Density-dependent recruitment rates in great tits: the importance of being heavier

摘要

In birds, individuals with a higher mass at fledging have a higher probability of recruiting into the breeding population. This can be because mass is an indicator of general condition and thereby of the ability to survive adverse circumstances, and/or because fledging mass is positively related to competitivestrength in interactions with other fledglings. This latter explanation leads to two testable predictions: (i) there is stronger selection for fledgling mass when there is more severe competition (i.e. at higher densities); and (ii) that besides absolute fledgling mass, relative mass of fledglings within a cohort is important. We test these two predictions in two great tit (Parus major) populations. The first prediction was met for one of the populations, showing that competition affects the importance of mass-dependent recruitment. The second prediction, that fledglings recruit relatively well if they are heavy compared to the other fledglings, is met for both populations, The consequence of the importance of relative rather than absolute fledgling mass is that the fitness consequences of reproductive decisions affecting fledgling mass, such as clutch size, depend on the decisions of other individuals in the population.