Reproductive Performance of Ring-Billed Gulls in Relation to Nest Location

Abstract

-We evaluated the reproductive performance of Ring-billed Gulls (Larus delawarensis) nesting on the periphery and in the center of a colony in northern Lake Superior in 1976 and 1977. The center and periphery of this colony were not subject to flooding, predation or differential human disturbance. Egg-laying, clutch size, hatching and fledging success, and nest attentiveness did not differ significantly between and clutches. The colony population has increased in size during the last five years and become stabilized, whereby and areas are now being occupied simultaneously. In this study, reproductive performance was related more to the timing than the location of nesting. The proportion of eggs hatched and young fledged often differ markedly in relation to nest location in colonies of many bird species. Birds nesting in the part of a colony normally hatch proportionately more eggs and raise more young than do birds with nests on the periphery (Coulson 1968, Dexheimer and Southern 1974). We wanted to determine if such differences existed at an island colony of Ring-billed Gulls (Larus delawarensis) that has shown a rapid increase in size in recent years. STUDY AREA AND METHODS We conducted the study during the nesting seasons (May, June, July) of 1976 and 1977 on Granite Island (48043'N, 88029'W), Black Bay, northern Lake Superior. Granite Island is a rock outcrop 402 m by 201 m with a summit 30 m above the surrounding water. During the study the colony consisted of 1,900 pairs. The nesting habitat was described by Ryder and Somppi (1977). The nesting area was on the summit of the island. The gulls nested in numerous shallow depressions in the granite rock surface. The dominant vegetation in the depressions was Kentucky bluegrass (Poa pratensis). The area formed the south and west edges of the area and consisted of the same type of habitat. Immediately adjacent to the area, off the colony, was a forest, primarily of balsam fir (Abies balsamea), white cedar (Thuja occidentalis), and white birch (Betula papyrifera). No gulls nested in the forest. We use the following terms: a peripheral nest was located on the edge of the colony, forming part of the border (Dexheimer and Southern 1974) and not surrounded by other nests. A central nest was located inside the border of the colony and was surrounded by other nests (Tenaza 1971). Hatching was the percentage of eggs laid that hatched (Gilman et al. 1977). Fledging was the percent of chicks that were estimated to fledge from eggs hatched (Gilman et al. 1977). A fledgling was a chick at least 21 days old (Dexheimer and Southern 1974). Reproductive was the number of chicks fledged per breeding pair. Daily nest histories were kept in 1977 for all clutches within the study area. We did not visit the colony during the hottest part of the day or during inclement weather because we assumed that such visits would hinder the survival of embryos. Nests were marked with a numbered wooden block placed beside the nest. Eggs were numbered, in the sequence they were laid, on the blunt end with a non-toxic felt pen. In 1977 we measured the distance to nearest neighbor for a total of 98 randomly chosen nests in order to assess the relationship of nest spacing to reproductive performance. Chicks were marked at hatching with a numbered fingerling fish tag fastened through the foot web. We recaptured them at 7 to 14 days of age, when their legs were large enough to retain a U.S. Fish and Wildlife Service aluminum leg band. To facilitate recovery of the chicks, we erected a 30.5-cm fence of chicken wire mesh around the study area. Nisbet and Drury (1972) found that the effect of this type of fencing on breeding success was negligible in their study of Common Terns (Sterna hirundo) and Roseate Terns (S. dougallii). Additionally, they stated that Pearson (1968) and Langham (1968) had fenced Arctic Tern (S. paradisaea) and Black-headed Gull (L. ridibundus) nests, respectively, without detrimental effects. We monitored attentiveness of early-nesting gulls in the center and periphery of the colony in both years using a camera mounted in a 5.5-m high observation tower located just off the colony. The camera was equipped with an automatic timer set for one shot every 3 min. In 1976, we determined from the film the presence or absence of attendants during early, mid and late incubation from 25 and 25 nests that were started during the peak of egg-laying. In 1977 this procedure was repeated for 30 early and 24 early nests. Additionally, in 1977 we recorded attentiveness at 16 late-starting nests and eight late-starting nests.