Behavioural and population responses to changing availability of Artemia prey by moulting black-necked grebes, Podiceps nigricollis

Abstract

We examined how availability of brine shrimps, Artemia parthenogenetica, influenced temporal aspects of foraging behaviour and population dynamics of moulting black-necked grebes, Podiceps nigricollis, from late August to early December in four salt ponds in the Odiel marshes, southern Spain, in 2008 and 2009. The moulting grebe population was higher in 2009, coinciding with an increase in shrimp biomass, with a peak of 2,500 birds in October. Grebes increased their time spent foraging as the season progressed, coinciding with decreases in shrimp biomass and water temperature. Foraging activity was lower in 2009, when shrimp biomass was greater. Diving was the most frequent feeding method, especially as the season progressed. Brine shrimps at the bottom of the water column were larger than those near the surface. Differences between years in grebe body mass suggest that changing shrimp availability and water temperature had an influence on body condition. The grebe population consumed an estimated 0.2–2.0% of the standing crop of Artemia per day, with this fraction increasing as the season progresses, thus contributing to the decline in the Artemia population. Our results suggest that moulting grebes are probably only able to adjust foraging effort within certain limits, especially at the end of moulting period when thermal stress is greatest and food supply is lowest. They may leave the study area when they can no longer meet their energy requirements.