaT6G2E9,Canada
ABSTRACT.--We examinedthe rolesof food abundanceand male parental carein the maintenanceof monogamyin Tree Swallows(Tachycineta bicolor).Mated, male Tree Swallowswere removed from territories to simulate the lack of male parental care that would be incurred by secondaryfemales.Removalstook place in lakeshoreand roadsidehabitatsin central Alberta, Canada. Insect sampling over three breeding seasonsindicated that the lakeshore had greater biomassof insectsthan the roadsideduring egg laying, but there was no difference during the nestling period. Bothour male removal experimentand natural casesof polygyny suggestedthat lossof male parental care had little effecton reproductivesuccess or survival of females or their offspring. Unaided females increasedtheir per-capita nest-visit rate in two of three years,so the rate of visitation per nestling did not differ significantlybetween unaided
females
and both
the male and female
at control
nests. Control
females
at the
lakeshoreproducedmore fledglingsthan controlfemalesat the roadside.This differencein productivitywasdue to earlier laying and larger clutchesat the lakeshorethan at the roadside and not to differencesin brood reduction or insect abundanceduring the nestling period. In both habitats,male parentalcarewasrelatively unimportantto femalereproductivesuccess and, therefore,within a given habitatsecondaryfemalesdid not incur a reproductivecost. All natural casesof polygyny occurredat the lakeshore.We suggestthat polygyny occurred at the lakeshore,and not at the roadside,becausefood abundancewasgreaterduring laying at the lakeshore,and this allowed secondaryfemales to lay more eggs and produce more fledglingsthan femalesmatedto monogamousmalesat the roadside.Received 11March 1991, accepted10 February1992. THE ABUNDANCE and distribution
Under these conditions, females should choose
of food are
thought to affect the mating systemsof birds (Emlen and Oring 1977, Oring 1982). For example, some models of avian mating systems (Orians 1969, Emlen and Oring 1977) predict higher levels of polygyny on territories that have higher food abundance. These models assume that females prefer to settle on territories with more food becausethey will be able to produce relatively more offspring. Food abundance may influence mating systemsand reproductive successin specieswith biparental
monogamy more frequently in habitats with lessfood becausemale parental care should be relatively more important to female reproductive success.Second, females may choosemating situationsbasedon food conditionsduring settlement or laying if reproductive successis influenced more strongly by laying date or clutchsize than by male assistance with feeding nestlings. If, early in the season, food abundance is greater on the territories of alreadymated than unmated males, then females may care via two mechanisms. First, food abundance choosesecondarymate statusbecausethey can may affect the cost to females of feeding their producemore offspring by laying earlier or by nestlings,and this may influence whether they laying larger clutches.These two mechanisms choose unmated or already-mated males as are not exclusive,but they suggestdifferent ways mates. In many birds secondary mates of po- that food abundance can affect both reproduclygynous males receive little or no assistance tive successand mating behavior in bird popwith feeding nestlings and, as a consequence, ulations. raise fewer young than either primary or moMany previous studies have suggestedthat nogamousfemales (reviewed in Dyrcz 1988). monogamy in birds is maintained by the importance of male parental care to female reproductive success(Winkler and Wilkinson 1988). • Presentaddress:Division of Botanyand Zoology, School of Life Sciences, Australian National
This interpretation is basedon a large number of male-removal
Univer-
studies that indicate
that male
parental care improves female reproductive
sity, GPO Box 4, Canberra ACT 2601, Australia. 488
July1992]
Food Abundance andMonogamy inSwallows
success(see reviews in Wolf et al. 1988, Bart and Tomes 1989, Dunn and Hannon 1989). How-
is a small
area around
489 the nest box. We considered
malespolygynousonly if they were seencopulating with two femalesand they defended the nest boxes ever, most male-removal studies cannot estimate the relative importance of male parental occupiedby thesefemalesthroughoutthe season.All femalesin our casesof polygyny were marked indicareto the maintenanceof monogamy,because vidually. Copulationsin Tree Swallowsoccuron the they rarely control for other factors, such as nest box or on nearby perches. food abundance, that also influence female reStudyarea.--Tree Swallows were studiedat Beaverproductivesuccess and mate choice.In contrast hill Lake,Toileld, Alberta (53ø38'N,112ø36'W)during to the male-removal experiments,studiesthat May through July 1986-1988. Beaverhill Lake is a have supplied supplementalfood suggestthat large (130 kin2),shallow lake surroundedby pastures the level of food early in the breeding season and aspen(Populusspp.) parkland. The lakeshoreis influencesreproductivesuccess and mating be- characterizedby large emergencesof midges (Chironomidae)in mid-May. Adult swallowsoften forage havior (Ewald and Rohwer 1982, Davies and
Lundberg 1984).To understandthe relative importance of food abundanceand male parental care to the maintenance of monogamy it will be necessaryto control for the effectsof both of these factorson female reproductive success. In this paper, we investigatethe influence of variation in food abundanceand male parental care on the reproductive successand mating behavior of female Tree Swallows (Tachycineta bicolor). We removed mated male swallows to
low over the lake surface
at this time.
