intraspecific interactions in jackdaws corvus

221

INTRASPECIFIC INTERACTIONS IN JACKDAWS CORVUS MONEDULA: A FIELD STUDY COMBINED WITH PARENTAGE ANALYSIS DORIT LIEBERS1,2 & HANS-ULRICH PETER1 Liebers D. & Peter H.-U. 1998. Intraspecific interactions in Jackdaws Corvus monedula: a field study combined with parentage analysis. Ardea 86: 221-235. As part of a long-term population study, we investigated the social and reproductive behaviour of Jackdaws in a colony with 70 nestboxes and about 180 birds near Jena, Germany. Three kinds of social relationships were recognized between breeding pairs and non-breeding birds: 'Followers' were associated with a breeding pair, functioning as potential replacement mates and behaving either neutrally or cooperatively with resident pairs. They were mostly young birds without breeding experience and of unknown sex. 'Visitors' were birds of either sex that did not complete, or had lost, their own clutch during incubation. They appeared to be of low social rank and interfered little with resident pairs. 'Usurpers' were also failed breeders, but they had lost their nestlings late during rearing. They remained paired and made aggressive attempts to take over nestboxes from resident pairs. A parentage analysis using multilocus DNA fingerprinting was carried out on 15 broods with 39 nestlings, mostly of pairs associated with followers. It revealed one extra-pair offspring and ruled out intraspecific brood parasitism and pseudo-parasitism. Key words: Corvus monedula - reproductive behaviour - usurpers - multilocus DNA fingerprinting - extra pair offspring - brood parasitism - mate replacement lInstitute of Ecology, Friedrich-Schiller-University, Domburger StraBe 159, D-07743 Jena, Germany; 2Present address: Ernst-Moritz-Amdt-University Greifswald, Vogelwarte Hiddensee, D-18565 Kloster, Germany, Email [email protected]

INTRODUCTION Although monogamy was originally considered to be the most successful breeding strategy in birds (Lack 1968), alternative mating strategies occur in many bird species (Birkhead & Moller 1992). Trivers (1972) suggested that males could increase their fitness through extra-pair copulations. Recently, biochemical and molecular techniques have been successfully employed to examine breeding strategies in terms of genetic advantages or disadvantages to breeding individuals (e.g. Krebs & Davies 1991; Pena et a1. 1993; Wetton et a1. 1992). Monogamy is not any longer seen

as a truly monogamous pair bond between one male and one female since extra-pair copulations and divorce are known to happen quite frequently (Ens et a1. 1996). Jackdaws Corvus monedula are socially monogamous, can live for at least 16 years and are fertile for 11 years (Peter 1992; Glutz von Blotzheim & Bauer 1993). Once a pair has formed and lasted for more than six months, partners usually stay together until one of them dies (Lorenz 1931; Zimmermann 1951). They can maintain the same breeding site for many years and may remain together even after several seasons of unsuccessful breeding (Roell 1978). Long-mated,

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ARDEA 86(2), 1998

experienced pairs enjoy a high social rank. Because their breeding behaviour is well synchronized and they are more efficient at rearing offspring these well established pairs have the highest breeding success in the colony (Peter & Steidel 1990; Henderson & Hart 1993). In contrast to other bird species (e.g. Sturnus vulgaris, discussed in Krebs & Davies 1991), both parents are necessary for successfully protecting and provisioning their offspring (Henderson et al. unpubl. data). Removal experiments in monogamous Oystercatchers Haematopus ostralegus (Ens 1992) showed, that the clutch is usually lost during removal of the male, which also seems to be the case for Jackdaws too (Peter 1992). However, mate changes in Jackdaws do occasionally occur (Coombs 1978). The first evidence of alternative mating strategies was discovered by Roell (1979) who described two cases of bigamy, were one male took care for two females and their hatchlings. This has been recorded also from other colonies (Nilsson 1990; GIutz von Blotzheim & Bauer 1993). In addition it is known that third birds can become associated with a breeding pair and form trios (Roell 1978; Strebel 1991). Depending on social rank and breeding history, nonbreeders may use different social strategies to interfere with resident pairs and either become involved in reproduction or acquire a new breeding site within the colony. In the colony near Jena (Germany), studied for more than ten years, bigamy seems to be relatively rare since the frequency of supernormal clutch sizes is very low (unpubl. data). In contrast, trios occur in almost every year but have never been studied in detail. Because monogamy as postulated by Lorenz (1931) is not the exclusive reproductive strategy in Jackdaws, this study addresses the following questions: What do third birds do at the nest of mated pairs and which social strategies do they pursue? Does extra-pair copulation or brood parasitism occur? How do supernumerary birds influence the reproductive success of mated pairs? We sought to resolve these questions by combining behavioural observations of individually marked birds with parent-

