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

In birds, individuals with a higher mass at fledging have a higher probabil ity of recruiting into the breeding population. This can be because mass is an indicator of general condition and thereby of the ability to survive ad verse circumstances, and/or because fledging mass is positively related to competitive strength in interactions with other fledglings. This latter exp lanation leads to two testable predictions: (i) there is stronger selection for fledging mass when there is more severe competition (i.e. at higher de nsities); and (ii) that besides absolute hedging mass, relative mass of fle dglings within a cohort is important. We test these two predictions in two great tit (Parus major) populations. The first prediction was met for one o f the populations, showing that competition affects the importance of mass- dependent recruitment. The second prediction, that fledglings recruit relat ively well if they are heavy compared to the other fledglings, is met for b oth populations. The consequence of the importance of relative rather than absolute fledging mass is that the fitness consequences of reproductive dec isions affecting fledging mass, such as clutch size, depend on the decision s of other individuals in the population.