g-Feng Shen

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

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 non-randomly 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.

JOINT NESTING IN TAIWAN YUHINAS: A RARE PASSERINE CASE

Abstract Joint nesting, in which more than one pair contributes eggs to a group nest, is rare among cooperatively breeding birds. Early reports of the breeding biology of Taiwan Yuhinas (Yuhina brunneiceps) described them as joint nesters; however, these studies were preliminary and did not involve color-banded individuals. We conducted a 7-year study on the breeding biology of yuhinas. We found that the frequency of joint nesting was extremely high (90% of groups). Breeding groups were composed of two to seven adults (mode = 4). Most of the juveniles (78%) disappeared from our study site the year after they hatched. Only 6% remained in their natal groups; therefore, our study groups appeared to be composed primarily of non-kin. Within each group, there was a linear hierarchy of socially monogamous pairs. Mean reproductive skew index, as determined by microsatellite markers, was low (0.19). Alpha males had longer wings than beta males, whereas females did not show any differences in body size. Breeding group size had no significant effect on nest survival rate. Overall clutch size of the group increased while the average number of eggs laid by each pair decreased with group size. However, average number of fledged young per pair per season was similar for all group sizes. Alpha females were the major contributors during both diurnal (65% of groups) and nocturnal (77%) incubation, although beta females and males also contributed substantially toward incubation. Nidificación Grupal en Yuhina brunneiceps: un Caso Raro en Aves Paserinas Resumen. La nidificación grupal, en la cual más de una pareja contribuye con huevos a un nido grupal, es muy rara entre las aves que presentan cría cooperativa. Estudios anteriores sobre la biología reproductiva de Yuhina brunneiceps describían a esta especie como con nidificación grupal; sin embargo, esos estudios eran preliminares y no involucraron individuos marcados con anillos coloreados. Realizamos un estudio de 7 años de duración sobre la biología reproductiva de Y. brunneiceps. Encontramos que la frecuencia de nidificación grupal fue extremadamente alta (90% de los grupos). Los grupos reproductivos estuvieron compuestos por dos a siete adultos (moda = 4). La mayoría de los juveniles (78%) desaparecieron del área de estudio luego de que eclosionaron. Sólo el 6% permaneció en sus grupos natales, por lo tanto los grupos estudiados parecen haber estado compuestos principalmente por individuos no emparentados. Dentro de cada grupo hubo una jerarquía lineal de parejas socialmente monógamas. La desviación reproductiva promedio, determinada por marcadores micro-satelitales, fue baja (0.19). Los machos alfa presentaron alas más largas que los machos beta, mientras que las hembras no presentaron diferencias en el tamaño corporal. El tamaño del grupo reproductivo no tuvo un efecto significativo sobre la tasa de supervivencia de los nidos. En general, el tamaño de la nidada del grupo aumentó mientras que el número promedio de huevos puestos por cada pareja disminuyó con el tamaño del grupo. Sin embargo, el número promedio de polluelos emplumados por pareja por época reproductiva fue similar para todos los tamaños de grupo. Las hembras alfa fueron las que más contribuyeron durante la incubación diurna (65% de los grupos) y la nocturna (77%), aunque las hembras y los machos beta también contribuyeron substancialmente a la incubación.

Unfavourable environment limits social conflict in Yuhina brunneiceps

Identifying the factors that modulate cooperative and competitive behaviours is the key to understanding social evolution. However, how ecological factors affect social conflict and their fitness consequences remain relatively unexplored. Here, using both a game-theoretical model and empirical data, we show that Taiwan yuhinas (Yuhina brunneiceps)--a joint-nesting species in which group members are unrelated--employ more cooperative strategies in unfavourable environmental conditions. Fighting duration was lower, fewer total eggs were laid and incubation was more likely to start after all females completed egg laying (which causes more synchronous egg hatching). Surprisingly, as a consequence, there were more surviving offspring in unfavourable conditions because the cooperative strategies resulted in fewer dead nestlings. To our knowledge, this study is the first theoretical analysis and empirical study demonstrating that an unfavourable environment reduces social conflict and results in better fitness consequences in social vertebrates.

Unity and disunity in the search for a unified reproductive skew theory

Sandra Vehrencamps (1983) Animal Behaviour paper provided a rigorous mathematical foundation for understanding reproductive partitioning within animal societies by focusing on the limits to reproductive competition over reproductive shares among group members. The central idea is that the degree of inequity in reproductive shares is limited by the option of group members to leave the group and reproduce elsewhere. This central idea has been retained in extensions of her model, and unification of these extensions with rival models has been developed to accommodate new data and alternative starting assumptions. Although some criticism has been directed towards skew theory, as presented both in the original Vehrencamp model and in subsequent syntheses, we show that many of these criticisms have been misguided. Synthetic skew theory, with Vehrencamp’s model as its cornerstone, stands as our most general and complete, yet still largely untested, framework for understanding the evolutionary forces shaping the evolution of reproductive partitioning in animal societies.