The roadside
habitat that we studied was 1.5 km southwest
of the
lakeshore site. The roadside site did not have large
emergencesof midgesbecauseit was 1 km southof the lake and the prevailing winds during the study were from the southwest (i.e. not from the lake; En-
vironment Canada 1986-1988). Individually marked
swallowsusually foraged lessthan 300 m from the nest box and marked
birds
from
one habitat
were
never observed foraging in the other habitat. We placed 80 nest boxesin lakeshorehabitat and 40 nest boxes in roadside habitat in 1986. These num-
simulatethe lack of male parental careincurred bers were increased to 100 nest boxes at the lakeshore by secondaryfemales.This was a valid assump- in 1987 and 1988, and to 55 and 52 boxes at the roadtion in our study, becausesecondaryfemales side in 1987 and 1988, respectively.All nest boxes rarely received male assistancewith feeding were of the same dimensions and all faced east. Most nestlings(P. Dunn, unpubl. data).The removals nest boxeswere spaced24 m apart at the lakeshore were done at two sites (lakeshore and roadside)
and roadside.As part of another experiment(Dunn
that differed in insect abundance during the laying, but not the nestling,period. Therefore, if male parental care was not important to female reproductivesuccessin either habitat, then any differences between the lakeshore and roadsidein femalereproductivesuccess or mate choice probably would be due to differences between the sitesin conditionsduring the lay-
and Hannon 1991), some nest boxes at the lakeshore
ing season. METHODS
had additional boxesplaced around them in a spiral at 1, 4, 8 and 16 m, and some boxes at the roadside
had additional boxesplaced 1 m eastand 4 m west. This differencein number and spacingof boxesbetween
the lakeshore
and roadside
did not confound
our analysis of reproductive success,because nest
density did not affect fledging success(Dunn and Hannon 1991);however, it may have affectedthe frequencyof polygyny in each habitat. We addressthis potential effect in the discussion.All boxeswere in the same location each year except at the roadside where boxes were moved 700 m west after 1986 (to
Study animaL--Tree Swallows are aerial insectivoresthat nestreadily in nestboxes.They are usually
keep boxesaway from a temporarypond). Estimation ofinsectabundance.--To estimatea relative
monogamous,but polygyny occursoccasionally(58% of males;Quinney 1983,this study).At our study area in central Alberta, Tree Swallows have a single
index of insect abundance, we used "tow nets" that
brood of altricial nestlings each year. Eggsare incubatedby the femalefor 13 to 15 daysand young fledge at 18 to 20 days of age. During incubation the male guards the nest, but does not incubate. In monoga-
mouspairs both parentsfeed the nestlings.Polygynous males usually feed young in the nest of their primary female(the first female to settle),so the secondaryfemale receiveslittle or no male parentalcare. Foragingoccursboth on and off the territory, which
have proven effective for sampling insectseaten by Tree Swallows(Quinney and Ankney 1985). The insect sampling technique and equipment were identicalto thosedescribedby Husselland Quinney (1987). There is a significant positive correlation between clutch sizeand insectabundanceasmeasuredby these nets (Hussell and Quinney 1987). These nets collect all of the taxa and size classes of insects delivered
to
nestlings,although not in the same proportion as in the nestlings'diet (Dunn 1989).Therefore, we believe that thesenetsprovide an index of relative food avail-
490
DUNNANDHANNON
[Auk, Vol. 109
ability. We emphasizethat our resultsare basedon shore (20% SY females; 31/156) and roadside (14% SY relative indices,and any potential biasesin sampling females; 12/86) in the proportion of yearling females are likely to be similar betweenhabitats.Insectsflew that nested at each site (G = 1.35, P = 0.2, df = 1). or were
blown
into the tow nets and were
collected
Male-removalexperiment.--In each habitat, control and male-removalnestswere chosenrandomly from of the net. Two nets were used in each habitat each monogamouspairs.Over three years,maleswere colyear.Netswere2 m abovethe groundandmanybirds lected from 47 nests:7 nestsduring laying; 25 nests foragednearthisheightat our studyarea.In southern during incubation;and 15 nestsbetweenhatch and Ontario, Holroyd (1972) found that Tree Swallows sevendaysof age (nestlingscan fledgeafter 16 days). spent 47% of their foraging time below 4.6 m. The This totaldoesnot includeunaidedfemalesthatgained nets were opened manually at dawn and closedat a replacementmate (n = 2 females;thesemateskilled dusk on almostevery day from 6 to 12 May (depend- the young at hatch). Six of the 47 unaided females ing on the year) through 2 July. Nets were not op- were yearlings.The number of visitsto nestsby oberatedon two daysin eachof 1986and 1987and one servers was similar for nests of control and unaided day in 1988 becauseof high winds or snow. Hourly females. We examined the data from nests of unaided wind speedswere calculatedfor eachnet at eachsite femalesfor differencesin timing of male removalthat to correct for variation in the amount of air sampled may have biased our analyses.Mean date of male per day by eachnet. Thesehourly wind speedswere removal (relative to hatch date) did not differ between estimatedfrom regressionsbasedon wind-speedmea- lakeshoreand roadsidehabitats (P = 0.16; two-way surements made at each net with hand-held anemomANOVA). However, timing of male removal did difetersand wind speedsrecordedat Environment Can- fer among years (P < 0.001) becausemaleswere reada weather stations (Elk Island National Park, 30 km movedfrom laying throughthe early nestlingperiod northwest and Edmonton International Airport, 80 in 1987, while we removed males only around the km west). A correctionfor low wind speedwas made time of hatch in 1986 and 1988. If male parental asfollowing Hussell and Quinney (1987). Insectbiomass sistancehad a cumulative effect on fledging success, was calculatedby countingthe number of insectsin then one would expect to see a positive correlation different size and taxon categoriesand multiplying between the timing of male removal and subsequent that numberby the mean dry biomassof eachsize fledging success.However, no suchrelationshipwas and taxon category.An insectbiomassindex (IBI) was found in this study (r2 = 0.001, P > 0.5). Therefore, calculatedfor eachday by dividing the daily insect male removaldata from throughoutthe breedingseabiomassby the kilometersof wind passingthrough son were pooledwithin years,but we controlledfor the net while it was operating that day. We did not year effectsin all analyses. include insectsover 13 mm long in the insectbiomass Estimationof femalesettlementdate.--We estimated index calculation,becausethey are rarely fed to nest- when pair bonds were established:(1) to determine lings (0.7%,Quinney and Ankney 1985,this study). which birds were the primary and secondarymates Insects less than 1 mm were excluded for the same of polygynousmales;and (2) to comparethe fledging reason. success of monogamousand secondaryfemalesfrom Capture,marking,and determination of ageandsex.different territories that paired at the sametime. SetSwallows were caught in the nest box (Cohen and tlement date and date of nest initiation were used to Hayes 1984,Magnusson1984),bandedand individ- estimatethe date pair bonds formed. Settlement date ually color-markedon the breastwith felt-tippedink was the first date of three consecutivedays during markers.We individually color marked 86%and 19% which two birds were seen on or near a nest box of all breeding females and males, respectively.For (methodssimilar to Stutchburyand Robertson1987). behavioral observations,we only usedbreeding pairs Ten 1-min scanswere madedaily at eachnestingarea in which at leastone member was individually color- in 1988 to determine if a nest box was occupied. Nest marked.Birdswere classifiedto sexby the presence initiation was estimatedas the first day that piecesof of a brood patch in females or cloacal protuberance grasswere found in the nestbox. Settlementdate was in males.Femalesthat were yearlings (SY = second significantlycorrelatedwith date of nestinitiation in calendaryear of life) and older (ASY = after second 1988 (r2 = 0.58, df = 63, P = 0.001). Stutchburyand calendaryear) were distinguishedby plumagedif- Robertson(1987) did not find a significantcorrelation, ferences (Hussell 1983). Other studies have found difbut the breeding seasonis shorter and more synferencesbetweenyearlingandolderfemalesin clutch chronousin our study population (Dunn and Robsize and fledging success(DeSteven 1978);however, ertson in press). In 1986 and 1987 settlement data we found no such differencesin this study. Never- were not collected on a daily basis,so we estimated theless,we have generally excludeddata from year- the date of pair formation from nest initiation date. ling femalesto facilitatecomparisonswith other stud- This procedure allowed us to unambiguously assign ies. Data from yearlings (n = 6) were only included femalesto primary or secondarystatusin five of seven in the analysisof return rates where samplessizes casesof polygyny. In the remaining two cases,the were small. There was no difference between the lakeprimary and secondaryfemales settled on the same
in a jar of 70%ethanolattachedto a sleeveat the end
July1992]
Food Abundance andMonogamy inSwallows
491
date, so we assignedsecondarystatusto the female that did not receive male assistancewith feeding nestlings.Our resultsremainedunaffectedwhen datafrom primary and secondaryfemaleswere switchedin these
visitsby the female and male inside the nestbox and, for the females, the time engaged in particular behaviors: inside the nest box; perched on the box or post;flying; and perched at the entrancehole looking
two
into or out of the nest box.
cases.