age analysis using multilocus DNA fingerprinting.

STUDY AREA AND METHODS The investigated colony is situated near Jena, Thtiringen (eastern Germany). It is comprised of 70 nestboxes that are located inside 12 pillars of a highway bridge, which has been used for breeding by Jackdaws and Kestrels Falco tinnunculus for more than 20 years. Nestboxes are placed behind lower or upper openings (see below) and were checked from within the pillars. The inhabiting bird families were named by a code symbolizing their location within the bridge. The quality of nestboxes varied greatly with respect to microclimate, predation risk and risk of take-over by kestrels. Nestboxes were characterized by three features: 'size of opening', which differed between 10 x 15 cm (small) and 20 x 30 cm (wide); 'draughtiness' of the nestbox and 'height', representing the distance from the ground (3 or 10 m in lower or upper openings, respectively). Beech Martens Martes foina were the main predators of Jackdaw broods in this colony. Birds have been colour-ringed since 1983 and individuals can be identified (Peter 1992). Within one breeding season the total number of adults fluctuates between 140 and 180. Strong competition takes place for the few high-quality nestboxes, even during winter when pairs visit their nestboxes regulary. Consequently, a considerable proportion of non-breeders, 20 - 60 birds, is present. Nonetheless, there is very high natal philopatry, perhaps enhanced by the fact that many other small colonies in the vicinity have been abandoned over the years, so restricting opportunities for dispersal (Peter 1994). The spring and early summer roost site for non-breeders and offduty breeding birds is only about 400 m away, while feeding grounds are located in an area of 6 km2 around the colony (Steidel et al. 1994). Behavioural observations Field observations were carried out from April

Liebers & Peter: INTRASPECIFIC INTERACTIONS IN JACKDAWS

223

Table 1. Stages of breeding periods and observation time in the Jackdaw colony Jena, 1993. date

breeding period

days

hours

01 Apr-20 Apr 21 Apr-IO May 11 May-19 Jun

egg-laying incubation rearing

5 5 14

18.0 15.5 47.0

mean (± SD) observ. time

----

to June 1993 (Table 1). Birds were directly observed with a telescope (40x magnification) from a distance of about 150 m. The whole south-facing side of the bridge contained 37 nestboxes. Nearly all nestboxes could be viewed at once and multiple data were recorded simultaneously. Particular attention was, however, paid to breedings pairs with a third bird present. Among the population of 62 breeding pairs within the total of 180 birds, 29 pairs inhabiting the south side of the bridge were studied. The overall observation time was 85.5 hours, mainly during the early morning. In 'normal' breeding pairs all activities were counted, e.g. flying in and out the nestbox, brooding, preening, feeding chicks and defending nest site. Pairs with an associated non-breeding bird were studied in greater detail and the following interactions were recorded, some with brief additional descriptions: male and/or female defending the nestbox (including attacks, fights or chasing away the intruder); third bird entering the nestbox and appeasement gestures or allopreening between a breeding and a non-breeding bird in the vicinity of the nestbox. Copulations almost always took place inside the nestboxes and were never directly observed. Copulation frequencies are poorly known (Antikainen 1978) and we could not establish this in our study. Quantitative measurements of mate guarding were not possible because the number of birds in the colony was too large, and foraging sites were too widely scattered, to permit the tracking of single mated pairs. Egg laying date, hatching rate and survival of chicks was recorded during nine nestbox checks. Because this study was

