by Melinda S. LaBranche
illustrations by Julie Zickefoose
Is mate choice anything
more than a random decision?
Project PigeonWatch was designed primarily to answer one major
question: "Why are there so many different colors of pigeons?"
We can start our investigation by examining the distribution
of pigeon color morphs. To determine how many of each morph are
seen in different flocks, we ask PigeonWatchers to record the
number of each color in the flock. But this doesn't explain why
there are so many colorsit is a preliminary step. After this
first step we must figure out what allows so many colors to continue
to exist together in the same flock.
There are a number of ecological explanations for why most wild species have only one or two color variations, making feral pigeons an exception. One well-accepted idea is that there are not many predators in typical feral pigeon habitats. Predators, particularly birds of prey, often target individuals that stand out within a group. As it swoops down upon a flock, a predator needs to be able to keep its eye on just one potential victim and follow it until it is separated from the flock. If all of the pigeons in the flock are similar, the predator may have difficulty following just one. Thus, individuals in a flock might appear camouflaged if they are similar to one another. But, for example, if there are many light-colored pigeons in a flock, a dark pigeon would stand out and make an easy target for a hawk or falcon.
Lacking a consistent set of predators, as do most cities, pigeon flocks are free from the removal of rare birds. This low-predation-pressure hypothesis suggests that because the rare-colored pigeons are not removed from flocks, they can pass their color genes on to future generations of that flock, and thus those colors remain in the flocks. To examine this, we can compare the color morphs of city flocks with those of rural flocks (where there are likely to be more predators). And since the reintroduction of Peregrine Falcons in many large cities, it would be possible to compare cities with large numbers of falcons to those with few or none.
But predation pressure is not the only ecological idea about
the persistence of color variation in pigeon flocks - behavior
may have an influence. Consider, for example, the choice of a
mate. Among pigeons it is the female who chooses - males strut
around and females choose from among the males (see the descriptions
of pigeon courtship behaviors in the Bird Watchers' Notebook
article). Once the choice is made, most pairs stay together
for life. If color influences the female's choice, then mate
choice may influence the colors of pigeons in future generations.
First, females may prefer mates that are similar to themselves. If you think about this, you may ask how a pigeon can figure out what color she is, since she doesn't have a mirror! Ecologists suggest that a female chooses a mate who is similar to her parents - and, of course, parents are likely to be similar to the female herself. Thus, if pairs in a flock are the same color and their offspring look just like them, then a "culture" might develop where pigeons always choose a mate of their parents' and their own color.
If pigeons of like color morphs always mate, then morphs that are genetically dominant will not "drown out" the other morphs over time. This hypothesis suggests that, all else being equal, color morphs will remain in fairly constant proportions over time. To test this hypothesis, we could examine the proportions of morphs in the same flocks over many years. If the proportions of colors remain constant, then some support for this idea exists. If, however, the proportions change over time, this doesn't mean that mate choice doesn't have an influence on pigeon-color variation.
A second suggestion is that females choose males that are unusualso-called "rare males." If females prefer rare males, then these males will always have mates, and thus they may have more offspring compared with males that are common. Unlike the first idea, this rare-male phenomenon will probably cause changes over time in the proportions of colors because the once-rare males will become common and females will choose the "new" rare color morphs.