Estimation of reproductive success andsurvivaL--Each nest box was visited every two to three daysaround the time of clutch initiation to determine date of laying the first egg (laying date) and clutch size. Nests were checked starting one or two days before the calculatedhatch date to determine hatching date (day the first egg hatched was nestling day 0). Nestlings were weighed with a Pesolascalejustbefore fledging (at 16 daysof age) to assessbody condition. Fledging successwas the number of nestlings in the nest at 16 days of age minus birds found dead in the box on visitsafter 20 daysafter hatch.The percentageof nests that produced at least one young (successfulnests) and fledging successof successfulnestswere used to determine if male parental care made a significant contribution to the reproductive successof the pair. For the purpose of comparing insect abundance with timing of breeding, we defined three periods during breeding: egg-formation/laying; incubation; and nestling. The laying period was defined as four daysbefore the date of the 10th percentile of clutch initiation to two days before the date of the 90th percentile of clutch completion. These values were chosenbasedon studiesof egg formation (Schifferli 1976) and timing of fertilization (Leffelaarand Robertson 1984). The nestling period was from 15 days (approximatelength of incubation) after the 10th percentile of clutchcompletionsto 30 days(approximate length of incubation and nestling periods) after the 90th percentileof clutchcompletions.The incubation period was the period between the laying and nestling periods. Femalescould compensatefor lossof male parental careby increasingtheir rate of food delivery to nest-
During the nestling period in 1986-1988, 31 control and
27 unaided
adult
females
were
observed
for a
total of 264 and 266 h, respectively. Becausethe same individuals were observed several times, we used av-
eragesof eachbehavior calculatedfrom multiple observationsof the same bird (only nestswith at least three observation sessionswere analyzed). This reducesthe problem of nonindependent samples.However, it assumesthat: (1) brood size and age have the same relative
effect on the behavior
of unaided
and
control females (i.e. similar slopes);and (2) observations of unaided
and control
females were distributed
similarly. Thesewere valid assumptionsin our case. Slopesof the regressionlines for nest visit rate and all other
behaviors
did not differ
between
unaided
and control females (P > 0.05; ANCOVA for each year and behavior, with brood size or age as the covariate and treatment asthe grouping variable). There was also no difference
between
unaided
and control
femalesin the distribution of brood sizesor ageswhen our observationswere made (Kolomogorov-Smirnov tests for each year; P > 0.32). Therefore, although brood size and age may have influencedparentalbehavior (Lombardo 1991),they did not have different effects on control
and unaided
females.
There
was
also no significant correlation between the number of observations/nestand the mean value of any behavior we measured (separate Pearson correlations for control and unaided females, P > 0.28).
We collectedfood bolusesfrom nestlingsfitted with pipe-cleaner collars (e.g. Walsh 1978) to examine whether massof food bolusesgiven to nestlings differed between
unaided
and control
females
and be-
tween habitats. Each nest was sampled once to reduce
lings. This may result in lower body massor return rate of unaided than control females. We weighed femalesbetween 1500and 1800when their nestlings
potential effectson nestling growth, and we made no parental care observationsat a nest on the day we sampledfoodboluses.Collarswere placedon all young
were 16 days old to determine if unaided females weighed less than control females. Rate of return to the study area was analyzed from banded swallows nesting in 1986 and 1987 to determine how many offspring and parents recruited into the breeding population.
in a nest during sampling. Collars were removed after the female made three to four trips inside the nest
Observations of parentalcare.--Observationsof individually marked females at control and male-removal nestswere madeat eachnest every two to three days.An initial nest was chosenrandomly for observation every two to three days (at the start of a new seriesof observations),after which nestswere picked systematically. Observationsessions were 20 rain long and were spread as evenly as possible among three periods of the day: morning (sunriseto 0900), midday (0900 to 1500), and late afternoon (1500 to 2000). During each observation session,observersrecorded all
box. Boluseswere retrieved from nestlingsand dried to constant massin an oven at 95øCprior to weighing.
Statisticalanalyses.--Weused analysis of variance (ANOVA) to examine insectabundance,laying date, clutch size, fledging success,and various behaviors. Interaction terms from ANOVA were nonsignificant unless noted otherwise.
Values
for the insect biomass
index were log-transformedprior to analysis.All tests of independence in two-by-two tables were performed with the G-test and William's correction (Sokal and Rohlf 1981).Testsof independencewith threeway tableswere performed using log-linear models in the CATMOD procedureof SAS(SASInstitute1985). Means are presentedwith standarderrors.All statistical tests were
two-tailed
unless noted
otherwise.
492
DUNN ANDHANNON
[Auk, Vol. 109
TABLE1. Insectbiomassindex (mg/100 km wind) a at Beaverhill Lake, Alberta, 1986-1988.
Laying Site
œ
(95% CI)
Incubation nb
œ
Nestling
(95% CI)
n
œ
(95% CI)
n
(4.0-20.2) (6.2-49.3)
9 9
11.7 18.7
(7.2-19.0) (12.0-29.2)
19 17
(1.6-34.0) (8.5-23.5)
6 12
8.9 9.8
(5.9-13.5) (5.8-16.6)
21 18
(6.4-15.7) (7.4-19.5)
10 7
7.6 10.2
(5.0-11.5) (6.4-16.2)
22 23
1986
Lakeshore Roadside
68.0 36.2
(30.3-152.9) (18.7-70.1)
16 16
9.0 17.5
Lakeshore Roadside
28.7 12.4
(15.9-52.1) (6.6-23.5)
17 12
7.5 14.2
Lakeshore Roadside
14.4 10.6
(7.1-28.7) (5.5-20.1)
14 17
10.1 12.0
1987
1988
aMean IBI estimatedby calculatingarithmeticmean of log-transformedIBI and then convertingbackto nag/100km wind by taking antilog of transformed mean. Calculation of 95% CI from Sokal and Rohlf (1981:420-421).
bDays of insectsampling.
