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Seed Preferences: East Versus
West
Why do birds in different places like different foods? Last winter more than 5,000 backyard birders recorded nearly half a million seed choices made by over 30 species of common birds. It was one of the largest volunteer-based bird experiments ever conducted-- the Lab's successful Seed Preference Test. In the previous issue of Birdscope we summarized Seed Preference Test results: Most ground-feeding species preferred white millet or red milo to black-oil sunflower seeds; many tree-feeding species preferred sunflower seed; and bird species in the West liked red milo more than birds in the East did. The East-West differences are especially interesting. Do they occur simply because different species of birds are found in each region? In this article, we'll explore the question of geographic variation in birds' seed preferences. First, let's compare the bird species reported by eastern and western observers. The eastern bird list contains any tree-feeding species such as chickadees and titmice, and finches such as the Northern Cardinal. In contrast, the western bird list includes many ground-feeding species such as sparrows, towhees, thrashers, and doves. So perhaps eastern species prefer sunflower seed not because they live in the East but because they're mostly tree feeding species. Maybe if we compared eastern tree-feeding birds to closely related western tree-feeding birds (for example, Mountain Chickadee and Plain Titmouse), we'd find that these western species also prefer sunflower seed. Unfortunately, Seed Preference test participants didn't record enough data about these species for us to draw any conclusions. We did compare other pairs of related species for which one member of the pair occurred in the East and the other in the West. We also compared eastern and western populations of widespread species (see the table below). For example, we compared the Steller's Jay, a western species that ate the most red milo of any species tested, to its eastern relative, the Blue Jay. Blue Jays in the Northeast, like many of their feeder companions, registered a strong preference for sunflower. But we also received data for a small sample of Blue Jays from more western regions (the species' range extends to the Rocky Mountains). These birds showed a shift in preference; they ate less sunflower and more milo than their eastern cousins but not much as Steller's Jays. Cornell ornithologist Kevin McGowan, an expert on jays and crows, points out that jays have an unusually high tolerance for tannins, the bitter-tasting chemicals that make milo unattractive to many birds. This information helps to explain why Steller's Jays were the number-one milo eaters. Why don't eastern Blue Jays share this taste for milo? It's still a mystery.
We also looked at the most widespread feeder bird in North America, the Dark-eyed Junco. This species has distinct geographic races, including the "Oregon Junco" and the "Slate-colored Junco"; at times, ornithologists have considered these races to be separate species. Even so, the Seed Preference Test results indicated that western juncos chose only slightly less sunflower and slightly more milo and millet than eastern juncos. Two other widespread species, Black-capped Chickadee and Mourning Dove, showed a clearer difference between eastern and western populations. In both species, western birds ate considerably less sunflower and more milo or millet than eastern birds. The distinct population of Purple Finches resident along the West Coast also ate less sunflower than their widespread eastern relatives; however these birds took up the slack by eating millet, rather than milo. At this point, we began to wonder whether our initial question --Why do western birds eat more milo?-- wasn't reflecting the Cornell Lab of Ornithology's eastern bias. What the western birds have in common isn't necessarily an increased appetite for red milo, but rather a decreased craving for sunflower. Western birds simply seem to be more generalized feeders that is, they eat a little of each seed type and don't show a strong preference for a single food. Perhaps we should not be asking why western birds eat more milo, but why eastern birds show such a marked preference for black-oil sunflower seed. Our final comparison sheds even more light on this question. When we looked at the data on House Finches in the Southwest and West Coast regions (where this species is native) we found a fairly strong preference for sunflower seeds (71 percent). House Finches were introduced to the East Coast in 1940, and their population has soared since then. Did eastern House Finches retain their original seed preferences over the past 50 years, or did these birds adopt the tastes of their eastern feeder-mates? A look at the bottom line of the table on page 2 shows that the introduced finches have indeed shifted their preferences. They made 85 percent of their visits to sunflower, and reduced their consumption of millet by one-half compared to the western finches. Does this difference reflect a rapid shift in the genetically controlled food preferences of the eastern population or a difference in the availability of the various seeds in the two regions? We don't know yet. But these results are especially intriguing in light of a recent study showing that eastern House Finches have larger bills, on average, than their western "ancestors." Could this be an example of rapid evolutionary change? Has finch bill size changed in response to the eastern birds' increased specialization on large sunflower seeds? Like any good scientific experiment, the Seed Preference Test has left us with more questions than we started with. Now, in addition to wondering which species prefer which kinds of seed, we wonder why western birds show a greater tolerance for red milo. And why do eastern specialize more on a single type of seed? Does a diet of fat-rich sunflower help eastern birds get through cold winters? Or are the habits of those of us feed birds different enough in different regions to influence the food of the birds themselves? We may be able to answer some of these questions with the data you have already provided. Others will require innovative new experiments. For now, we'll wait for a second year's results from the Seed Preference Test. We hope the new data will give us larger samples from western states and provinces, and also allow us to check for year-to-year stability in birds' seed preferences. We hope that even more of you will take part this winter, as the world's largest natural science experiment. Copyright 1995 Cornell Laboratory of Ornithology. |
