Site selection and resource depletion in black‐tailed godwits Limosa l. limosa eating rice during northward migration

Abstract

Summary1. During migratory stopovers, animals are under strong time stress and need to maximize intake rates. We examine how foragers react to resource depletion by studying the foraging ecology and foraging site selection of black-tailed godwits Limosa l. limosa staging in rice fields during their northward migration stopover (January-March 2007). 2. We analysed godwit abundance and foraging behaviour, sampled the availability of rice in the fields and used the functional response model to predict the giving-up density (GUD) of rice kernels when godwits should give up a rice field. Sightings of individually colour-marked birds were used to verify whether individuals moving between rice fields confirmed the predicted GUD. 3. Black-tailed godwit intake rates at different rice densities fitted Holling's functional response curve. The predicted GUD of rice necessary to balance allometric estimates of daily energy expenditure (DEE) and measured time budgets were confirmed by GUD measured in the field. 4. Individually marked birds moved towards rice fields with higher rather than lower rice densities more often than randomly expected. These birds increased the measured intake rates after this move. 5. Godwit foraging caused a decrease in the rice density of individual fields during the stopover period. Despite this, overall intake rates remained constant as godwits reacted to resource depletion by moving to a new foraging site as soon as their intake rate falls below the required levels to achieve DEE.

Ways to be different: foraging adaptations that facilitate higher intake rates in a northerly-wintering shorebird compared to a low-latitude conspecific

“…Such observations conform to the more general patterns first derived by Holling (1959) and elucidated in shorebirds by others (e.g. Duijns et al, 2014;Lourenço et al, 2010;Piersma et al, 1995). In its simplest form, observations are modeled by the equation:…”

“…following Lourenço et al (2010) and van Gils et al (2005b). We determined T s for each Macoma that was consumed and accounted for the depletion of prey when integrating density (D) in our estimates of a.…”

Underuse of stopover site by migratory swans

“…Underuse of stopovers has been reported before, but in those cases it was used in the sense that migratory birds leave more food at stopover sites than at wintering sites because on the latter they should reflect energy balance rather than a positive energy budget (Van Eerden et al 1997a; but see Lourenço et al 2010;Alonso et al 1994). Here, we show that migratory birds may also underuse a stopover site in the sense that they leave more food behind in some years than in others, because of yearly differences in fuelling conditions.…”

“…At the stopover, carrying and maintaining a given fuel load already comes at a cost, and this will further slow down the increase in flight range with fuelling time (Klaassen and Lindström 1996). Moreover, food depletion might cause diminishing returns of potential flight range with fuelling time, because depletion often leads to lower fuelling rates (Nolet and Drent 1998; but see Lourenço et al 2010). Together, fuel load and food depletion determine the optimal fuelling time at a stopover (Alerstam and Lindström 1990;Alerstam 1991).…”

The Future for Research on Waterbirds in Rice Fields

“…Placing data collection on foraging behavior in a broader theoretical context would also provide important insights (e.g. Amano et al 2006aAmano et al , 2006bLourenço et al 2010), especially if linked directly to measures of body condition or survival.…”