= 5) was as high as the fledging successof monogamousASY femalesat the lakeshore(5.9 _+ Insect abundance.--Mean insect biomass index 0.2 young, n = 21; t = 0.92, P = 0.37) and road(IBI) did not differ between the lakeshoreand side (5.0 _+0.7, n = 8; t = 0.6, P = 0.58). Thereroadside when data from the entire breeding fore, secondaryfemalesdid not suffer a reproseasonwere analyzed (Table 1; P = 0.37, three- ductive cost from loss of male parental care. way ANOVA). However, there were significant Male-removalexperiment.--Reproductivesucinteractionsbetween nesting period and year, cesswas analyzed from three years of data on betweenhabitatand year, and betweenhabitat 41 unaided and 149 control ASY females. Males and nesting period. Year effectswere probably were removed from 16 nests at the lakeshore causedby greatermean IBI in 1986than in 1987 and 25 nests at the roadside. There were no or 1988 (Table 1). The habitat-yearand habitat- differencesin laying dateor clutchsizebetween nesting-period interactions probably were control and unaided females (Tables 2 and 3). causedby a greater mean IBI during laying at However, the earliest male removals were made the lakeshore than at the roadside in 1986 (Table late in the egg-laying period, so we did not 1). When analyzedby nestingperiod, meanIBis expect an effect. There were significant interonly differed between the lakeshoreand road- actions between treatment and site and besideduring the laying period(Table1;P = 0.027, tween year and site in the analysisof laying two-wayANOVA). In general,the meanIBI was date (Table 2). However, theseinteractionswere 1.4 to 2.3 timesgreaterat the lakeshorethan at not presentin the analysisof clutchsize (Table the roadsideduring the laying period (yearly 3), or in the subsetof datausedto analyze fledgmeans; Table 1). ing success(see below). Natural polygyny.--All six casesof polygyny Sample sizeswere too small to comparethe occurred at the lakeshore. There were two cases frequencyof nestfailure of controland unaided of natural polygyny in 1986 (6% of malesat the femalesbetween habitats.After pooling study RESULTS
lakeshore), four casesin 1987 (7% of males at
sites, there was no difference between control
the lakeshore)and none in 1988.Secondaryfemalessettledfrom 7 to 13 May (10 May _+1.3, 5 ASY, 1 SY), while monogamousASY females at the lakeshoresettledfrom 5 to 14 May (8 May
(32% failures, 41/128) and unaided (29%, 12/41)
_+0.4, n = 35; t = 1.55, P = 0.13). Data from 14 monogamousASY femalesat the lakeshorewere
higher in 1988 (46%)than in 1987(16%)or 1986 (19%),becauseof predation by weasels(Mustela ermina) on almost one-half of the nests at the lake (P = 0.01 for year effect). Only one to two nestswere depredatedin 1986 and 1987,when
excludedto comparethe fledging success of monogamousand secondaryfemales that settled concurrently. For females that settled concurrently and nestedsuccessfully,the fledging successof secondaryfemales (5.4 _+ 0.7 young, n
femalesin the frequencyof neststhat produced no fledglings (log-linear model; P = 0.66 for treatment effect). The rate of nest failure was
most nest failure
donment.
was due to starvation
or aban-
Since the rate of nest failure
was sim-
July1992]
FoodAbundance andMonogamy in Swallows
493
TABLI•2. Laying date at nestsof control and unaided female Tree Swallows, Beaverhill Lake, Alberta, 1986-
1988."Only includesadult birds with known clutchsize. Doesnot include data from nestsof primary or secondaryfemales, or renests.
Site
1986
1987
1988
œ+ SE (n)
œ + SE (n)
œ + SE (n)
23 May + 0.7 (27) 27 May + 0.7 (11)
24 May + 0.4 (48) 24 May + 0.8 (20)
Control
Lakeshore Roadside
26 May + 0.6 (8) 28 May + 1.1 (10)
Lakeshore Roadside
26 May + 2.2 (3) 28 May _+1.5 (7)
Unaided
20 May _+0.8 (5) 28 May + 0.7 (9)
22 May + 0.4 (6) 25 May _+1.1 (6)
P-valuesfor eacheffectin three-wayANOVA: treatment,P = 0.28;year, P = 0.0001;site,P -< 0.001;treatmentx site,P = 0.047;year x site, = 0.007;treatment x year, P > 0.60.
ilar between control and unaided females, but
wasgreaterfor unaidedfemalesat the lakeshore (6.2 +_0.3 young, n = 11) than control females ing successfor successfulnests. at the roadside(5.2 _+ 0.3 young, n = 38; t = Fledgingsuccess at successful nests.--Mean 2.5, P = 0.02). For a given clutch size, adult fledgingsuccess of unaidedand controlfemales femalesproducedsimilarnumbersof fledglings did not differ within a habitat or amongyears at the lakeshore and roadside (ANCOVA with (Table 4). The interaction between treatment clutch size as the covariate;P for intercept = and habitatwas alsonot significant(P = 0.43), 0.97; df = 1 and 84). However, there was a sigwhich suggeststhat the effectof male removal nificant difference between the slopes of the
differentamongyears,we only analyzedfledg-
did not differ
between
the lakeshore
and road-
side. Overall, femalesnesting at the lakeshore producedmore fledglings than femalesat the roadside(Table4). This occurredprobablybecausemean clutch size of monogamousadult femaleswas greaterat the lakeshorethan at the roadside(Table 3) and fledging success was related positively to clutchsize at both sites(lakeshore, r 2 = 0.48, P = 0.004; roadside, r 2 = 0.75,
lakeshoreand roadsideregressionlines (P = 0.014),becausein one year (1988) femaleswith the most common
clutch
sizes of five and six
eggstended to producefewer fledglingsat the road than at the lake. Therefore, in at least two
of three yearsthe differencein fledgingsuccess was probably due to differencesin laying date or clutch size and not due to differential
loss of
eggs or nestlings. Bodymassof nestlings.--Weused body mass
P < 0.001). Brood reduction (loss of eggs or young) was similar between the two sites.As- asan indexof nestlingbodyconditionjustprior suming that experimental male removal simu- to fledging (16 daysafter hatch).In this analysis latessecondarymatestatus,it appearedthat sec- a mean nestling masswas calculatedfor each ondary matesat the lakeshorewould produce brood to reduce dependenceamong samples. more young than females mated to monoga- Prior to examining the effectof male removal mous malesat the roadside.Fledging success on meannestling mass,we testedfor and found TABLE3. Clutch size at nestsof control and unaided female Tree Swallows,Beaverhill Lake, Alberta, 1986-
1988."Doesnot includedatafromnestsof primaryor secondary femalesor renests.Fivenestswith unknown laying datesare included. 1986
Site
œ+ SE (n)
1987
1988
œ+ SE (n)
• _+SE (n)
6.6 + 0.3 (27) 5.8 + 0.3 (12)
6.5 + 0.1 (43) 6.4 + 0.2 (20)
7.2 _+0.2 (5) 5.9 + 0.4 (9)
6.8 + 0.2 (6) 6.3 + 0.2 (6)
Control
Lakeshore Roadside
6.8 ñ 0.2 (8) 5.9 ñ 0.3 (12)
Lakeshore Roadside
7.0 _+0.0 (3) 6.1 + 0.3 (9)
Unaided
P-valuesfor eacheffectin three-wayANOVA: treatment,P = 0.16;year,P = 0.?9;site,P = 0.0001;all interactions, P > 0.16.
