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 Namibian Desert is an unforgiving place full of strange creatures that must all survive alongside one another.
I spend much of my time in Namibia observing sociable weavers (Philetairus socius) building and maintaining the largest tree-nests in the world. These birds spend a lot of energy searching out suitable nest material and then weaving the material into the nest. Not only are these sociable weavers cooperative, but so are other animals—termites and ants are ubiquitous on the Namibian landscape, and both of these groups also have hyper-cooperative individuals that devote the majority of their time and energy towards helping the group. In all honesty, I personally witness the cooperative aspects of nature a lot more than the competitive side, despite nature often being labeled “red in tooth and claw.”
However, there are times when cooperation gives way to self-preservation, and it can often be due to the freezing nights we experience in Namibia. Recently, we had a cold snap where temperatures dropped below freezing for multiple hours. The sociable weavers compensate for these challenging temperatures by foraging more in the morning. In fact, I have found in previous field seasons that colder temperatures significantly reduce how much time individuals spend cooperatively constructing the nest. Additionally, weavers will crowd into specific nest chambers, and I have found up to seven adult birds in a single chamber during cold nights. This undoubtedly helps conserve heat but also seems to have an unfortunate consequence. During the cold snaps, I find young chicks or very young juveniles that were crowded out of the chamber and fell from the nest during the night. Unfortunately, most of these young birds do not make it to morning.
Given the icy extremes, weavers will find material such as feathers and soft grass heads to line the inside of their chambers to improve insulation. Interestingly, I accidentally dropped a cotton ball on the ground (we use cotton in the process of drawing blood samples from the sociable weavers), and soon enough, a weaver brought the cotton into the nest to use as nest lining.
The cold temperatures also change the behavior of other animals by bringing out desperate foraging tactics. For instance, I recently saw a Gabar goshawk (Melierax gabar) rip open hanging nests of southern masked weavers (Ploceus velatus) looking for a meal. I have seen common scimitarbills (Rhinopomastus cyanomelas) spend considerable time searching for insects to eat, instead of the other components of their diet, like nectar. The yellow mongoose (Cynictis penicilatta) pictured above would also forage below the nests, looking for eggs that had fallen out. Perhaps the most striking predatory behavior I’ve seen is that of the yellow hornbill (Tockus leucomelas). For three straight days, a hornbill (quite possibly the same one) returned to the sociable weaver nest and scavenged for babies that had been pushed out of the colony. While these yellow hornbills are thought to feed primarily on ants and termites during the dry season, they try to find the most protein-packed and nutritious items around, even if it’s at the cost of sociable weaver parents.A yellow hornbill at a nest of weavers. (Photo by Gavin Leighton)