Nest site selection and its survival value among laughing gulls

Abstract

1. The nesting strategy as determined by nonrandom variation in environmental features at laughing gull (Larus atricilla) nests in a salt marsh was studied (Fig. 3). Gulls tended to nest on mats in tall grass that grows on low ground (just above high tides) near water (Figs. 4–7). Grass height was inversely related to ground elevation and distance to water (Fig. 8). Throughout the season, gulls selected nest sites in grass about 35 cm in height; due to continued grass growth, early breeders had taller grass around nests (Fig. 9). Pairs in the colony center nested earlier and in taller grass than pairs in a peripheral area. 2. Mats apparently stabilize nests during flooding, and by settling on mats gulls may conserve energy in the collection of nest material. Tall grass around nests afforded chicks protection from predators and weather, and held floating nests in place during flooding. 3. Gulls spend about 4 weeks (two spring tidal cycles) on the nesting grounds before egg laying. During this time they perform virtually no nest building and probably gain important information about nest site suitability. 4. Tidal flooding, the greatest threat to reproductive success—destroying 70–100% of the nests in the colony — occurred on average once every 2 years over 10 years. Floods occurred during spring tides accompanied by sustained NE winds. 5. Following a flood that destroyed 70% of the nests in the colony, it was shown that a significantly greater proportion of successful pairs nested on mats and in significantly taller grass than unsuccessful pairs. Grass height, especially that on the SW side of the nest, was the most important predictor of success during flooding. 6. More pairs in the central area were successful than those in the peripheral one: the result of nesting in taller grass and the greater protection of the central area from tides and winds. Though not differing among successful and unsuccessful nesters, females in the peripheral area laid smaller eggs and clutches, and laid later than females in the central area (over 3 years), suggesting that females in the peripheral area were on average younger than females in the central area. It was speculated that, on average, younger pairs will experience during flooding lower reproductive success as a result of nesting inexperience and nesting in suboptimal habitat. The smaller reproductive investments of younger pairs in eggs and clutches can be interpreted as an adaptation to conserve energy during a period of the life cycle when new behavioral adjustments and nesting areas are being explored.