494
DUNNANDHANNON
[Auk,Vol. 109
TABLE 4. Number of fledglingsat successful nests(fledgedat leastone young)of controland unaidedfemale Tree Swallows, Beaverhill Lake, Alberta, 1986-1988. a 1986
Site
1987
1988
• _+SE (n)
• _+SE (n)
5.7 + 0.2 (25) 4.0 _+0.7 (9)
5.8 + 0.4 (17) 5.7 _+0.3 (19)
6.4 _+0.4 (7) 4.3 _+ 0.8 (6)
5.0 (1) 4.7 _+0.7 (6)
• -+ SE (n) Control
Lakeshore Roadside
5.8 + 0.2 (6) 5.5 _+0.2 (10)
Lakeshore Roadside
6.0 + 0.6 (3) 4.7 _+0.4 (6)
Unaided
P-valuesfor eacheffectin three-way ANOVA: treatment,P = 0.60;year, P = 0.62;site, P < 0.014;all interactions,P > 0.05.
no effect of timing of male removal, year or habitat on mean nestling mass.We also found no correlationbetweenmean nestling massand percent brood reduction. A positive relationship would be expectedif there were greater mortality among lighter nestlings.Only brood sizesof five, sixand sevenyounghad morethan three samplesin each treatmentcategory(Fig. 1). Among these broods, mean nestling mass was not affectedsignificantly by male removal (two-way ANOVA; P = 0.15; df = 2 and 70). Broodsize tended to have an effecton fledging
of the 10 returning fledglingswas the heaviest nestling in its brood on day 16. Among ASY females that nested in 1986 or 1987, 28% (18/ 65) of control and 28% (8/29) of unaided females
returned to the study area the next year (G = 0.0001,df = 1, P = 0.99).For all femalesnesting in 1986 or 1987, 29% (24/84) of control and 26%
(9/35) of unaided femalesreturnedthe next year (G = 0.05, df = 1, P > 0.8). For females that did
not changebreeding habitats,fledging success in the next year was not affected by male removal in the previous year (F = 0.03, P = 0.87,
mass (P = 0.059).
df = 1 and 23; data included
Body mass of female parents.--Control and unaided ASY female parents weighed 16 days after hatch had similar body mass(ANOVA; P
nests and 5 SY females).
= 0.17 for treatment effect, P = 0.49 for site effect). At the lakeshore, control and unaided
cantly between control and unaided females, although unaided femalestended to make more visits than control females (P = 0.11; Table 5).
femalesweighed 21.0 _+0.3 g (n = 24) and 20.8 _+0.2 g (n = 7), respectively;while at the roadside,control and unaided femalesweighed 21.1 _+ 0.4 g (n = 9) and 20.2 _+ 0.5 g (n = 9), respectively.Mean body massof ASY femalesalso did not differ
between
unaided
and control
fe-
males at either the lakeshore (P = 0.30) or road-
side (P = 0.64) after controlling for brood size with
ANCOVA.
Return rate of fledglingsand parents.--Ten fledglingswere recapturedon the studyareain subsequentyears,and all but two (from control nests) subsequentlybred. Eight of the 10 returning birds were from control nests (1.24% returning), and two were from nestsof unaided females (1.18% returning). These return rates were not significantly different (G = 0.003, df = 1, P > 0.95). Mean nestling massof control broods
that had at least one individual
return
in subsequentyears (22.5 _+ 0.7 g, n = 4) did not differ
from
broods
that had no individual
return (23.5 _+0.2 g, n = 49; P = 0.07). Only 1
10 unsuccessful
Nest-visitrate of controland unaidedfemales.Rate of nest visitation did not differ signifi-
The nest-visitrate of all femaleswas greaterat the roadside than at the lakeshore (P = 0.04 for site effect; Table 5). In contrast, the rate of nest
visitation washigher for controlpairs(maleand female combined) than for unaided females, and
the differencebetweensiteswasnot significant (Table 5). Interaction termswere not significant in these analyses,which suggeststhat the relative
effect of male
did not differ
removal
between
on nest visitation
unaided
females
at the
lakeshoreand roadside.Although unaided females did not increasesignificantly their total number of nest visits, from the perspective of individual nestlings (per-capita visits) it appeared that, in two of the three years, unaided females visited them as often as control pairs (Fig. 2; P > 0.27 for 1987 and 1988). However, in 1986 the per-capita nest-visit rate was lower for unaided femalesthan control pairs (Fig. 2, a differenceof 1.1 visitsnestling-• h-'; P = 0.04). Other
behaviors
were
examined
for differ-
July1992]
FoodAbundance andMonogamy in Swallows
27/] 2 3
••
'--- 23
9
[]Control ß Unaided !1
3 78 27 e 20 •
2.0
495
13
• 1.5
9
t5
'• 1.0
g•
ß
Female visits
[]
Male visits
11
It
U
C
• 0.5 17
2
3
4
5
6
7
0.0
8
C
Brood size
Fig. 1. Mean bodymass(g) and SEof Tree Swattow •esttin•s o• day ]6 0•st before •ed•i•) at •ests o• controt (paired) and u•aided •emales,Beaverhit]Lake, Alberta, [986-]988. Data •rom •ests o• ASY •emales.
N•mbe•s above bars are •umber o• •ests sampied.
ences between unaided
U
C
1986
1987
U
1988
Fig. 2. Mean nest visits per nestling and SE (during 20-min observationperiods) at nests of control (C) and unaided (U) female Tree Swallows, Beaverhill Lake, Alberta,
1986-1988.
Numbers
above bars are
number of nestssampled.
and control females us-
ing ANCOVAs with brood size asthe covariate.
females in 1987 and 1988. There was no differcontrol and unaided females in
ence between
There were no differences
between the lakethe mean brood size or brood age during the shoreand roadside,so habitatswere pooledin daysthat thesenestswere sampled(t-tests,P > theseanalyses(Table6). Unaidedfemalesspent 0.24, df = 14). Mean dry biomass/food bolus lesstime perched at their nest boxesthan did did not differ between unaided (0.054 + 0.008 control females. However, it was not obvious g) and control (0.040 + 0.007 g) femalesat the what
other behaviors
unaided
females were
lakeshore, or between unaided (0.037 + 0.006
spendingmore time doing, if they were spend- g) and control (0.050 + 0.004 g) femalesat the ing less time perched (Table 6).
roadside (P = 0.82 for treatment effect; P = 0.56
Food bolus mass at the roadside and lakeshore.-
for habitat effect;two-way ANOVA).
