Group provisioning limits sharing conflict among nestlings in joint-nesting Taiwan yuhinas

Abstract

Offspring often compete over limited available resources. Such sibling competition may be detrimental to parents both because it entails wasted expenditure and because it allows stronger offspring to obtain a disproportionate share of resources. We studied nestling conflict over food and its resolution in a joint-nesting species of bird, the Taiwan yuhina (Yuhina brunneiceps). We show that adult yuhinas coordinate their feeding visits, and that this coordination limits competition among nestlings, leading to a 'fairer' division of resources. Transponder identification and video-recording systems were used to observe adult feeding and nestling begging behaviours. We found that: (i) yuhinas feed nestlings more often in large parties than in small parties; (ii) feeding events occurred nonrandomly in bouts of very short intervals; and (iii) food distribution among nestlings was more evenly distributed, and fewer nestlings begged, during large-party feeding bouts compared with small-party feeding bouts. To our knowledge, this is the first study in a cooperative breeding species showing that adults can influence food allocation and competition among nestlings by coordinating their feeding visits. Our results confirm the hypothesis that the monopolizability of food affects the intensity of sibling competition, and highlight the importance of understanding the temporal strategies of food delivery.

The Adaptive Significance of Provisioning and Foraging Coordination between Breeding Partners

“…Pair synchrony, and therefore the equity in partners' work rate, increased rather than decreased with experimental brood size, which suggests that partners coped with additional offspring demand by increasing cooperation rather than intensifying sexual conflict. This pattern is also consistent with the hypotheses that nest visit synchrony is an adaptive response to counter offspring competition (Shen et al 2010), nest predation risk (Raihani et al 2010), or to improve the assessment of other individuals' care (Doutrelant and Covas 2007;McDonald et al 2008), as suggested for cooperative breeders. Accordingly, even though nest visit synchrony followed foraging synchrony, it also responded directly to the brood size manipulation, unlike the response to nestling age, which was mostly driven by synchrony while foraging.…”

“…"Experimental brood size" was the number of nestlings in the nest after manipulation, whereas "brood manipulation" was the deviation from the original brood size calculated as (postexperimental brood size -original brood size)/original brood size. Food partitioning was computed as in Shen et al (2010) using the "skew index S," which varies from 0 to 1, for increasing inequality among nestlings. For the between-pair comparison, parental behavior and seed count were standardized per brood size by using the residuals of their linear regression with experimental brood size.…”

“…Indeed, better partner coordination has long been suspected to contribute to the increase in reproductive success with time observed in many species with long-term pair bonds ("mate familiarity effect," reviewed in Black 1996). In addition, as emphasized more recently in several cooperatively breeding birds, caregivers synchronize their visits to the nest (i.e., visit the nest together rather than independently; Doutrelant and Covas 2007;McDonald et al 2008;Raihani et al 2010;Shen et al 2010), and this synchronization appears to improve nestling growth and survival (Raihani et al 2010;Shen et al 2010). Specifically, synchronizing provisioning visits may improve nestling growth by facilitating food partitioning among nest mates (Shen et al 2010), perhaps because more food is brought to the nest at once, or because partners can better assess and adjust to each other's food allocation to individual nestlings.…”

“…In addition, as emphasized more recently in several cooperatively breeding birds, caregivers synchronize their visits to the nest (i.e., visit the nest together rather than independently; Doutrelant and Covas 2007;McDonald et al 2008;Raihani et al 2010;Shen et al 2010), and this synchronization appears to improve nestling growth and survival (Raihani et al 2010;Shen et al 2010). Specifically, synchronizing provisioning visits may improve nestling growth by facilitating food partitioning among nest mates (Shen et al 2010), perhaps because more food is brought to the nest at once, or because partners can better assess and adjust to each other's food allocation to individual nestlings. In addition, pooling nest visits should reduce the likelihood of the nest being detected by a predator (Raihani et al 2010) by reducing the number of bouts of parental activity and noisy chick begging at the nest (Martin et al 2000).…”

A high-accuracy, time-saving method for extracting nest watch data from video recordings

“…The duration of recorded video can vary depending on the scale of the experiment. For example, a study by Shen et al (2010) examining provisioning in Taiwan Yuhinas (Yuhina brunneiceps) analyzed 122 h of video, while Gladbach et al (2009) analyzed 720 h of Wilson's Storm Petrel (Oceanites oceanicus) video. Analyzing this amount of video is arduous and also has a high potential for error due to observer fatigue.…”