Offspring body condition and immunocompetence are negatively affected by high breeding densities in a colonial seabird: a multiscale approach

Why avian colonies vary in size and how food competition among nearby colon ies affects offspring quality are still not completely understood. We simul taneously examined the effects of four scales of breeding density on two me asures of offspring viability (body condition and T-cell-mediated immunity) in the colonial Magellanic penguin. Body condition of fledglings was inver sely correlated with breeding density within 100 m(2) of nests, and decreas ed with increasing numbers of breeding pairs competing within the parental foraging ranges (100 km), probably as a result of density-dependent feud de pletion. The T-cell-mediated immune response was positively correlated with body condition, reflecting, to some extent, the previous breeding-density effects, and was negatively correlated with colony size, which may be relat ed to social stress. However, given the effect of protein intake on cell im munity, this result could also indicate a thus far neglected cost of coloni ality, namely the consumption of low-protein food to compensate for the dep letion of optimal prey. These results were not influenced by other traits, nor by the current exposure of birds to parasites and diseases. as measured by serological variables. Since body condition and the T-cell-mediated imm une response of fledgling birds are indicators of their surgical and recrui tment prospects. the costs we have identified can explain variability in co lony size in relation to food competition with surrounding colonies, as wel l as the skewed distribution toward small colonies in this species.