A Mixed Brood of Coal Tits Periparus ater and Blue Tits Cyanistes caeruleus in Central Portugal

Mixed-species broods appear to be an uncommon phenomenon in altricial birds. In secondary hole-nesting birds, such as tits (Paridae), mixed-species clutches occur as a consequence of facultative interspecific brood parasitism or as a byproduct of nest takeover. Here, we report a case of a mixed-species brood composed of one Coal Tit Periparus ater and three Blue Tit Cyanistes caeruleus nestlings that was successfully raised by a Blue Tit pair. The foreign Coal Tit nestling received more food than its Blue Tit nestmates, possibly resulting in a larger tarsus length compared to other Coal Tit nestlings from a regular single-species brood in the same forest.