Bi‐parental vs. cooperative breeding in a passerine: fitness‐maximizing strategies of males in response to risk of extra‐pair paternity?

Abstract

In socially monogamous species, males that risk cuckoldry more than others might gain inclusive fitness by yielding paternity to relatives. We tested this prediction in the Tibetan ground tit Pseudopodoces humilis, an unusual facultative cooperative breeder wherein most helpers (87% males) join a mated pair shortly before clutch completion. Extra-pair paternity among bi-parental broods occurred less often (26% of broods, 9% of young) compared with cooperative broods (68%, 25%). In the former, most extra-pair sires (88%) were pair breeders unrelated to the cuckolded males, whereas in the latter, sires (87%) were mainly helpers related to the dominant males. Brood productivity did not differ between the bi-parental and cooperative breeders, but helpers' partitioning over group paternity reduced the realized reproductive success of helped males. After taking inclusive fitness into account, however, there was no difference in success of dominant males between the two social systems. One possible explanation for the differences in the rates of cuckoldry in the two systems was body size, because pair-bond males in bi-parental situations were significantly larger than those in cooperative ones. We propose two alternative strategies for males to maximize fitness: breed as a pair if large to avoid cuckoldry from helpers, or breed cooperatively if small but compromise some paternity to relatives. Our results provide an unusual route to the incidence of cooperative groups, based on constraints imposed by low competitive ability of breeding males rather than some external ecological or demographic factors.

Breeding Biology and Variable Mating System of a Population of Introduced Dunnocks (Prunella modularis) in New Zealand

“…Both the ground tit [7], and the moustached warbler [8] exhibit variable mating systems, with some females breeding with more than one partner. However in these latter species, the formation of the polyandrous group differs from that of the dunnock.…”

“…However in these latter species, the formation of the polyandrous group differs from that of the dunnock. While in dunnocks, the polyandrous males generally form the bond with the female at the beginning of the breeding season, in ground tits and moustached warblers, polyandrous group formation occurs after the female has initiated incubation [7], [8]. It seems reasonable that differences in adult sex ratio among these species (see discussion above) create scenarios in which males: (i) are constrained into forming breeding coalitions (e.g., as in dunnocks) and thus contribute more to paternal care; and (ii) have opportunities to breed monogamously (e.g., as in ground tits and moustached warblers), but when faced with perturbations, such as nest failures, can join another pair and try to secure residual reproductive success.…”

“…However, studies that investigated the direct benefits of additional male help in species with variable mating system have provided mixed findings. There is little evidence that the additional help provided in polyandrous groups of the ground tit, Pseudopodoces humilis [7], and of the moustached warbler, Acrocephalus melanopogon [8] is converted into fitness benefits for these polyandrous females. On the other hand, in another species with a variable mating system, the dunnock, Prunella modularis , socially polyandrous females fledge more offspring than socially monogamous ones [9].…”

Nest helpers improve parental survival but not offspring production in a high‐elevation passerine, the Ground Tit Pseudopodoces humilis

“…We used courtship behaviour and the brood patches of females to determine the gender of adult birds. We then validated these determinations from blood samples (Du & Lu , ). The sex of nestlings was also determined from blood samples.…”

Female ground tits prefer relatives as extra-pair partners: driven by kin-selection?

“…The ground tit ( Parus humilis ) is a passerine inhabiting alpine meadows at 2500–5500‐m elevation throughout the Tibetan plateau. Our previous studies on a population in the northeast of the species’ range revealed that the annual turnover of individuals is very high (Du & Lu 2009). When breeding, the birds establish socially monogamous pairs, of which 20% are helped mainly by male individuals that fail to find a social mate or finish a nesting attempt during the breeding season.…”