Seasonal fluctuations in the mortality rates of three Northern and three Southern Hemisphere Gulls

Abstract

A comparison was made of the seasonal distributions of mortalities of silver gull, Larus novaehollandiae Stephens; black-billed gull, Larus bulleri Hutton; southern black-backed gull, Larus dominicanus Lichtenstein; black-headed gull, Larus ridibundus L.; herring gull, Lavus argentatus Pontoppidan; and glaucous-winged gull, Larus glaucescens Naumann. Seasonal mortality peaks are considered to be mainly due to gull numbers exceeding those which can be supported by the environmental resources available. Mortality tends to be low during the main breeding season and at that time it is usually higher in older than in yearling gulls. After the main breeding season a mortality peak in older birds tends to precede a post-fledging mortality peak in juveniles. Only in the glaucous-winged gull do winter mortalities exceed summer mortalities.