Hooded Crows Coi,ews cornix, Great Black-backed Gulls Lnrtts marintts and Herring Gulls L. nrgentutzts were the main nest predators in an Eider population in ...
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Nest predation and nest site selection among Eiders Somateria mollissima: the influence of gulls FRANK GOTMARK & MATTI AHLUND Department of Zoology, University of Gothenburg, P.O. Box 25059, S-300 31 Gothenburg, Sweden
Accepted 20 April 1986 Hooded Crows Coi,ews cornix, Great Black-backed Gulls Lnrtts marintts and Herring Gulls L. nrgentutzts were the main nest predators in an Eider population in southwest Sweden. T h e clutch sizes of Eider nests within gull colonies did not differ from those outside gull colonies. 'The proportion of Eider nests destroyed by predators was significantly lower within than outside gull colonies, especially on islands with Lesser Black-backed Gulls L. fuscus. Although the difference was not significant, the survival time of simulated Eider nests was higher within than outside gull colonies. O n Eider islands with gull colonies, foraying crows spent more time within the colony area than expected by chance. However, crows apparently avoided an area around each gull nest, and we suggest that the colonies, to some extent, protected Eider nests against predation. T h e density of Eider nests was higher on gull islands than on gullfree islands, and higher within than outside the gull colonies. However, the association with gulls was weak compared to that displayed by some other waterfowl.
Some ducks, grebes and waders nest in colonies of aggressive gulls and terns, and this is often interpreted as an adaptation to reduce nest predation (e.g., Hilden 1965). In the Tufted Duck Aythya fulzgula and in grebes, nests in colonies of gulls and terns seem to suffer less predation than nests outside the colonies (e.g., Newton & Campbell 1975, Burger 1984). Hatching success may thus be improved in the vicinity of larids, although ducklings can be subject to high predation at colonies of some gull species (e.g., Dwernychuk & Boag 1972, Munro & Bedard 1977). For the Eider Somateria mollissima, there is conflicting evidence as to the influence of larids on hatching success. For instance, in the Baltic Sea, Eider nests on islands with nesting gulls suffered lower predation than nests on gull-free islands (Olsson 1951). On the other hand, hatching success of Eiders did not differ within and outside a large Herring Gull colony in south Sweden (Andersson 1968). Bellrose (1980) concluded that the larger gull species are the main predators of Eider nests over most of the species' range. In this study, we examine nest predation and Eider hatching success in relation tcr presence of Lams gulls. Further, we examine the foraging behaviour of Hooded Crows Corms cornix (potential nest predators) on Eider islands and identify the main predators on Eider nests.
Study area T h e area is situated on the Swedish west coast (57"50'N, 1l"40'E) and covers about 50 km2,with a total island area of 3.8 km2(Fig. 1). T h e islands are low (0-25 m a.s.l.), usually presenting a mixture of low vegetation (mostly grass) and bare rocks. At protected sites, there are often some shrubs, but rarely trees. During the incubation period of the Eider, human disturbance is rare on the nesting islands. There is little tidal fluctuation in the area (spring tide 20 cm).
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Figure 1 . T h e study area at the coast of southwest Sweden. Islands were censused for Eider nests in the whole area, but observations of Eider islands were performed in the inner archipelago. Dots mark inhabited nests of Hooded Crow on islands in 1981, and numbers refer to the islands included in Table 2. Three of the larger islands are inhabited by people.
In 1981, we censused the breeding populations of potential nest predators. All cliff- and tree-nests of crows were checked, while the number of gull pairs on each island was estimated by counting all adults and dividing b y two. Potential nest predators identified were: Hooded Crow, 26 pairs; Magpie Pica pica, 1 1 ; Great Black-backed Gull L . marinus, 360; Herring Gull L. argentatus, 700; Lesser Blackbacked Gull L .fuscus, 725; and Common Gull L. canus, 325. Mammalian predators were rare: we observed one fox Vulpes vulpes (on a large Eider-free island), two stoats Mustela erminea and one mink Lutreola lutreola. Crows nested on both small and large islands (Fig. l ) , while Eiders and gulls nested on small or isolated islands. T h e first crow egg hatched 4 May (1981) and 6 May (1982); most eggs hatched before 15 May. T h e four Larus gulls start egg-laying in mid-April (marinus),late April (argentatus),early May (fuscus) and about 10 May (canus) (pers. obs.).
Methods Field work was performed between 14 April and 4 June 1981,s April-10 June 1982, 2-6 May 1983, and 25 April-1 5 May 1985. I n 1981, we visited eight islands 2-6 times in the latter part of the Eider's incubation period (for details, see Gotmark 8z Ahlund 1984). On 8 May, just before the Eider nests hatched, we carefully searched 1 1 other islands, recording robbed nests, a few hatched nests, and clutch size of active nests. A nest was classified as robbed if it contained down but no eggs, or eggshells with membranes tightly attached; hatched nests contained many small shell fragments, loosely or not at all attached to membranes. Concealment of each nest was classified on a 1-3 scale (1; < 10%cover of herbs, shrubs or cliffs, 2; 10-60% cover, 3; > 60yo cover).
