Gavin Leighton is a graduate student at the University of Miami conducting studies among weaver birds in Africa to try to understand the evolution of their amazing societies. Studying any animal in its own habitat can be trying, however, and the timeless expanse of the Namibian Desert is an unforgiving place. It is full of strange creatures that must all survive alongside one another, and which sometimes come in to colorful conflict.
I’ve previously discussed how living in a big, communal nest helps sociable weavers avoid the frigid temperatures of Namibian nights and how individuals in the colony benefit simply by roosting inside the nest. While this seems like an especially strong benefit to group living, there are other benefits that should not be ignored when we consider why some animals tend to live together. In many other animal groups, i.e. ants, bees, and wasps, individuals will defend their nest from intruders. The stings of these animals are a painful memory for those that get a little too close to the nest. In contrast to these insects, sociable weavers do not have special morphological adaptations for defense, and have to rely on behaviors to deter potential enemies.
Sociable weavers engage in multiple types of behavior depending on other species of birds that intrude on the nest. For instance, weavers will typically surround small birds of prey that arrive at the nest and give short, repeated, and high-pitched alarm calls. This behavior is quite common when pygmy falcons (Polihierax semitorquatus) arrive at the nest. In these instances the sociable weavers will gather in large groups near the pygmy falcon, but will not swarm or dive-bomb.Vigilant sociable weavers monitoring a pygmy falcon (bottom left) at the nest. (Photo by Gavin Leighton, 2014)
In addition to pygmy falcons, other species such as the acacia pied barbet (Tricholaema leucomelas) and ashy tit (Parus cinerascens) also investigate the sociable weaver nest and will often harass the weavers. The barbets are larger than the weavers, but adult weavers can effectively chase the barbets away. Although a barbet is significantly stronger than any single weaver, the weavers collectively prevent any barbets from monopolizing the benefits of the their titanic nests by sequentially chasing the barbet whenever it gets too close to the nest. Therefore, these acacia pied barbets would have to tolerate dozens or hundreds of chases if they persisted in trying to stay close to the weaver colonies.
It is important to consider these benefits of group living when studying the motivation behind it, as we do not want to scientifically push over-simplified theories as to why animals join and stay in groups. Multiple benefits can cause animals to live in social collectives, and these benefits often include the increased defense of living in a group. It’s one more piece of the puzzle as to why these birds evolved to cooperate in such large colonies.