Facultative interspecific brood parasitism in tits: a last resort to coping with nest-hole shortage

Abstract

We studied the occurrence of facultative interspecific brood parasitism (eggs from two species incubated by a single female) in two bird species, the blue (Cyanistes caeruleus) and the great tit (Parus major). These species are secondary cavity nesters. We monitored 38 forest plots of variable size over 3 years. We found a total of 39 mixed-species clutches in 1285 nests, representing a prevalence of 3.0 %, but it reached 7.2 % in small woodlands. Seventeen mixed-species clutches involved blue tit facultative interspecific brood parasitism, with the same number of great tits usurping and directly laying in blue tit clutches. The higher the nest-box occupation rate, the greater the prevalence of mixed-species clutches of any origin. However, the two tit species behaved differently when faced with nest-hole shortage, with blue tits dumping one or two eggs into clutches incubated by great tits and these taking over the entire blue tit clutch. Nest takeovers were more frequent at the end of the season. These differences in behaviour are likely mediated by differing dominance status, with great tits being larger. The difference in size could also explain why great tit chicks presented larger hatching and fledging rates than their blue tit broodmates. These rates were lower in blue tit chicks from mixed-species broods compared with pure ones, and no advantages were found in usurper great tit chicks compared to pure broods. Mixed-species clutches appear to be a response to nest-hole shortage, a concept that we have termed the ‘last resort hypothesis’.