I am sure that this question has been keeping you up at night, but rest assured science is working hard to resolve the question. I came across this recent research paper in the scientific journal Evolutionary Ecology. The paper is not publicly available, it is restricted to subscribers.
Alexandra T. Stang, Susan B. McRae (2008). Why some rails have white tails: the evolution of white undertail plumage and anti-predator signaling Evolutionary Ecology DOI: 10.1007/s10682-008-9283-z
Rails, or Rallidea, is a family of small to medium sized birds that includes the Crakes, Coots, and Gallinules. I personally am most familiar with the American Coot which a common water bird in our area. The study was trying to evaluate the evolutionary significance of white on the under-tail coverts of some rail species.
The researchers were evaluating four possible hypotheses regarding the evolutionary development of these white feathers. Their studies produced some surprising results.
The four hypotheses include:
- White under-tail coverts may have been favored by sexual selection.
- White under-tail coverts may have developed to facilitate signaling in territorial interactions.
- White under-tail coloration might be used in intraspecific communication other than agonistic or sexual displays.
- Contrasting white tail feathers serve to accentuate tail-flicking signals directed at predators.
An important foundation assumption of the study is that non-white under-tail coverts is the ancestral state and that white under-tail coverts evolved as a result of natural selection, and not the other way around. There is ample previous research available on this topic to validate the assumption.
Regarding the first hypothesis, the researchers assumed that if the trait was sexual selected it would be more prevalent on species with greater mate competition. Those that utilize polygamous mating systems. Other research has indicated that species with intense mate competition are more likely to utilize displays involving plumage. The cross-species comparisons did not provide any evidence for a relationship between polygamous species and white under-tail coverts. The team then evaluated whether sexual dimorphism could play a role, but this too proved inconclusive.
Regarding hypothesis two, the researchers assumed that species that used their tails in territorial displays should be more likely to have white under-tail coverts if this trait evolved specifically for this purpose. Once again, there was no evidence for this hypothesis.
Regarding hypothesis three, the belief was that if white under-tail coverts were used for another type of signaling such as alarm or status, then they would be more common in birds that flock for at least part of the year. The analysis did indicate that this was significant.
For the last hypothesis, a number of interesting results were considered. Based on past research with some bird species such as warblers, its has been determined that some birds that exist in darker environments have more conspicuous plumage patterns. The team investigated the possibility that these conspicuous patches might have evolved to increase the efficacy of communication in darker environments. The results were the opposite of what was expected. They found a highly significant correlation that birds that live in open habitats were more likely than those in dense habitats to have white under-tail coverts. Also, those with crepuscular or nocturnal habits are no more likely to have white tails.
These results indicate that flocking behavior and open habitats are the most significant variables. This is where the statistical analysis went a bit deep for me. The researchers used software to perform contingency change tests to evaluate the evolutionary order of development. The results of this indicate that white under-tail coverts evolved before flocking, but after the move to open habitat. This order is very important as the evolution of white tails before flocking tends to indicate that the white tails evolved as a pursuit-deterrent function instead of of a social communication function. So now you know why white under-tail coverts evolved in rails!
I found this particular study interesting as it required deep analysis of a complex evolutionary situation. The white patch on these birds seem very insignificant overall, but is just one of many traits on many species which at one point in their past played a very significant role in their survival and thus their fitness to reproduce. It is an example which helps our overall understanding of evolution in general and bird evolution more specifically.
I was further intrigued by this particular study as some of their results came as a surprise. The results were the opposite of what was expected further showing that just because we understand how a particular mechanism works with a particular species, it does not mean that it works the same way with the next species.
I look forward to performing this type of research myself. I have been loosely contributing to a few research projects over the past few years. With my personal studies in Biology/Ecology I have made some new connections and will be accelerating my participation. I am also looking to apply for some research internships once I get my class and work schedule sorted out.