Bald Eagles: A Fission-Fusion Species?

While some websites state that Harris Hawks are the only raptors that hunt communally, social hunting – sometimes in mated pairs, and sometimes including unpaired birds, depending on the species – has also been documented in Bald Eagles, Golden Eagles, Striated Caracaras, Red-tailed Hawks, Ferruginous Hawks, Red-shouldered Hawks, Peregrine Falcons, Lanner Falcons, Aplomado Falcons, Philippine Eagles, and Verreaux’s Eagles. Some of these species are loners, but others – including bald eagles – appear to be a fission-fusion species: i.e., they navigate between living independently or in breeding pairs, and coming together in groups. While eagle breeders might defend territory year-round, non-breeders roam through large areas and often form large assemblies at food sources or night roosts, especially during the winter.

How might these assemblies impact bald eagle success and intelligence? The fusion portion of an eagle’s life appears to be socially complex: eagles often form multi-age groups for foraging and challenge one another for food. Even though group members compete – think 30+ eagles after one fish! – they also play, socialize, and roost together. We don’t know whether this results in the formation of social bonds, but we do know that social bonds are advantageous in conflict and group hunting: bonded birds tend to support one another in fights and win more conflicts than non-bonded birds, while group hunting allows individuals to expend less energy than they would hunting alone. We also know that eagles show high fidelity to their winter and summer grounds, which means that migratory birds are probably meeting many of the same individuals year after year.  

Do bald eagles recognize one another in communal groups?

Would bald eagles be able to recognize one another across years? We don’t know. Plumage and vocalizations change as eagles mature from juvenile to adult, and the calls of individual eagles vary significantly within and between years. However, eagles tend to have a recognizable style within any given year, according to research performed at the University of Arizona. Vocalizations are common in communal roosts when new individuals enter the roost or change perches, and very common at feeding sites, which suggests they have some sort of social function.

While eagles might not recognize one another year over year – we truly don’t know – they could absolutely recognize one another within any given year by sight and sound. In my opinion, this makes it more likely that stable congregations – I’m thinking places like Eagle Valley here – are largely composed of individuals that recognize one another versus anonymous crowds aggregating at a resource. This means that socially bonded eagles could also engage in forms of post-conflict affiliation, indicating relationship repair and support mechanisms. Nesting pairs and siblings establish social relationships (the pecking order is a real thing) and appear to engage in social maintenance, so why wouldn’t those behaviors carry over into wintering eagles?

Social complexity, social creatures

Another factor contributing to social complexity can be seen in a species’ fission-fusion dynamics. Eagle ‘flocks’ are very fluid: members come and go based on birth, death, migratory timing, and breeding status. But that doesn’t mean that individuals aren’t repeatedly meeting and interacting with one another. Brett Mandernack has published several papers which found that some eagles wander more widely than others. If eagles are social creatures, it stands to reason that individuals would have varying social capacities, with some eagles being more social than others. Perhaps wanderers simply don’t fit into large wintering groups.

In addition to group foraging and food squabbling – a quarrel of eagles – we’ve also seen eagles perch together, explore ice and sticks together, ‘play’ with coots (not that the coots enjoy it), and seem curious about what other eagles are doing. In my opinion, eagles meet much of the criteria for a fission/fusion species and are clearly not the loners they are often portrayed as. Repeated encounters and interactions are the basis for social relationships, even if those relationships don’t persist over an eagle’s entire lifetime, and perhaps the evolution of social cognitive skills. Are we watching eagles return to behaviors that would have been common before their population crashed? To what degree does the little-studied social stage of their fission-fusion life impact survival and later nesting success? I would love to know more.

Things that helped me learn about this topic

Curious about eagle vocalizations? We blogged on HM and HD’s morning song last December: