Competition among European Robins Erithacus rubecula in the winter quarters: sex is the best predictor of priority of access to experimental food resources

Abstract

Competition for resources by individuals of different sexes, ages, body sizes or geographical origins may have varied implications, from demography to the evolution of migratory systems. European robins Erithacus rubecula display segregation by habitat in relation to sex, age, body size and geographic origin in the winter quarters. Here we report on an experiment to investigate the existence of asymmetries in the priority of access to food resources among unconstrained individuals in their natural environment. Feeders with meal worms were provided in an are a with a high density of wintering robins and, after 15 days of provisioning, were replaced by a spring trap that was operated during a whole day. The first individuals to be caught in each trap were heavier and had higher fat and muscle scores than last-caught individuals. Based on this and on behavioural observations, we conclude that first-caught individuals had priority of access to feeders. First- and last caught individuals did not differ in body size, wingtip shape or age ratios. However, whilst 35% of the first-caught individuals were males, only 6%of the last-caught individuals were males. This suggests that sex is an important predictor of the outcomes of competition in an area where multiple robin populations coexist in winter, while, at least on sites where no strictly sedentary individuals are present, geographic origin may be relatively unimportant.