3.6 ± 0.3 hrs 3.1 ± 0.8 hrs 3.4 ± 1.9 hrs

part of an ongoing project, additional data on pair formation, breeding success and intraspecific interactions were available for previous and subsequent breeding seasons. Classification of non-breeders We differenciated three types of intraspecific interactions between resident breeding pairs and interfering non-breeders (Table 2). Followers were non-breeding birds, associated with one breeding pair and forming a trio which lasted at least one week. The sex of followers could not be determined. Followers were easily distinguished from breeding pairs by differences in behaviour. Breeding birds responded to an intruder either by expelling or tolerating it. In the first case threatdisplays and fights occurred to either intimidate or drive off non-resident birds. Followers responded with appeasement gestures, including a crouching body position, with head lowered and bill pointing down, wings half-spread and drooping, combined with lateral quivering of the tail. Additionally, allopreening of the resident's nape and head feathers took place. Visitors were single birds that either had not completed or lost their clutch during incubation due to take-over of nestboxes by Kestrels, predation, effects of bad weather or food shortage. Presumably most visitors bred in the colony for the first time and can be classified as unexperienced recruits with low social rank. Apparently their pair bond had temporarily dissolved. After they had lost their nest they visited numerous nestboxes seemingly at random and disappeared intermittently. In contrast to 'usurpers', visitors never appeared in pairs during their 'inspection

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ARDEA 86(2), 1998

Table 2. Supposed classification of intraspecific interactions between third birds and breeding pairs in Jackdaws. Ring codes: a = aluminium, r = red, y = yellow, w = white, b = blue. Results of DNA analysis for breeding pairs, third birds interacted with: MR - mate replacement, EPY - extra-pair young, 0 all hatchlings are highly related to their social mother and their social father. Note, not all individual visitors and aliens are listed. breeding history ring code

Followers -r/-a

--I-a yb/ra

--lay --/wa --I-a Visitors yb/ra --/ba

--/wa --I-a Usurpers yr/ba

-y/-a --/wa --/ba

age sex

compi. hatch clutch lings

time of appearance

fledg- breed. nest mcu- rearing entering lings pre buil- bation nestbox ding year

action results of towards DNAone nestbox analysis

ly ? 5y 2y 3y ?

? ? ? ? ? ?

no no ? no no no

no no no no no no

no no no no no no

no ? ? no no ?

yes yes no yes yes yes

yes yes no yes yes yes

yes no yes no no yes

yes yes yes yes yes yes

13S1 6S1 6S1 9S1 lOS3 lON3

6y 4y 3y ?

f m f m

yes yes yes no

no no no no

no no no no

? no no ?

no no no no

yes yes yes yes

yes no no no

no no no no

no no no no

>6y f >2y m 3y f 4y m

yes yes yes yes

yes yes yes yes

no no no no

yes yes yes? yes?

no no no no

no no no no

yes yes yes yes

yes yes yes yes

13S1, 14S1 13S1, 14S1 9S3 9S3, 12S3

flights'. Although they disturbed incubating females during these visits, serious fights or harmful attacks were never seen. Usurpers were mated birds that had lost their offspring late during the rearing period. Reasons were the same as for Visitors. Usurpers remained in pairs. Although they had lost their young they maintained a high social rank. They were very aggressive and harmful towards neighbouring breeding pairs by chasing them off, attacking fledglings and preventing breeding adults from either defending the nestbox or searching for food, which resulted in numerous severe fights. Their appearance in the colony from nearby roost sites triggered alarm calls and nest defence by breeding pairs. Followers and visitors are collectively termed non-residents, as opposed to usurpers, which did

0 0 MR EPY

0 0

0 0 0 0

maintain their resident position in the colony. Despite the clearcut division between the behavioural categories described above, occasionally birds could not be assigned to any category because only half of the colony was studied (south side) and birds that were probably from the north side frequently showed up in our study area. DNA fingerprinting During the breeding seasons of 1992 and 1993 blood samples were collected in the colony. We concentrated on breeding pairs associated with a third bird but collected blood from young of 'normal' pairs too. Predominantly breeding females were caught with a net when l~aving their nestbox. Males were trapped while feeding nestlings inside the nestboxes. Nestlings were sampled at the minimum age of 14 days. All blood samples