Females might be able to compensatefor the
lossof the maleby increasingthe massof each DISCUSSION foodbolusfed to nestlings.We examinedmean dry biomass/bolusby collecting food boluses Male parental care did not improve the re(n = 42) from collarednestlingsin nestsof seven productive successor survival of females breedcontrol (five lakeshore and two roadside) and nine unaided (five lakeshore and four roadside)
ing at Beaverhill Lake.There was no significant effectof maleremovalon nestsuccess, fledging
TABLE 5. Mean nestvisitsof adultTree Swallowsper 20-minobservation periodduringthe nestlingperiod, Beaverhill Lake, Alberta, 1986-1988. a 1986
1987
1988
œ+ SE (n)
œ+ SE (n)
7.2 + 0.7 (6)
5.8 + 1.5 (7)
5.9 + 0.6 (5)
4.5 + 0.3 (6) 5.6 + 0.8 (3)
3.4 + 0.8 (7) 4.8 + 0.5 (6)
3.1 ___ 0.3 (5) 4.3 + 1.0 (4)
8.1 + 1.7 (3)
4.9 + 0.8 (6)
+ SE (n) Lake
Pairs
Femalesonly Control Unaided
Road
Pairs
9.3 + 0.4 (4)
Femalesonly Control
6.4 + 0.6 (4)
3.9 + 1.0 (3)
3.4 + 0.8 (6)
Unaided
6.1 + 1.1 (5)
6.6 + 0.4 (3)
3.6 + 0.6 (6)
aP-valuesfor comparison of nestvisitsof controlversusunaidedfemalesin three-wayANOVA: treatment,P • 0.! 1;year,P = 0.0004;site,P = 0.038.P-valuesfor comparison of totalnestvisitsat nestsof controlversusunaidedfemales:treatment,P = 0.005;year,P = 0.003;site,P = 0.14.
496
DUNNANDHANNON
[Auk, Vol. 109
TABLE6. Behavior of control and unaided adult female Tree Swallows during the nestling period, Beaverhill Lake, Alberta, 1986-1988. Sample sizesare number of nests.Times are out of 20-rain observationperiods (in seconds).
1986 + SE (n)
1987
1988
œ + SE (n)
œ + SE (n)
P-values from two-way ANOVAs Treatment
Year
Interaction
In box
Control Unaided
278 + 51 (10) 300 + 78 (8)
238 + 57 (10) 318 + 32 (9)
Control Unaided
28 + 13 (10) 3 + 2 (8)
8 + 9 (10) 8 + 4 (9)
Control Unaided
840 + 48 (10) 871 + 78 (8)
806 + 59 (10) 792 + 39 (9)
Control Unaided
54 + 13 (10) 23 + 6 (8)
116 + 37 (10) 67 + 14 (9)
404 + 44 (11) 425 + 55 (10)
0.35
0.02
0.82
0.001
0.001
0.16
0.92
0.0001
0.91
0.34
0.005
0.33
Perched
84 + 26 (11) 46 + 19(10)
Flying 600 + 43 (11) 596 + 54 (10) At hole
success,female body mass,nestling body mass at fledging or rates of return of adults or fledglings to the study area. These results indicate that females were not choosingmonogamous mating situationsbecauseof the importanceof male parental care to female reproductive successor survival. Therefore, we suggestthat the importance of male parental care mechanism was not a likely explanation for the maintenanceof monogamyin thispopulation.The most important influence on female reproductive successappeared to be food abundance (mean IBI) during the laying season.Nestling season food abundance
did not differ
shore and roadside
between
the lake-
sites, and in two of three
years there was also no difference in brood reduction between sites. This suggeststhat the lakeshoreand roadsidesiteswere of equal quality during the nestling season.During the laying season,however, mean IBI was 1.4 to 2.3 times greater at the lakeshorethan at the road-
113 + 23 (11) 134 + 31 (10)
methods). However, in another experiment in which
we added
nest boxes to some territories
(Dunn and Hannon 1991), we found that males
increasedtheir probability of becoming polygynous when they had two or more nest boxes on their territories that were over 5 m apart. Additional
nest
boxes
at
the
roadside
were
placeda maximum of 5 m apart, and this could have limited the potential for polygyny to occur at the roadside. However, five males at the road-
side defended extra boxes 24 m apart yet did
not becomepolygynous.At the lakeshore,25% (3/12) of males with similar territories (two or
more boxes 24 m apart) became polygynous. This suggeststhat polygyny occurred at the lakeshore, and not at the roadside, because of
female choiceof breeding locationswith more food rather
than limited
numbers
of extra nest
sites. In summary, we suggestthat polygyny occurredbecauseof female choiceof better laying seasonconditionsat the lakeshore,and not side. This difference in food abundance was asbecausesecondaryfemalesat the lakeshorewere sociatedwith clutches that averaged 0.5 eggs compensatedsomehow for their loss of male greaterat the lakeshorethan at the roadside.As parental care. Most other male-removal studiesin typically a consequenceof the greater clutch size, fledging successwas also greater at the lakeshore monogamousspecieshave found that male pathan at the roadside (mean = 0.7 more fledg- rental care is helpful, but not a necessity (relings at the lakeshore).Thesedifferencesin food viewed in Wolf et al. 1988, Bart and Tornes 1989, abundance and reproductive successwere also Dunn and Hannon 1989). In these cases moassociatedwith a differencein mating behavior. nogamy may be maintained in the population, All six casesof polygyny occurredat the lake- becausethe lossof male parental careassociated shore.The absenceof polygyny at the roadside with secondarymate statusmakes monogamy could have been due to differences between sites almost always more advantageous to females in the number and spacing of nest boxes (see than polygyny (Wittenberger and Tilson 1980).
July1992] However,
Food Abundance andMonogamy in Swallows some
male-removal
studies
have
found that lossof male parental care had little or no effecton reproductivesuccess(Richmond 1978, Gowaty 1983, Hannon 1984, Martin et al. 1985, Martin and Cooke 1987, this study). In thesecases,monogamycannotbe explainedby the polygyny-thresholdmodel or other models that assumepolygyny is costly to the reproductive successof females (see reviews in Da-
vies 1989,Searcyand Yasukawa1989).The mating behaviorof Tree Swallowsat BeaverhillLake is probablybestcharacterizedunder the Searcy and Yasukawa (1989) classification as femaledirected choice, when there is no cost or benefit
associatedwith polygyny. That is, femalesare choosing mates based on the quality of the breeding situation (laying seasonfood abundance in our case),and mating statushas little or no effect on female reproductivesuccess. It is important to know the mechanism used by femalesto choosemates,becausethis information may help us to understandthe relative importance of male parental care in the maintenanceof monogamy(Dunn andHannon 1989). In the past,it hasgenerally been assumedthat the importanceof male assistancewith feeding nestlingsdetermines the choiceof mates.However, there is evidence that both laying and nestling seasonconditions may influence female reproductive successand mate choice.For example, Pleszczynskaand Hansell (1980) increasedlevels of polygyny in Lark Buntings
(Calamospiza melanocorys) by increasing the number of shaded nest sites. Greater nesting coverincreasedthe proportion of nestlingsthat fledged,presumablybecauseof greaterprotection from hyperthermia.Femalebuntingsalso appeared to show directed choice of better
quality nesting locations,but in this casenesting seasonconditionsappearedto influence re-
productivesuccess moststrongly.Foodsupplementation experiments that have examined mating behavior (Ewald and Rohwer 1982,Davies and Lundberg 1984) are more difficult to interpret,becausein thesestudiesfoodwassupplied throughout the breeding season.Nevertheless, in Red-winged Blackbirds (Agelaius phoeniceus), supplementalfeeding generally advanced egg laying and increasedmale mating success(Ewald and Rohwer 1982), suggesting an effect of early seasonconditionson female
497
seasonand, asa consequence,maleswere better able to defend
more
than
one mate.
