Competition among european robins Erithacus rubecula in the winter quarters: Sex is the best predictor of priority of access to experimental food resources

Campos, Ana R.; Catry, Paulo; Ramos, Jaime; Robalo, Joana Isabel

http://hdl.handle.net/10400.12/1360

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Competition for resources by individuals of different sexes, ages, body sizes or geographical originsmay have varied implications, from demography to the evolution ofmigratory 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 mealwormswere provided in an areawith a high density of wintering robins and, after 15 days of provisioning, were replaced by a spring trap thatwas operated during a whole day. The first individuals to be caught in each trap were heavier and had higher fat andmuscle 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 lastcaught individuals did not differ in body size, wingtip shape or age ratios. However, whilst 35% of the first-caught individuals weremales, only 6%of the last-caught individuals weremales. 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.