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Food for Thought
High-fat meals may keep birds warm
We’ve already reported one important discovery—that birds’ food preferences vary from east to west (Birdscope, Winter 1995). For some widespread species, western populations ate significantly more red milo than sunflower or millet seeds, whereas in the Northeast or Midwest regions, populations of the same species ate proportionately more sunflower. This pattern was apparent in 12 of the 19 species we tested for regional variation in food choice. We developed a hypothesis about why birds eat more sunflower in the Northeast. Northeastern birds may prefer sunflower seeds because they’re rich in fats and oils that provide extra calories to help birds survive the severe winters. To test this hypothesis we compared sunflower consumption from north to south for eight species in eastern states and provinces for which we had sufficient data. For six of the eight species, birds in northern states and provinces chose sunflower seeds more often than birds living farther south. This north-south variation was statistically significant. For example, Black-capped Chickadees chose sunflower seeds in nearly 100 percent of the observations in eastern Canada but in less than 80 percent of the observations at the southern edge of their range (Figure 1A). Mourning Doves chose sunflower 40 percent of the time in northern regions but less than 20 percent of the time in Florida and the Gulf Coast (Figure 1B). House Finches (Figure 1C) are native in the West and introduced in the East. Finches from the Rocky Mountain and Southwest regions and eastern finches at the same latitudes ate sunflower seeds at nearly identical rates. Finches on the West Coast, however, ate fewer sunflower seeds than we would have predicted based on their latitude alone—probably because the climate is milder on the coast than it is in the mountains. House Finches in British Columbia ate about as much sunflower seed as birds in the Carolinas. Our findings suggest that for some species, the food an individual bird eats is probably influenced by its local climate. Birds in northern regions may cope with the cold by eating plenty of oil-rich seeds. We would need to conduct more experiments to find out whether these regional preferences are constant or whether birds actively select their menu according to the weather. Possible experiments include offering birds sunflower seeds with different oil contents or testing the food preferences of individual birds on cold versus warm days. Figure 1
A footnote: one species for which we found no correlation between seed choice and latitude was the Dark-eyed Junco (Figure 1D). Juncos sometimes make partial migrations in midwinter; presumably, they’re adjusting their energy balance by moving into areas with warmer weather instead of by eating high-calorie seeds. Kenneth Rosenberg is chief scientist in Bird Population Studies.
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