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During the periods 10-1 8 May 1982 and 2-6 May 1983, we made one visit to each of 5 1 ( 1 982) and 49 (1 983) islands, recording Eider nests in the same way as in 198 1 . In 1982,6’, of 1488 recorded nests had already hatched, while in 1983 l o , of 1677 nests had hatched. These nests were included in analyses of nest predation, but not in analyses of clutch size. We specifically selected islands with different species of gulls or with no gulls at all, and on each island counted all adult gulls present. An Eider nest was classified as ‘within colony’ if the nest was < 10 m from a gull nest or a vantage point in the colony often used by gulls (indicated by copious gull faeces); otherwise it was ‘outside colony’. T h e areas of nine gull colonies were measured from maps (scale 1 : 10,000) upon which the approximate limits of the colonies had been drawn. Seven of the 5 1 islands censused in 1982 had few Eider nests or were otherwise unsuitable, and in 1983 we therefore excluded them, adding five new ones. In the analyses below, all censused islands were used. T o make sure that inclusion of different islands in different years did not influence the results, we repeated all analyses using only islands censused in both years; the results were almost identical for the two sets of data. In 1985, we used simulated nests to examine the risk of predation during the Eider’s incubation period. Females rarely leave their nests in daylight (Anderson 1975), resulting in a low density of unattended nests during the day. We therefore only set out 1-2 simulated nests per island (mean density 0.9 simulated nests/ha). For each such nest, three hens’ eggs painted to resemble Eider eggs were laid in a depression in the ground. T h e eggs were first covered by Eider down, then by grass and sprigs so that hardly any down was visible. Six active but unattended Eider nests found during the incubation period had been similarly covered by the hen. All simulated nests had concealment value 1 . We conducted 46 tests during two weeks in 1985 (29 April-3 May and 6-10 May). In each test, we exposed two simulated nests, one in a gull colony < 10 m from an active gull nest, the other outside the colony 2 2 0 m from any gull nest (mean distance 5 5 m, range 2&350 m) on the same (33 cases) or on a nearby island ( 1 3 cases). Each colony was used only once. Twelve tests were conducted for each species of gull, except for canus (10 tests; only 10 colonies were available). For all species, the tests were evenly distributed over the two-week period. We specifically chose colonies where one particular gull species nested exclusively or predominated; nests of that species always surrounded a simulated nest. In each colony and gull-free area we selected the nest site by using random numbers, given the restrictions described above. We checked the nests each day for five days after they were set out. From hidden positions on nearby islands, we recorded the activity of crows on Eider islands as well as predation on Eider nests. Observations were performed during 21 days (on average 5.5 man-hourslday) between 12 April and 1 3 May 1982. T h e islands were checked on opposite sides by two observers, communicating b y radio. Crows were scored either as ‘foraging’ (walking, pecking on the ground, or eating) or ‘inactive’. A crow was considered to be ‘within colony’ if the distance to the nearest gull nest or an adult gull on its territory was < 10 m. T o quantify the abundance of crows on an island, we use ‘crow-time’ (the summed time of residence of all individual crows), a measure adequate for our purposes (Gotmark & Ahlund
1984). Statistical analyses were conducted on the SAS package (SAS Institute Inc. 1985) at ‘Goteborgs Datacentral’. Unless otherwise stated, statistical tests are twotailed.
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Table 1. Observations of predation on Eider nests in the study area in 1981* and 1982; fgures refer to depredations initiated by a given predator (cf. text) Depredations ( n ) Predator ~
__
1981 ~
1982 _ _
Lorus marinus L. argentntits
L. fusctts I.. ranus Coreus rornix C . corns C . rornixlrorax L . marinusll. argentatitslC. cornix Notes: * Observations made during disturbance experiments (Giitmark & Ahlund 1984). t Number of nesting gull pairs on the study islands (no corvids nested there).
Results Predators on Eider nests I n 1981, observations were made during disturbance experiments (Gotmark & Ahlund 1984), while in 1982 we observed islands that were not disturbed b y people. Observations were made in the same area (Fig. 1) and partly on the same islands in the two years. T h e larger gulls (marinus and argentatus) and crows were the main nest predators (Table 1). L. marinus destroyed more nests than did argentatus, although argentatus was more abundant on the islands studied (Table 1). I n 1982, large gulls and crows were equally important as nest predators, while in 1981 gulls predominated, probably a result of the disturbance experiments in that year (see Gotniark & Whlund 1984). There was no difference in the composition of nest predators within gull colonies (large gulls robbed two nests, crows also two) and outside them (large gulls robbed seven nests, crows six; data from 1982). Predators were never observed to drive a female off the nest; all or most nests probably were unattended before they were preyed upon. Often several species of predators gathered at the site of egg predation. I n the two years, marinus took over depredations begun by argentatus (five cases), Hooded Crow (six) and Raven (one), while argentatus took over nests or eggs initially robbed by crow (three). Thus, larger predators were generally dominant over smaller ones. In 1982, crows lost six of 17 depredations to large gulls.
