How animals use food resources according to their relative availability is essential for our understanding and predictions of ecosystem interaction. Non-linear functions between use and availability are described with the feeding functional response, a key tool to describe such functions that is still poorly understood in some cases. For example, in species with a marked sexual size dimorphism it is not clear how such non-linear functions differ between males and females. Here, we used likelihood methods and model selection to model non-linear functions in legume use by male and female great bustards (Otis tarda) based on diet composition data from droppings. The great bustard shows the highest sexual size dimorphism among birds, and it has opportunistic and generalist foraging habits, with a strong selection for legumes. Simple functional-response models described the proportion of legumes in great bustard droppings as a non-linear function of legume availability. Functional response type II fitted to data better than type III in autumn and winter, whereas in spring, type III was the best-fitted model, describing a dietary switch of great bustards feeding on legumes. The best-fitted functional response included an effect of sex in the asymptote of the curve; the female diet included a higher proportion of legumes than the male diet, suggesting a possible effect of sexual size dimorphism. We concluded that great bustards behaved as opportunistic foragers, whose sexual size dimorphism might play a role in the functional response to legumes.

This study was supported by the Spanish Administration (grant numbers CGL2005-04893, CGL2008-02567, CGL2012-36345 and CGL2017-87206-P; AEI/FEDER, UE) and HENARSA-CSIC contract. The first author would like to thank Spanish SEPE from Spanish Ministry of Labour and Social Insurance for its funding support.

Peer reviewed