Influence of age and body condition on breeding performance in Common Kingfisher Alcedo atthis

Theory predicts that organisms are apt to optimize their resource allocation strategy and since rearing offspring is costly, condition might predict individual fitness as individuals in better condition hold more resources to allocate towards maximizing their reproductive success. We use a 3 year study (29 pairs, 58 broods, 366 nestlings) of Common Kingfisher Alcedo atthis to examine how parents’ age, scaled mass index (SMI) and heterophile-to-lymphocyte ratio (H/L ratio) influence breeding success of this species. Fieldwork was carried out on the Danube River system from Bratislava to Gabcikovo (south-western Slovakia) during 2016–2018. We observed that males and females differed in values of H/L ratio and SMI with better overall physical condition for females. At the same time, these two variables neither correlated with each other nor was there a difference between age groups. Further analyses revealed that females mated with males of lower H/L ratio tended to initiate their clutches earlier in the season. Concurrently, pairs which had laid their first brood earlier in the breeding season reared nestlings from more broods during that season. Number of broods was also significantly affected by age of female and male SMI, with older females and males in good condition more likely to produce more broods. Contrary to that, females in higher condition tended to have fewer nestlings per brood. In addition, neither SMI nor H/L ratio of females were connected to any other aspects of breeding success. Altogether, our results suggest that body condition and age can be relevant to breeding performance of Common Kingfishers but their predictive value is only partial.