225


(ca. 0.3 ml) were taken from the Vena ulnaris and kept in 1.0 ml APS-buffer (10% EDTA, 1% NaF, saturated with Thymol, pH 8.0; according to Arctander 1988) and stored at 4°C. The genomic DNA was isolated and purified by high salt precipitation (Miller et al. 1988). Ten mg of DNA were digested overnight with 25 units Hinf I or Hae III (New England Biolabs), extracted with phenol/chloroform and recovered by ethanol precipitation. Electrophoresis was performed in a 0.9% agarose gel and TBE buffer at either 30 or 50 V for 42 h. After electrophoresis the DNA fragments were transferred to a nylon membrane (Hybond-N+, Amersham) by alkaline capillary transfer as outlined in Sambrook et al. (1989). Membranes were prehybridized in hybridisation mixture for I h at 38°C and hybridized for 3 h using the probes (GGAT)4 or (GACA)4 (Fresenius). Subsequently the membrane was washed and incubated with an antibody against digoxigenin (Boehringer). This antibody was coupled to a phosphatase which in tum produced a coloured precipitate at the sites of hybridisation (for details see Mathe et al. 1993). The fingerprint profiles were analysed according to Westneat (1990) and Wetton et al. (1987). Normally the band-sharing coefficient fluctuates between 0.17 - 0.27 for unrelated individuals and approximates 0.5 for related individuals (Burke 1989). If unique mismatched bands of offspring were found but all other bands could be assigned to one or both parents, novel fragments were treated as mutation events. The mutation frequency (number of unmatched bands/total band number of all offspring) varies between 0.01 - 0.001 (Burke & Bruford 1987). If more than five mismatched bands were present and the number of parental bands was low, novel fragments were considered to be a result of an extra-pair fertilization or intraspecific brood parasitism (Wetton et al. 1992). Thus, a reliable parentage analysis is possible only by studying both putative parents.

1.5

::l

~ 1.0 en

c

o

·u ~

2 0.5 c

followers visitors usurpers

0.0 '----........." - - - -

egg-laying incubating

rearing

Fig. 1. Seasonal pattern of observed interaction rates between breeding pairs and followers (including all actions where the follower and one or both mates of the associated breeding pair were involved), visitors ('inspection flights') and usurpers (attacks towards breeding pairs) during egg-laying, incubation and rearing.

RESULTS Seasonal pattern of interfering non-breeders Non-breeders interacted with breeding birds in a seasonal pattern, as depicted in Fig. 1, where the number of interactions per hour is summarized for all observed nestboxes. Followers were in the colony during the whole breeding season, visitors appeared during incubation and rearing, while usurpers showed up only during the rearing stage. Interaction rates of followers did not differ between egg-laying, incubation and rearing (Kruskal-Wallis test, H = 0.129, df = 2, P = 0.938), while interactions with visitors were less frequent during rearing than during egg-laying (Mann-Whitney V-test, V 25 , 21 = 1009, P = 0.0017). Spatial pattern of interfering non-breeders To check whether non-breeders had certain spatial preferences within the colony, e.g. for nestboxes with high egg numbers, we compared all interactions of followers, visitors and usurpers during the different stages of the breeding period. Overall, there was a significant positve correla-

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ARDEA 86(2), 1998

0.5 0.0 0.5 0.0

~

.s:::

0.5

~ 0.0

o

:g ~ 0.5

~

0.0 0.5 0.0 0.5 0.0

clutch size / nestbox Fig. 2. Spatial pattern of interaction rates in the colony. Each column represents one nestbox (n = 37), arranged according to the clutch size. Row 1 shows the egg numbers, ranging from zero to six eggs (dark shading). Each following row (bottom to top) shows the interaction rates, starting with those for followers during egg-laying, incubation and chick rearing, followed by visitors during incubation and rearing and the top row stands for usurpers during chick rearing only.

