 |
|
|
The Competition Factor
What birds want to eat and what they get aren't always the same The Seed Preference Test asks a simple question: what seeds do birds prefer? The accompanying article uses the data you collected to answer that question (see also the Autumn 1994 issue of Birdscope). As in most scientific experiments, however, a confounding variable exists—one that slightly complicates how we interpret our results. SPT participant Monroe Ringis of Kirkwood, Missouri, summed up the problem well. "The pecking order among the species and within a species influences the seed preference. When a pair of Northern Cardinals came to feed, the male brazenly took his favorite, black-oil sunflower. But he would not let his mate approach those seeds…. She had to content herself with milo until the cad had his fill and left. Also, when the bully of the neighborhood, the Blue Jay, is at a seed, none of the other birds dare to approach." William Ross, of Riverside, Connecticut, adds, "There appears to be a major flaw in this test—there is no way to identify second-choice selections….We noted that an aggressive bird would chase off other birds, which would either leave without taking seed or take seed from a second choice." This phenomenon has been examined in a controlled study by Purdue University biologists Tom Langen and Kerry Rabenold. Their results show that what birds like and what they get aren’t necessarily the same (Behavioral Ecology, vol 5: pp. 334-338; 1994). The subject of their study, Dark-eyed Junco, is the most widespread feeder bird in North America, according to Project FeederWatch. Juncos form flocks in the winter; within each flock there’s a "dominance hierarchy"—dominant birds get to feed first, feed in the best location, or eat the best food. Langen and Rabenold predicted that, because of these dominance hierarchies, solitary juncos would choose different foods than juncos in flocks. To test this prediction, they placed seven male juncos in separate outdoor cages, each stocked with a variety of seeds. Then they counted how often each bird visited each food during timed observation periods—much like the Seed Preference Test. Next, they put all the juncos together, figured out which birds were dominant by watching to see which birds won aggressive encounters, and repeated their counts. What happened? Alone, each bird preferred sunflower, millet, canary, and thistle seeds to corn and oats, which are less nutritious and harder to handle. In flocks, each bird visited its favorite foods significantly less often than when it was alone. But dominant birds only made 7 percent fewer visits; subordinates visited the preferred foods 25 percent less often. In a cold winter, that could mean the difference between survival and death. The moral of the story: a bird’s diet isn’t just a matter of taste. A bird may choose one food because competitors are keeping it away from another food it really likes better—or because it’s trying to save energy by avoiding a fight. We have no way of studying this phenomenon accurately with the data we collected in our Seed Preference Test. But we are thinking about taking a closer look at pecking orders at feeders in future experiments. Meanwhile, see if you can detect competition in action at your feeders. |
