Helper effects in the azure‐winged magpie Cyanopica cyana in relation to highly‐clumped nesting pattern and high frequency of conspecific nest‐raiding

Abstract

In avian cooperative breeding systems, many benefi ts obtained by social pairs from the presence of helpers have been uncovered. However, until now, the factors that determine the type of assistance helpers provide and the responses of social pairs have not been well illustrated. We examined the contribution of helpers to cooperative groups and the relevant responses of dominant pairs in the azure-winged magpie Cyanapica cyana which breeds on the Tibetan Plateau. We used the capture -mark -recapture method to identify helpers. Results showed that helpers were mostly the yearling sons of dominant pairs. Th ey mainly contributed to the cooperative group in three ways, courtship-feeding the incubation female, provisioning the brood, and defending the nest. For responses of dominant pairs, we unexpectedly found that clutch size was not infl uenced by the presence of helpers at the nest. However, cooperative groups had higher brood feeding rates than biparental nests and their feeding pattern also diff ered to that of the latter. Consequently, nestlings in cooperative groups had larger fl edging body mass than that in biparental nests. By examining reasons for nest failure, we revealed that conspecifi c nest-raiding contributed to more nest failure than any other natural predators. Because of the contribution of helpers in defending against both predators and conspecifi c nest-raiders, cooperative groups had higher survival rate than biparental nests. Th us, our fi ndings suggest that in a highly-clumped nesting pattern, factors concerning the risk of nest predation, rather than that infl uencing food supply, play an important role in determining helper eff ects and responses of aided dominant pairs.

Azure‐winged Magpies Cyanopica cyanus trade off reproductive success and parental care by establishing a size hierarchy among nestlings

“…In a high‐altitude environment with a high risk of nest predation and conspecific nest‐raiding (Ren et al . ), these added nestlings may serve as replacements when brood reduction occurs (insurance egg hypothesis; Lamey et al . , Hunt & Evans ) or may provide short‐term thermal benefits (sib facilitation hypothesis; Mock & Parker , Mock & Forbes ).…”

“…The captured adult was disentangled immediately to avoid injury (Ren et al . ). Each adult was ringed with a numbered aluminium ring and a unique combination of two plastic colour‐rings.…”

“…The proportion of cooperative groups in the Tibetan population, less than 15% (Ren et al . ), is significantly lower than that in western populations, 38–50% (Valencia et al . , Carranza et al .…”

“…A recent study on a Tibetan population revealed that Azure‐winged Magpies bred in a highly clumped pattern (20–180 nests/ha, Ren et al . ), compared with those populations in Spain (0.3–5 nests/ha, Canario et al . , Valencia et al .…”

“…As conspecific nest‐raiding is the main cause of nest failure of the Tibetan population, helpers play a key role in nest defence rather than in brood provisioning (<5%, Ren et al . ). By contrast, helpers contribute 42–45% to brood provisioning in western populations (Valencia et al .…”