Facultative interspecific brood parasitism in tits: a last resort to coping with nest-hole shortage
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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’.
KeywordsCyanistes caeruleus Forest fragmentation Last resort hypothesis Nest parasitism Parus major Sibling rivalry
We thank E. S Ferrer and V. García-Navas for their help during sample collection and B. Álvarez and I. Pozo for figure editing. J. Morales, B. E. Lyon, M. Leonard and two anonymous reviewers greatly improved the first version of the manuscript with their comments. S. Young checked the English language of the article. We thank the Council of San Pablo de Los Montes and the board of ‘Centro Quintos de Mora’ for the permit allowance to develop our work. RB benefited from the JCCM-FSE 2007/2013 post-doctoral programme and from a ‘Juan de la Cierva’ post-doctoral contract (JCI-2011-10945). JBE was supported by a pre-doctoral fellowship from Junta de Comunidades de Castilla-La Mancha and ESD enjoyed a pre-doctoral fellowship from Ministerio de Ciencia e Innovación (BES-2011-047046). This study was funded by the Ministerio de Ciencia e Innovación (grant CGL2010-21933-C02-01) and Junta de Comunidades de Castilla-La Mancha and European Social Fund (grant POIC10-0269-7632).
Catching and ringing protocols and the general ethics of our research were approved by the Junta de Comunidades de Castilla-La Mancha, Consejería de Agricultura y Medio Ambiente (licenses avp_11_1467, avp_12_061 and avp_13_059) in accordance with current Spanish laws.
Conflict of interest
The authors declare that they have no competing interests.
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