In a five-
year studyof Northern Harriers (Circuscyaneus), Simmons et al. (1986) found that microtinerodent
abundance
was correlated
with
female
reproductivesuccessand male mating success. They suggestedthat male provisioningof food was important to female reproductive success throughout the breeding season.However, femalesmay basetheir mating decisionson male courtshipfeeding and condition, which appear to be influencedby food abundanceearly in the nesting season (Simmons et al. 1986). Studies of matingsystemsand parentalcareshouldask why male parental careis important (Dunn and Hannon 1989), not just whether it is important to female reproductive success. If male parental caredoesnot improve female fledgingsuccess in Tree Swallows,then: (1) Why do males provide any parental care?(2) Why are not more malespolygynous?Bartand Tornes (1989)presentedthree hypothesesfor the maintenance of monogamouspair bonds in cases where the value of male parental care is low. Only one of these hypothesesis applicable to Tree Swallows.They suggestedthat, if male parental careis generally not costlyto males,then there might not be strongselectionagainstproviding male parental care.This hypothesismay have some application in Tree Swallows because, although male parental care does not measurably improve female fledging success, there do not appearto be any betteralternatives for malesduring the nestling period (suchas finding another mate),and assistingthe female with feeding appearsto improve the condition of nestlings in larger broods (e.g. Dunn and Robertsonin press).A high rate of nest failure during the nestling period (from predation or weather) may also favor remaining with one's mate and assistingwith feeding if it facilitates renesting (Martin and Cooke 1987). It is also possible that male parental care could be important in years of very low insect abundance and that because of fluctuations
in food abun-
dance monogamymaximizesthe long-term fitnessof females(Lyonet al. 1987,Bartand Tornes 1989).During our three-yearstudy,insectabundancewasrelatively high (Table 7) and constant (Table1) during the nestlingperiod,sowe could not examine the role of uncertainty in mainmate choice. In female Dunnocks (Prunella mod- taining monogamy(see Rubenstein 1982). ularis),food supplementation led to a decrease Food abundance and intraspecific competiin femalehome-rangesizeearlyin the breeding tion for nest boxesprobably limit polygyny to
498
DUNN AND HANNON
[Auk, Vol. 109
TABLE7. Insect biomassindex (IBI) in Ontario and Alberta. Values are arithmetic means,standard errors,
ranges,andsamplesizesof back-transformed yearlymeansfor 1977-1984in Ontarioand 1986-1988in Alberta. Data collectedin sameway in Ontario and Alberta.
Location
Laying season
Nestling season
œ_+SE (range,na)
œ+ SE(range,n)
Sewage lagoon Backus field
39.1 + 7.6 (7.2-77.2, 8) 3.1 + 0.4 (1.6-5.2, 8)
17.9 + 3.3 (7.2-38.2, 8) 3.3 + 0.3 (1.7-4.5, 8)
37.0 + 6.0 (14.4-68.0, 3) 19.7 + 8.3 (10.6-36.2, 3)
9.4 + 1.5 (7.6-11.7, 3) 12.9 _ 2.9 (9.8-18.7, 3)
This study
Alberta
Lakeshore Roadside
Source Hussell and Quinney (1987)
Ontario
Number of yearsof sampling.
varying degreesin differentpopulationsof Tree the Alberta Division of Fish and Wildlife. Male swalSwallows.In many locations,polygyny may be lows were collectedunder scientificpermits WSA-14, uncommon
because food abundance
is relative-
WSA-7/87 and WSA-8/88 from Environment Canada, in accordance with Canadian Council on Animal Care
ly low and, asa consequence, maleparentalcare guidelines.Financialsupportwas providedby the may be more important to female reproductive Boreal Institute for Northern Studies,University of successand mate choice. For example, in pre- Alberta;a Natural Sciencesand EngineeringResearch vious studies of Tree Swallows in Ontario (Lef-
Councilof Canadagrant to S.J.H.and a Paul Stewart
felaar and Robertson1986,Quinney 1983,1986),
Award from the Wilson Ornithological Society to
male parentalcareappearedto be importantto female reproductivesuccess.Comparedwith
P.O.D.
Ontario sites, food abundance was relatively LITERATURE CITED high at BeaverhillLake(Table7; Dunn and Robertson in press).This may explain why male parental caredid not appearto be importantto BART,J., AND A. TORNES.1989. Importance of monogamousmale birds in determiningreproducfemale reproductivesuccessor survival at our tive success. Evidence for House Wrens and a study area. In areaswhere food abundanceis review of male-removal studies. Behav. Ecol. So~ relatively high and polygyny is advantageous ciobiol. 24:109-116. to some females,polygyny might still be unCOHEN,R. R., AND D. J. HAYES. 1984. A simple uncommon becauseintrasexualcompetition for attachednestbox trapping device. N. Am. Bird nestboxespreventsmostmalesfrom gaininga Bander 9:10-11. territory large enough that two femalescan DAVIES,N.B. 1989. Sexualconflictand the polygamy
breed without one female excluding the other (see Dunn and Hannon 1991, Dunn and Robertson in press).
threshold.
Anim.
Behav. 38:226-234.
DAVIES,N. B., AND A. LUNDBERG. 1984. Food distri-
bution and a variable mating systemin the Dunhock, Prunella modularis.J. Anim. Ecol. 53:895-
ACKNOWLEDGMENrrs
We thank R. Ebel,J.Faragini,S. Lord,C. McCallurn, R. Osterwaldt, D. Padilla, M. Quinn, J. Roberts, P.
Stubbs,L. Wakelyn and S. Westoverfor assistancein the field. C. McCallurn alsoassistedwith data analysis and preparationof the manuscript.We thank D. Hus-
912.
DESTEVEN, D. 1978. The influence of age on the breeding biology of the Tree Swallow Iridoprocne bicolor. Ibis 120:516-523.
DUNN, P. O. 1989. The maintenance of monogamy
in Black-billedMagpiesand TreeSwallows.Ph.D. dissertation, Univ. Alberta, Edmonton.
sellfor supplyingthe insectnets,providingthe com- DUNN, P. O., AND S. J. HANNON. 1989. Evidence for obligatemale parentalcarein Black-billedMagputerprogramsfor estimatingIBI and readinga draft pies. Auk 106:635-644. of this paper.Variousversionsof the manuscriptwere improvedby the commentsof D. Boag,D. Bryant,P. DUNN, P. O., AND S. J. HANNON. 1991. Intraspecific competitionand the maintenanceof monogamy Gowaty,G. Holroyd, D. Hussell,D. Jones,J. Lifjeld, M. Lombardo,R. Robertson,J. Spence,L. Whitting-
in Tree Swallows.
