What determines the time-activity budgets of Avocets (Recurvirostra avosetta)?

Abstract

Time-activity budgets of birds are known to be affected by many different factors. The aim of this study is to explain the intra-specific variation of activity patterns (in particular foraging activity) of one particular wader, the Avocet. Sixty-seven series of scan observations of 12 h to 12.5 h length were made at several sites on the flyway of the northwest European population and at various stages in the species’ annual cycle. In estuarine habitats the activity pattern was mainly influenced by the tide. As soon as the conditions allowed (neap tides) Avocets abandoned the tidal rhythm. No time of day effects on activity patterns could be detected. Activity patterns by day and at night were essentially the same, except during very dark nights (owing to artificial illumination at some of the study sites such nights were a rare event), when foraging activity was reduced. The breeding season induced considerable changes of the activity patterns, including a reduction of foraging time to less than 20% of the budget at the end of the breeding season. Outside the breeding season, activity patterns were mainly influenced by the type of food (fish: reduced foraging time, Chironomid larvae: prolonged foraging time), by temperature (increase of foraging time with decreasing temperature), by windspeed (reduction of foraging time at wind speeds above 10 m/s) and by the darkness of the previous night (compensatory feeding after dark nights).