Time and energy costs of different foraging choices in an avian generalist species

Abstract

BackgroundAnimals can obtain a higher foraging yield by optimizing energy expenditure or minimizing time costs. In this study, we assessed how individual variation in the relative use of marine and terrestrial foraging habitats relates to differences in the energy and time investments of an avian generalistic feeder (the Lesser Black-backed Gull, Larus fuscus), and how this changes during the course of the chick-rearing period.MethodsWe analyzed 5 years of GPS tracking data collected at the colony of Zeebrugge (Belgium). Cost proxies for energy expenditure (overall dynamic body acceleration) and time costs (trip durations and time spent away from the colony), together with trip frequency, were analyzed against the relative use of the marine and terrestrial habitats.ResultsThe marine habitat was most often used by males and outside weekends, when fisheries are active. Marine trips implied higher energetic costs and lower time investments. As chicks became older, terrestrial trips became more prevalent, and trip frequency reached a peak towards 20 days after hatching of the first egg. Over a full chick rearing period, energy costs varied widely between individuals, but no trends were found across the marine foraging gradient. Conversely, a higher use of marine foraging implied lower overall amounts of time spent away from the colony.ConclusionsForaging habitat choice was related to overall time costs incurred by gulls, but not to energy costs. The effect of chick age on foraging habitat choice and effort may be driven by energy expenditure constraints on the amount of marine foraging that can be performed. If time is less constraining to them, Lesser Black-backed Gulls may meet the increasing chick demand for food by switching from high to low energy demanding foraging strategies.

Context‐dependent specialisation drives temporal dynamics in intra‐ and inter‐individual variation in foraging behaviour within a generalist bird population

“…This suggests that decreased agricultural resources driving changes observed in breeding females was unlikely. The invertebrates collected in agricultural areas are, however, typically low‐caloric compared to fishery discards and anthropogenic food sources (Sibly and McCleery 1983, van Donk et al 2017, Sotillo et al 2019a) and the occurrence of ploughing and mowing events is spatiotemporally unpredictable (Camphuysen 2013, van den Bosch et al 2019, Sotillo et al 2019b). Hence, agricultural resources could simply be too energy‐ or time‐demanding for raising chicks.…”

Spatial and temporal variation in foraging of breeding red‐throated divers

“…Animals foraging during a breeding season have to adapt their foraging effort temporally in order to accommodate the demands of growing young (Tulp et al 2009, Sotillo et al 2019. However, at all three sites, the foraging effort for RTDs did not change through the season as we expected.…”

“…On the other hand, RTDs in Finland and Scotland showed a more notable decrease in proportion of IDZ dives. This could relate to following vertical prey movement, foraging in different habitat or switching to prey occupying a different area of the water column (Sotillo et al 2019), to fulfil the food size demand of the chicks (Reimchen and Douglas 1984). The latter could especially be true in Finland, as evidence suggests black-throated divers Gavia arctica foraging in freshwater can provision benthic invertebrates to chicks in the early stages of rearing (Jackson 2003).…”

Dynamic space use of Andalusian rice fields by Lesser Black‐backed Gulls (Larus fuscus) is driven by flooding pattern

“…ODBA, a proxy for energy expenditure and activity (e.g. Sotillo et al 2019), as well as the proportion of time spent on different behaviours, did not change during overwintering period, despite an increase in daily commute distances. This implies there might not be substantial differences in the dietary or metabolic needs of Lesser Black-backed Gulls throughout the winter, and thus that habitat use changes were not driven by changes in need but in availability.…”