Behav.
Ecol. 2:258-266.
In press. Geoham,andanonymous reviewers.Thisstudybenefited DUNN, P. O., •u'qDR. J. ROBERTSON. graphicvariation in the importanceof male pagreatly from the cooperationand assistance of the
Beaverhill Bird Observatory.Funding for nest boxes was provided by the Bucksfor Wildlife Programof
rental careand mating systemsin Tree Swallows. Behav. Ecol.
July1992]
FoodAbundance andMonogamy in Swallows
499
DYRCZ,A. 1988. Adaptive significanceof facultative OPaNG,L.W. 1982. Avian mating systems.Pages 192 in Avian biology, vol. VI (D. S. Farner and J. polygyny in birds. Pages283-291 in Proceedings of the international centennial meeting of orniR. King, Eds.). Academic Press,New York. W. K., AND R. HANSELL. 1980. Polygthology (R. van den Liven, K.-L. Schuchmann PLESZCZYNSKA, and K. Schmidt-Koenig, Eds.). Deutsche Orniyny and decision theory: Testing of a model in thologie Gesellschaft,Bonn. Lark Buntings(Calamospiza melanocorys). Am. Nat. 116:821-830. EMLEN,S. T., ANDL. W. OPaNC. 1977. Ecology,sexual selection, and the evolution of mating systems. QUINNEY,T.E. 1983. Tree Swallows crossa polygyny Science
197:215-223.
threshold.
ENVIRONMENT CANADA. 1986-1988. Monthly meteorological summary at Edmonton International Airport, Alberta. Atmospheric Environment Ser-
Auk
100:750-754.
QUINNEY,T.E. 1986. Male and female parental care in Tree
Swallows.
Wilson
Bull.
98:147-150.
QUINNEY,T. E., AND C. D. ANKNEY. 1985. Prey size
vice, Environment Canada, Edmonton, Alberta. selectionby Tree Swallows.Auk 102:245-250. EWALD,P. W., AND S. ROHWER.1982. Effectsof sup- RICHMOND,A. R. 1978. An experimental study of plemental feeding on timing of breeding, clutch advantageof monogamyin the Cardinal. Ph.D. size and polygyny in Red-winged Blackbirds, dissertation,Indiana Univ., Bloomington. Agelaiusphoeniceus. J. Anim. Ecol. 51:429-450. RUBENSTEIN, D. I. 1982. Risk, uncertainty and evoGOWATY, P.A. 1983. Male parentalcareand apparent lutionary strategies. Pages 91-111 in Current monogamyamongEasternBluebirds(Sialiasialis). problemsin sociobiology(King's College SocioAm. Nat. 121:149-157. biology Group, Eds.). Cambridge Univ. Press, HANNON,S.J. 1984. Factorslimiting polygyny in Cambridge. the Willow Ptarmigan. Anim. Behav.32:153-161. SASINSTn'LrrE.1985. SASuser'sguide:Statistics.SAS HOLROYD,G. L. 1972. Resourceuse by four avian Institute, Inc., Cary, North Carolina. speciesof aerial insect feeders.M.S. thesis,Univ. SCHIFFERLI, L. 1976. Factors affecting weight and Toronto, Toronto. conditionin the HouseSparrowparticularlywhen HUSSELL,D. J. T. 1983. Age and plumage color in breeding. Ph.D. dissertation, Univ. Oxford, Oxford. female Tree Swallows. J. Field Ornithol. 54:312318.
SEARCY,W. A., AND K. YASUKAWA. 1989. Alternative
HUSSELL,D. J. T., AND T. E. QUINNEY. 1987. Food abundance
and
clutch
size
of
Tree
Swallows
Tachycinetabicolor.Ibis 129:243-258. LEFFELAAR, D., AND R. J. ROBERTSON.1984. Do male Tree Swallows guard their mates?Behav. Ecol. Sociobiol.
16:73-79.
LEFFELAAR, D., ANDR. J. ROBERTSON. 1986. Equality of feeding rolesand the maintenanceof monogamy in Tree Swallows. Behav.Ecol. Sociobiol.18: 199-206.
LOMBARDO, M.P. 1991. Sexual differences in parental effort during the nestling period in Tree Swallows (Tachycinetabicolor).Auk 108:393-404. LYON, B. E., R. D. MONTGOMERIE,AND L. D. HAMILTON.
1987. Male parental careand monogamyin Snow Buntings. Behav. Ecol. Sociobiol. 20:377-382. MAGNUSSON, A. 1984. Ny fangstmetod for holkfaglar. Var Fagelvarld 43:318. MART•N, K., F. G. COOCH, R. F. ROCKWELL,AND F.
models of territorial polygyny in birds. Am. Nat. 134:323-343.
SIMMONS, g., g. MACWHIRTER, P. BARNARD,AND G. L. HANSEN.
1986.
The influence
of microtines
on
polygyny, productivity, age, and provisioning of breedingNorthern Harriers:A 5-yearstudy.Can. J. Zool. 64:2447-2456. SOKAL,R. R., AND F. J. ROHLF. 1981. Biometry, 2nd ed. W. H. Freeman, New York. STUTCHBURY, B. J., AND R. J. ROBERTSON.1987. Do
nest building and first egg dates reflect settlement patterns of females?Condor 89:587-593. WALSH,H. 1978. Food of nestling Purple Martins. Wilson
Bull. 90:248-260.
Wn,•KLER,D. W., AND G. S. WILKINSON. 1988. Parental
effort in birds and mammals:Theory and measurement. Pages185-214 in Oxford seriesin evolutionary biology, vol. V (P. H. Harvey and L. Partridge, Eds.). Oxford Univ. Press,Oxford.
COOKE. 1985. Reproductive performance in WITTENBERGER, J. F., AND R. L. TILSON. 1980. The Lesser Snow Geese: Are two parents essential? evolution of monogamy: Hypotheses and eviBehav. Ecol. Sociobiol. 17:257-263. dence. Annu. Rev. Ecol. Syst. 11:197-232. MARTIN,K., AND F. COOKE. 1987. Bi-parental care in WOLF,L., E. D. KETTERSON, AND V. NOLAN, JR. 1988. Willow Ptarmigan: A luxury? Anim. Behav. 35: Paternal influence on growth and survival of 369-379. Dark-eyed Juncoyoung: Do parental males benORIANS, G.H.
1969. On the evolution of avian mat-
ing systemsin birds and mammals.Am. Nat. 103: 589-603.
efit? Anim.
Behav. 36:1601-1618.
