Many organisms send signals to other individuals by means of various outward physical or biochemical characteristics and behaviors. The technical term for such observable characteristics is phenotype. The phenotype is the expression of a trait determined by an individual’s genetic makeup and/or a variety of environmental factors. Common examples of what one might call a phenotype include things such as eye color, height, and blood type. Evolutionary biologists have a long history of trying to understand what information certain phenotypes might convey to others. For instance, it has been demonstrated that the color of a hummingbird’s throat patch or the length of a flycatcher’s tail, for example, can be signals of quality. In evolutionary terms, this is an individual’s fitness.
There is also something called the extended phenotype, which is an external characteristic that is directly influenced by the individual, such as the bowers of Australian Bowerbirds or the nests of many other birds. Black Kites (Milvus migrans) decorate their nests with all manner of white plastic objects. This curious behavior may be classified as part of the species’ extended phenotype. But why would they do such a thing? What kind of a signal are these birds trying to send? A recent study on Black Kites in Spain suggests that these raptors decorate their nests as a way to communicate their level of fitness to would-be competitors. The authors propose that white plastic nest decorations “act as reliable threats to conspecifics, revealing the viability, territory quality, and conflict dominance of the signaler.”
The researchers studied 127 nests over a period of five years and ultimately reached the conclusion that evolutionarily more fit birds are signaling that they have no fear of potential threats by other birds. In other words, stronger birds are investing time and energy into finding white plastic objects and placing them in their nests in order to send a clear message that they are more than able to defend their nests to any challenger. Weaker birds on the other hand, typically the youngest and oldest of the population, don’t decorate their nests at all, or at least do so to a much lesser degree. The researchers did experiments where they added decorations to certain nests and found that weaker birds would actually reject them. They concluded that they must do this to avoid competition all together by not drawing attention to themselves or their nests from stronger birds that might aspire to evict them and takeover a better nest site. They consider this behavior to be an honest signal. This means that birds with lots of decorations are not bluffing about their fitness because there is nothing to be gained from doing so, other than a potential battle that might leave them injured or without a nest to reproduce that season.
Of course there is debate about whether these conclusions are accurate. Many other scientists believe that is does not make sense for a bird to advertise nest and that the white plastic objects may actually serve to help camouflage the eggs to predators looking for a meal according to an egg search image. All good science deserves to be challenged, so stay tuned for more interesting research that may enlighten us to the true meaning of this bizarre behavior.