The Brave Leader Game and the Timing of Altruism among Nonkin

The evolution of cooperation among nonkin remains a puzzle, and almost no theoretical work has examined the timing of altruism, that is, when a behavior that benefits others at one’s own fitness expense should be expressed in time. We present an evolutionary dynamic‐game model to address the question of when, if ever, an altruist would voluntarily emerge in time in groups of nonrelatives. Our model shows that when the benefit of having an altruistic leader decays with time, leaders will eventually emerge and will emerge later (i) in larger groups, (ii) when the cost of leadership increases, and (iii) when the assessment interval increases. The model applies to diverse situations in which time‐decaying group benefits are obtained only after a group member assumes a leadership role at some cost to itself, including leader roles in foraging flocks and migration groups in birds and spiny lobsters and in high‐risk foraging in desert ants.

GROUP-SIZE EFFECTS AND PARENTAL INVESTMENT STRATEGIES DURING INCUBATION IN JOINT-NESTING TAIWAN YUHINAS (YUHINA BRUNNEICEPS)

Abstract We investigated the effect of group size on incubation effort in Taiwan Yuhinas (Yuhina brunneiceps) at the Highlands Experimental Farm of National Taiwan University at Meifeng, Nantou County, central Taiwan, during 2000 and 2001. The Taiwan Yuhina is a joint-nesting, cooperatively breeding species endemic to Taiwan. We compared differences in parental investment among individuals of different sexes and status, explored the effect of group size on group incubation effort, and investigated whether individuals show compensatory reductions in care with respect to the number of females laying. Constancy of incubation increased as group size increased. Alpha females exhibited a significantly greater incubation effort than other individuals, but effort was similar among other group members. Both alpha males and females decreased their relative and absolute incubation effort as group size increased (i.e., there was a compensatory reduction in parental effort). However, beta pairs maintained a consistent but low incubation effort when either gamma pairs or an extra individual joined the group. Our study also demonstrated a new potential group-size benefit for cooperatively breeding birds—an increase in the constancy of incubation.

SEXUAL DIMORPHISM, DISPERSAL PATTERNS, AND BREEDING BIOLOGY OF THE TAIWAN YUHINA: A JOINT-NESTING PASSERINE

Abstract We studied the breeding ecology of Taiwan Yuhinas (Yuhina brunneiceps) at the Highlands Experiment Farm at Meifeng, National Taiwan University, in 1995 and from 1997–2002. The Taiwan Yuhina is a joint-nesting, cooperatively breeding species endemic to Taiwan. Males had significantly longer wing chords and tail lengths than females, probably due to sexual selection. Males also had a longer residence time at Meifeng than their female mates, which could be explained by philopatry being greater in males. Alpha males had a significantly longer residence time at Meifeng than beta males, but this was not the case for females, because females did not remain in the same group as males did after their mates disappeared. The breeding season was approximately 6 months long and multiple brooding was common. Nest building took 3 days, egg laying occurred over 3–4 days, the average incubation period was 14 days, and the nestling period was 12 days. Breeding success did not decrease later in the breeding season. Maximum longevity was 12 years, and the estimate of average annual overwinter survival rate for adults at Meifeng was 74%.

Environmental Uncertainty and Social Behavior

Ecology plays a critical role in shaping social evolution. Here we explore how environmental factors influence cooperative breeding behavior, a complex form of social behavior that involves more than two individuals caring for young. Both harsh and benign environments can promote cooperative breeding, which constitutes a paradox of environmental condition and sociality. Using the dual benefits framework to distinguish between different types of grouping benefits – resource defense benefits derived from groups defending critical resources versus collective action benefits resulting from social cooperation among group members – can help resolve this paradox. Instead of simply focusing on the behavioral decisions of why individuals stay at home or why they help parents from only the offspring’s perspective, the dual benefits framework considers the fitness consequences of cooperation from both the perspectives of current group members (insiders) and of potential joiners (outsiders). The different grouping benefits of resource defense and collective action profoundly affect insider-outsider conflict resolution, resulting in predictable differences in the per capita productivity, group size, kin structure, and stability of the social group. Ultimately, the dual benefits framework allows researchers to make testable predictions about how the environment influences social evolution, as well as explore the ecological and evolutionary consequences of sociality.