Non-kin Reciprocal Cooperation in the Superb Starling

Across the vast grasslands of central Kenya lives a strikingly colorful bird that has surprised scientists studying cooperation in animals. This species, the superb starling (Lamprotornis superbus), belongs to the order Passeriformes and is easily recognized by its metallic plumage that glints with iridescent color. Superb starlings are highly social birds that live in groups typically consisting of several dozen individuals, sometimes reaching as many as sixty. These groups are not simply extended families composed of close relatives. Instead, they form complex social communities in which related and unrelated individuals coexist. Over two decades of continuous field observations have revealed an intricate network of long-term cooperation among these birds, exposing subtle social interactions that are not immediately obvious.

Superb starling（Image source：Dennis Irrgang， CC BY 2.0 ）

In many animals, helping behavior—such as assisting in raising another individual's offspring or defending the nest from predators—is usually explained by kin selection. Under this principle, individuals increase the survival of shared genes by aiding close relatives. However, the social life of superb starlings cannot be fully explained by this mechanism alone. Research shows that even when birds have the opportunity to help relatives, many still choose to assist unrelated group members. At first glance, such behavior appears inconsistent with classical evolutionary expectations emphasizing self-interest. Yet when the interactions among individuals are followed for many years, or even decades, a different pattern emerges.

Researchers investigated these relationships by collecting blood samples for genetic analysis and marking birds with uniquely identifiable leg bands. This allowed them to determine kinship and track individuals across time. The data revealed long-lasting patterns of mutual assistance between specific birds. For instance, one individual might serve as a helper during a breeding season, assisting another bird in raising its chicks. Several seasons or even years later, the roles might reverse, with the former recipient becoming a helper in return. Such role reversals are unusual in cooperative breeding systems. In many species, helpers are subordinate individuals that consistently assist dominant breeders, and once these roles are established they rarely change. Superb starlings, however, display remarkable flexibility. Both males and females—whether born within the group or immigrating from elsewhere—can alternate between breeder and helper roles multiple times throughout their lives. Individuals of this species can live between fifteen and twenty-five years, providing ample time for such exchanges to unfold.

These long-term cooperative relationships occur both among relatives and among unrelated individuals. Approximately half of the observed helping events involve birds with no genetic relationship. In several cases, two completely unrelated birds maintained cooperative partnerships simply because they had helped each other previously. This pattern resembles the principle of reciprocity: individuals that assist others may receive help in return at some point in the future.

One reason this phenomenon remained largely unnoticed for so long is that it is difficult to detect. Kin-biased helping can often be identified relatively easily, since related individuals are expected to assist one another. Reciprocal cooperation among unrelated animals, however, can only be recognized by following the same individuals for many years and reconstructing their interactions across time. In this study, scientists assembled more than twelve thousand recorded instances of helping behavior across nine social groups and forty breeding seasons. Only with such an extensive dataset could the underlying structure of these relationships become visible.

Superb starling（Image source：Diego Delso， CC BY-SA 4.0 ）

Further analysis revealed that patterns of helping differed according to sex and origin within the group. Females that were born in their natal groups almost never became breeders. As a result, they rarely participated in reciprocal exchanges. Their helping behavior largely reflected kin preference, and outside of assisting relatives they often remained inactive with respect to breeding and helping. In contrast, birds that immigrated into the group—both males and females—formed reciprocal relationships with unrelated individuals far more frequently.

Helping unrelated birds may also provide additional advantages beyond the possibility of future repayment. In superb starling societies, group size strongly influences survival and reproductive success. Larger groups provide greater protection against predators and increase the likelihood of successful breeding. By assisting unrelated individuals, a bird may help stabilize or expand the group, indirectly improving its own chances of survival and reproduction. For immigrants, receiving assistance from established group members may also encourage them to remain in the group, preventing the community from shrinking through mortality or dispersal.

Researchers also propose that, under certain circumstances, individuals might even manipulate social cues to be mistaken for relatives in order to receive assistance. Although such behavior carries an element of deception, it can also be viewed as an extension of reciprocal cooperation within a mixed social network. In these communities, behavioral strategies are not rigid but continuously adjusted in response to social conditions.

Although reciprocity has long been discussed in evolutionary theory, clear empirical demonstrations remain rare. In many animal societies, limited observation periods make it difficult to detect long-term exchanges of assistance. Consequently, the importance of reciprocity may have been underestimated. Kin-based helping is easier to identify and measure, which has often led researchers to regard it as the dominant mechanism of cooperation. The findings from superb starlings suggest that understanding animal cooperation requires moving beyond a simple opposition between kin and non-kin interactions.

Instead, cooperative societies may arise through a combination of mechanisms operating simultaneously. Kin selection, reciprocal exchanges, and the benefits of maintaining larger social groups can all contribute to the stability of these complex social systems. The case of the superb starling demonstrates that cooperation among animals may rely on a dynamic balance of strategies, shaped by long-term relationships and the shifting roles individuals play throughout their lives.

Author: Shui-Ye You

Reference:

Earl AD et al. (2025). A cryptic role for reciprocal helping in a cooperatively breeding bird. Nature.

（Paid content. Unauthorized reproduction or use is prohibited.）