"Males and females can mate with up to seven different partners in any one breeding season and frequently vary their attitude to childcare between different clutches.
Over the course of a breeding season more than half of all clutches are cared for by the female, with up to a fifth cared for by the male. The rest, roughly a third, are abandoned by both."
Clutches are abandoned in order to seek new sexual conquests. Bird in question? The Penduline Tit.