Kin do not always help: testing multiple hypotheses on nest feeding in a cooperatively breeding bird

Abstract

In cooperatively breeding species, group members may derive multiple benefits from helping to raise other individuals’ offspring, yet not all individuals do so. In this study, we tested predictions from the “kin selection”, “pay-to-stay”, “group augmentation” and “skills” hypotheses, to explain why group members feed nestlings of breeding placid greenbuls (Phyllastrephus placidus). In our study population, about 70% of the breeding pairs were accompanied by subordinates, and in 60% of these cases at least one subordinate helped in provisioning nestlings. In total, 80% of the subordinates were related to one or both breeders. In accordance with the “kin selection” hypothesis, and contrary to the “pay-to-stay” hypothesis, all the helpers were first-order kin of the breeding female (although relatedness to the breeding male did not explain variation in helping) and the presence of helpers was associated with increased survival of the breeding pair. However, the propensity to help varied among group members, as 46% of group members related to the breeding female did not feed nestlings. Younger helpers fed offspring more often than older ones, supporting the “skills” and “group augmentation” hypotheses. However, support for the “group augmentation” hypothesis was mixed since subordinate sex and group size did not explain additional variation in helping propensity and effort. We argue that in addition to indirect and direct benefits, also the costs of helping as well as other types of helping aside from provisioning must be considered to better understand variation in helping behavior.

Cooperative breeding alters physiological and behavioral responses to habitat fragmentation

“…We have previously shown that cooperatively breeding males and females have higher annual survival than pair breeders. 40 Whether these fitness benefits are retained in low quality territories remains to be tested, but the present results suggest that through reduced physiological costs, as evidenced by low baseline corticosterone levels, breeders from cooperative groups may be able to avoid the survival costs of breeding in small fragments. 48 …”

“…In placid greenbuls and other cooperative species, subordinates commonly contribute less to nestling provisioning than the breeding pair. 40 , 49 Lower baseline corticosterone levels in subordinates than breeders indicate that they can keep the physiological costs of helping low. 47 Although some subordinates do not provide any help 40 and therefore may have low corticosterone levels, because subordinates were caught at the nest, it is likely that they were indeed helping to feed the offspring of the breeding pair.…”

“… 40 , 49 Lower baseline corticosterone levels in subordinates than breeders indicate that they can keep the physiological costs of helping low. 47 Although some subordinates do not provide any help 40 and therefore may have low corticosterone levels, because subordinates were caught at the nest, it is likely that they were indeed helping to feed the offspring of the breeding pair. Furthermore, long return latencies after capture suggest that subordinates may only help if their own survival is not jeopardized, 50 i.e., when confronted with stressors, subordinates may suspend helping behavior.…”

“…Related subordinates, i.e., offspring of the breeding pair, may help the breeding pair to feed nestlings while both related and unrelated subordinates defend the nest against predators. 40 , 60 Breeding females have the highest reproductive burden as they deliver more food to nestlings than breeding males or subordinates. 39 , 40 Breeding females and breeding males that receive help during offspring provisioning reduce their own feeding rates 32 , 40 and survive better than breeding females without subordinates 40 indicating that subordinates may reduce reproductive workload.…”

“…We tested the following predictions: (1) Higher foraging effort of individuals breeding in lower quality territories is reflected in higher baseline corticosterone levels; (2) Breeding females, which have the highest foraging effort, 39 , 40 have higher baseline corticosterone levels than subordinates and breeding males; (3) Breeding pairs that receive help from subordinates (and therefore have lower foraging effort 32 ) have lower baseline corticosterone levels than pairs without subordinates in low quality territories; (4) Subordinates have the highest baseline corticosterone levels if resource competition is more important 37 ; (5) Individuals from lower quality territories have higher stress-induced corticosterone levels and take longer to resume feeding their nestlings (return latency) in response to capture and handling; (6) At nests with only one nestling parents mount a higher hormonal stress response and take longer to return than at nests with two or three nestlings, in particular in low quality territories. This was expected because hormonal and behavioral responses to stressors can be modulated by the value of the breeding attempt 41 , 42 and one-egg clutches have reduced expected fitness gain, which has been shown to lead to infanticide in another cooperative breeder 43 ; and (7) Individuals with higher corticosterone levels take longer to resume feeding after capture.…”