A New Avian Mating System: Ambisexual Polygamy in the Penduline Tit Remiz pendulinus

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Abstract
A study of colour-ringed population of the penduline tit in Southern Sweden has revealed a new mating system, ambisexual polygamy, in which sequential polygyny and polyandry occur simultaneously. All parental duties were carried out by one mate only. Of 140 clutches, 48% were attended by females and 18% by males, while 34% were deserted by both parents before incubation. Polyandry was exhibited by 31% of the females, while the remainder attended their first brood. Clutches incubated by females were larger than those deserted. Of the males, 30% assumed cubated by females were larger than those deserted. Of the males, 30% assumed parental care, most of them in the later part of the breeding season. Polygamy could give each mate the opportunity of increasing its breeding success, but due to individual differences in behaviour, the outcome is varied. We argue that the unique mating system of the Penduline Tit has developed as a consequence of an increasingly skewed sex-ratio among individuals available for new pair-bonds as the breeding season progresses.