tion of interaction rate and clutch size (Spearman rank, r = 0.62, df = 36, P < 0.001). However, this does not holds true for followers' interaction rates during the egg laying (r = 0.30, df = 36, P = 0.07) and incubation period (r = 0.09, df = 36, P = 0.58), while during chick rearing there was a positive correlation (r = 0.40, df = 36, P = 0.015). Visitors did not appear to preferentially attack nest boxes with higher clutch size during the incubation (r = 0.14, df = 36, P = 0.41) and the chick rearing (r = 0.27, df = 36, P = 0.11) period. In contrast, usurpers showed a strong correlation

of attack rates and clutch size (r = 0.52, df = 36, P < 0.001). From Fig. 2 it appears that this effect might be due to a concentration of interactions upon certain nest boxes. We used the Standardized Morisita's Index of Dispersion (Krebs 1989) in order to investigate whether the attacked locations showed an aggregated pattern. With IP = 0.54 the distribution of interactions of followers indeed showed a slightly clumped pattern on certain nest boxes (interactions of all three periods combined). Considering interactions from all supernumerary birds (followers + visitors + usurpers), the pattern is still slightly aggregated (IP = 0.52). Nests in boxes with a wider opening had a significantly lower median clutch size (3 eggs) than nests in boxes with a small opening (median 5 eggs), (Mann-Whitney V-test, Vn 21 = 127, P = 0.032). There was no difference in 'clutch size between nests with low (median 4.0) and with strong (median 4.5) draughtiness (U3 32 = 57.5, P = 0.86), whereas the trend for differ~nt clutch sizes in lower (median 0 eggs) and upper (median 5 eggs) heights from the ground approached significance (U4 ,33 = 38.5, P = 0.07). Note however, that there were only four samples from the lower height category. The formation of trios During the whole breeding season six 'stable' trios were observed in the colony. These followers belonged to all age classes but did not breed themselves in the colony (Table 2). Before and during the egg-laying period breeding males, but not females, reacted very aggressively towards followers. Usually a follower sat in front of the nestbox while the resident female was incubating or brooding. If no breeding bird was present, the follower entered the nestbox for 10 - 20 seconds (nine observations), but we could not determine whether it fed the young. Faecal sacs were removed four times. The follower remained subordinate to the breeding pair and showed appeasement gestures. Within trios, social relations of followers were closer to the breeding female than to the male, although fights occurred with both


sexes. Allopreening took place twice on neutral ground, distant from the nest territory. In both cases the follower started to preen the resident female. Overall trios were observed 82 times at 20 different nestboxes. 20 of those interactions occurred before or during the egg-laying phase, therefore increasing the potential for extra-pair copulations (Fig. 1). While most trios split up after one or two weeks, one trio lasted over the whole breeding season (see appendix, family 13Sl; Liebers 1994). The breeding male defended its nestbox most severely before and during egg-laying and all offspring were highly related to their social mother and their social father.

Visitors and usurpers Seven pairs lost their nestbox either before completing their clutch or during incubation. Such adults were observed 26 times as visitors in the colony, each time involving consecutive 'inspection flights' of up to four different nestboxes (Fig. 1). Visitors did not influence breeding pairs since they only 'hang around' in the colony. Due to the complete occupation of potential nest sites, there was no opportunity for them to immediately take over a nestbox. Another five pairs lost their complete brood during the chick rearing period. As usurpers they intruded into neighbouring nestboxes whether or not the breeding pair was inside. A total of 55 attacks by usurpers on 11 breeding pairs were seen. In contrast to visitors, each usurper pair focused its intrusions and attacks on only two or a maximum of three neighbouring nestboxes. Breeding pairs lost time due to aggression and nest site defence which was never necessary when dealing with visitors. Especially fledglings suffered from usurpers either by direct attacks or by being caught between fighting adults, through starvation or by leaving the nest too early. In one case a fledgling died as a consequence of such attacks, in several other cases nestlings were injured, some seriously, by conspecifics. After young had fledged, usurpers were still often seen at these nestboxes, which they apparently had tried to take over. Indeed, two of such usurper

227

pairs actually bred in these boxes during the following breeding season (own observations). There was a clear difference in the behaviour of visitors and usurpers and we point out that the time of breeding failure is most important for the future impact on the reproductive success of pairs still breeding.