Activity of Hooded Crows on Eider islands with gulls Crows are important predators on Eider nests (e.g., Tenevuo 1963; Hildkn 1964, this study). If Eider nests within gull colonies are protected against crow predation, crows may be expected to avoid the colonies. However, on seven Eider islands where no crows nested (Fig. I), crows spent more time within the gull colony than expected by chance (Table 2). T h i s applied to all five islands (1, and 4-7) with many gull pairs,
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Table 2. Activity of Hooded Crows and attack rate by gulls within ( W C O L ) and outside ( O C O L ) gull colonies on seven Eider islands in 1982
Island number and predominant gull species
L . marinus L . marinus L . marinus L. argentatus L . argentatus L . argentatus L. canus All islands (1-7)
1. 2. 3. 4. 5. 6. 7.
Crow-time spent foraging
Crow-time* (min) WCOL
(
OCOL
281 (120)t 264 4.2 (18) 196 6.3 (7.3) 346 262 (181) 131 352 (181) 152 46 (18) 0 93 (77) 85 1044 (602) 1174
(425)t (182) (345) (212) (322) (28) (100) (1614)
WCOL 72 0
21 48 93 85 100 76
"."
)
OCOL
57 70 32 65 58 -
77 54
Gull attacks/min of crow-time WCOL 0.11 0 0 0.09 0.09 0.02 4.00
OCOL
(29)f 0.02 0.01 0.01 (23) 0.03 (30) 0.01 (1) (370) 1.11
(5)f (1) (2) (4) (1) (94)
Observation time(min) 1172 852 852 895 708 708 922 6109
(9)5 (7) (7) (6) (4) (4) (5) (17)
Notes: The summed time of residence of all individual crows. t Expected values (04 area with gull colony x total crow-time on an island). f Total number of attacks observed. 4 Number of days with observations.
but not to islands 2 and 3, where only 1-2 gull pairs nested. Crows appeared to be equally attracted to colonies of marinus and argentatus, but less attracted to the canus colony (Table 2). Owing to dependence within and between islands, the data cannot be tested statistically, but there was no tendency for crows to avoid gull colonies. T h e colonies and the areas outside them did not seem to differ with regard to topography or amount of vegetation (pers. obs.). On three of four islands with many gull pairs, crows spent a higher proportion of their time foraging within than outside the colony (Table 2). Attacks on crows by gulls were more common within the colony (Table 2), but the larger gulls rarely attacked crows (Table 2) and appeared to do so only when crows were close to the gulls' nests. In the colony of canus, the attack rate was much higher than in colonies of the larger gulls. During an attack by a large gull, a crow usually flew away a few metres, while during attacks by (several) canus a crow usually crouched; sometimes it flew away from the colony.
Clutch size and predation of Eider nests Injluence of nest concealment In both years, clutch size increased strongly with the degree of nest concealment (Fig. 2; P 6 5 O / , of all gulls). Eider nests ‘within colony’ of other gull species on these islands were excluded. In neither of the two years were there any significant differences in Eider clutch size within and outside the colonies of the four types of gull islands. However, for all four types of gull islands and in both years (with exception for marinus islands in 1982),
(xf
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1982
II
570 835
559684
Outside colony
Within colony
52
245 271
L. marinus
125
65
L. orgentotus
93
84
100
L. conus
L . fuscus
Figure 3. Proportion ( O 0 ) of robbed Eider nests outside colonies (both islands with and without gulls included) and within colonies of different Lams gulls in 1982 and 1983. 'Within colony' is the pooled data for all gull species, including Eider nests where the adjacent gull nest or territory could not be identified to species ( < 20"" of all Eider nests in the colonies). Only T y p e 1 nests (see Fig. 2 and the text) are included here and in Figures 4-6. T h e number of islands where nests were found is given above and sample size (nests) below each column.
35 O/O -82
-83-82
Q -
I;
83
-82
-83
-82
-83
...
... ... ... ... ... ... ... ...
- - - - - -- 8 201
39 9
36 I2
19
16
L. marinus
4
7
L. orgentotus
3
3
L. fuscus
6
4-
5
L. canus
Figure 4.Proportion ("")of robbed Eider nests on islands with and without nesting gulls, and within and outside the colony on islands where different Larus gulls predominated in 1982 and 1983 (see text for further information). Below columns are given sample size (nests) and (below brackets) number of islands included in the comparison.
the proportion of robbed Eider nests was lower within than outside the gull colony (Fig. 4), although the difference was significant only for fuscus islands in 1983 ( x f = 7 . 8 , P
