Reproductive Ecology of the Western Gull: The Importance of Nest Spacing

George L Hunt,,Molly W Hunt

doi:10.2307/4084556

Abstract

REPRODUCTIVE ECOLOGY OF THE WESTERN GULL: THE IMPORTANCE OF NEST SPACING GEORGE L . H UNT, JR., AND MOLLY w. H UNT GULL colonies usually exhibit considerable variation between pairs in both territory size and timing of breeding. This variation is subject to natural selection, and timing of breeding and territory size can be cor- related with reproductive success ( Paynter 1949; Paludan 195 1; Ver- meer 1963, 1970; Brown 1967; Kadlec and D rury 1968 ; Kadlec et al. Patterson ( 1965) and Kruuk ( 1964) showed that for the small Black- headed Gull (Larus ridibundus ) the principal source of egg and chick mortality was predation. For these birds, optimum predator defense re- sulted from nesting during that period when the maximum number of birds were breeding. Pairs breeding either earlier or later suffered greater loss of chicks to predation. Patterson ( 1965) also postulated that there should be an optimal spacing of nests, based on a balance between clump- ing of nests for enhancement of group defense and scattering of nests for effectiveness of the cryptic coloring of eggs and chicks (Tinbergen et al. For the larger Larus gulls neighboring pairs may be a more important source of chick loss than predators (Paynter 1949, Emlen 1956, Parsons 1971). For these species large territory size may reduce the chance that a chick will trespass into a neighbor's territory. It has also been found that adults defend their territories more actively when they have chicks than when they are incubating (Hunt and Hunt MS, also this study, see below). Thus in these species early nesting may be advantageous as a means of minimizing chick loss to neighboring pairs. In light of the conflicting selective pressures on territory size and tim- ing of breeding, it is reasonable to expect that the optimum territory size and time for breeding will vary from one colony to another, depending upon the relative importance of predators or neighbors for chick loss. The colony of Western Gulls (Larus occidentalis wymani) nesting on Santa Barbara I sland, California, provided an opportunity to test this hypothesis, as the only potential chick predators there are adult Western Gulls. In the present study we identified the causes of chick mortality and related the reproductive success of pairs to aspects of spacing of nests and timing. STUDY SITE AND METHODS We conducted our study on Santa Barbara Island, Channel Islands National Monu- ment, Santa Barbara County, California. Santa Barbara Island is a 260-ha (1 square mile) island 61 km (38 miles) from the nearest mainland and 39 km (24 miles) from The Auk 92: 270-279. April 1975

Highlights

With the knowledge that the Western Gull colony on Santa Barbara Island was subject to no predation by other species and that neighbors were a potential source of chick mortality, we expected that birds nesting early in the season and holding large territories would enjoy the greatest reproductive success
This prediction was based on our hypothesis that opposing selective pressures on territory size and timing of breeding caused by predators and dangerous neighbors will be reflected in patterns of reproductive success as discussed in the introduction
Unlike the situation with Glaucous-winged Gulls (Larus glaucescens) on Mandarte Island, British Columbia (Hunt and Hunt MS), we found no correlation between chick survival and either date of hatching or territory size for the Western Gulls on Santa Barbara Island

Summary

RESULTS

Chick survival.- the chicks in 'our study sites hatched over a period of 38 days (22 May to 28 June), 85% of the broods hatched during the first 2 weeks (Fig. 1). Of the 99 chicks hatched within our study grids, 84 (84.8%) survived to a weight of 500 g, our criterion for survival (see Hunt 1972). The numbers are too few for statistical analysis, it is clear that killing by neighbors and starvation (as judged by loss of weight or failure to grow) were the most important causes of mortality. Causes of mortality were not uniformly distributed with respect to the date of hatching of the first chick in a brood (Table 2). It appears that more chicks were killed from late broods than from early broods and that starvation may have been less frequent in the late broods than in the early. No statistically significant correlation was evident between time of hatching and either chick survival (Fig. 2) or the number of chicks killed by neighbors

22-26 May 27-31 May 1-5 June

DISCUSSION

SUMMARY