Parentage analysis DNA fingerprinting focused on those six pairs which were associated with a follower, although followers were impossible to catch. In addition, paternity in broods of nine 'normal' pairs was studied. Altogether the paternity of 18% of successful breeding pairs in 1992 and 1993 was analysed. Fingerprinting data for probe (GACA)4 and (GGAT)4 are listed in Table 3. Band-sharing rates for presumably unrelated birds were low (0.2 and 0.21, respectively), for first order relatives in the range of 0.5. Two chicks revealed band-sharing rates different from the expected value: One chick (EPY - extra-pair young) was the first-hatched nestling of a clutch containing six eggs. Bandsharing values between the chick and the social mother were high (0.58 and 0.50, respectively), but low between the chick and the social father (0.26 and 0.36, respectively). A high number of unmatched fragments was detected with both probes. The offspring appeared to be highly related to their social mother but unrelated to their social father. This was interpreted as a case of extra-pair fertilization which corresponds to 2.6% extra-pair offspring within the whole sample. In the other case (MR - mate replacement) the genetic father had disappeared during incubation and was replaced during the rearing phase by a different adult, not related to the chick. Again, the bandsharing values between this chick and the social mother were high (0.51 and 0.54, respectively), but low between the chick and the social father (0.17 and 0.22, respectively). The number of novel fragments varied between 6 and 7 according to the probe used and it was proven that the social father had no genetic relation to the offspring he was rearing.

228

ARDEA 86(2), 1998

Table 3. Results of fingerprinting analysis using probes (GACA)4 and (GGAT)4' EPY - extra-pair young; MR mate replacement. (GACA)4

(GGAT)4

9 7 35

8 7 29

mean number of scorable bands

21.35 SD 4.55, n = 51

20.42 s.d 3.89, n = 44

average band-sharing between unrelated birds

0.20 SD 0.05, n = 11

0.21 SD 0.05, n = 11

social mother - offspring (with EPC and MR)

0.54 SD 0.09, n = 33

0.55 SD 0.10, n = 26

social father - offpsring (without EPY and MR)

0.52 SD 0.06, n = 20

0.50 SD 0.08, n = 20

EPY - social mother EPY - social father

0.58, n = 1

0.50, n = 1

0.26

0.36

0.51, n = 1

0.54, n = 1

0.17

0.22

investigated number of birds mothers fathers offspring

MR - social mother MR - replacement bird

Fig. 3 illustrates the different band-sharing values between unrelated birds, social motheroffspring and social father-offspring. Using both probes the extra-pair young (EPY) and the young raised by a replacement mate (MR) fall into the band-sharing range typical for unrelated birds. Fig. 4 shows the number of novel fragments against the band-sharing values for all offspring and their putative parents (unrelated birds are excluded). Only one nestling appeared to have a unique mismatched band using probe (GACA)4' This corresponds to 1/1026 = 0.00097 and solely represents a mutation event. Each picture appears to have two groups: 2 chicks with 0 or I novel fragment, 2 chicks with 7 or 6 novel fragements. The low band-sharing values and the high number of novel fragments prove the occurence of extrapair young and mate replacement in our colony.

Since all chicks are highly related to their social mothers, broodparasitism can be excluded. Background sharing (between unrelated birds) was significantly lower than band-sharing values between social mother-offspring and social fatheroffspring while inbetween the latter two no difference is found (Student's t-test; Table 4). The resolution of both probes is high and each can be used independently.

DISCUSSION

Permanent territoriality and considerable longevity characterize our study population, as is typical for Jackdaws (Peter 1994). Intense competition for nestboxes resulted from severe limitation of nest sites as well as from demographic fea-

229


10

8

(GACA)4

0.8

Dunrelated birds (n=9)

(GACA)4

o social mother-offspr. (n=35)

0

II social mother-offspr. (n=35) I social father-offspr. (n=22)

0.6

0.4

Q)

:::>

~

• social father-offspr. (n=22)

• EPY.

0.2

MR.

~ g' 0.0

.~

.