Birdscope Winter 1996- A Redpoll Perspective

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WINTER 1996/VOLUME 10, NUMER 1

A Redpoll Perspective
BY JEFFREY V. WELLS

Please cite this Page as:
Wells, J.V. 1996. A Redpoll Perspective. Birdscope, Winter 1996, Volume 10, Number 1.

FeederWatch site descriptions shed light on redpoll movements

If you are a FeederWatch participant you might wonder why, year after year, we ask you to describe your backyard on a Site Registration Form. You know, the form that asks whether you have coniferous trees near your yard or live in a rural neighborhood.

Your feeder counts tell us how many birds show up in a particular place at a given time—but not why they show up there. With site data, however, we can examine this question.

Consider the winter of 1993-94, which eastern FeederWatchers dubbed "The Year of the Redpoll." Every two years or so, redpolls leave their usual winter haunts in Canada and Alaska to invade northern regions of the United States. The 1993-94 invasion was unusually big. We used site-registration data to find out what sites redpolls preferred and how these preferences changed over the course of the season.

Feeder counts showed that redpolls started arriving in the Northeast in November 1993. The invasion continued until January; after that, the percentage of feeders with redpolls leveled off. But even though redpolls stopped moving in, the average flock size continued to increase as birds began to aggregate into large flocks at preferred feeding sites.

What constitutes a "preferred feeding site"? That’s where the site registration forms came in handy. We found that, in general, redpolls preferred rural sites to urban sites. Within the Northeast, over 70 percent of rural sites had redpolls, but less than 40 percent of urban sites did. However, in areas where the redpoll population density was high, such as Canada, the birds showed less selectivity, visiting only a slightly smaller percentage of urban feeders than rural feeders (Figure 1).

This finding fits with the theory of habitat selection developed by biologists Stephen Fretwell and Harry Lucas. Fretwell and Lucas argue that an individual bird will do better in poor but uncrowded habitat than in a high-quality but crowded habitat. They predict that at low population densities, a high proportion of the birds should use the prime habitat. At high population densities, however, birds have to compete aggressively for resources. This strife becomes a "cost" of living in that habitat, and some birds should start to spill over into poor-quality habitat.

Figure 1
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Figure 1. The distribution of Common Redpolls at FeederWatch sites in the winter of 1993-94. When the redpoll population density was low, birds preferred rural sites to urban sites. At high population densities the birds filled most available sites.

Our findings were consistent with this theory. Redpolls seemed to prefer high-quality rural sites. But when their numbers increased, more and more birds moved to poor-quality urban sites (Figure 1). Redpoll behavior is also consistent with this theory. Anyone who has seen redpolls at a feeder knows they pugnaciously defend favored feeding spots.

The sites that redpolls found least attractive shared certain characteristics: many buildings, lots of nearby pavement, and few trees. Still, sites like these did attract some redpolls. As the redpoll flocks came streaming down from the north, they spread out over the landscape. Hungry birds probably stopped at the first feeding site they encountered regardless of whether or not it was a high-quality site. Redpolls didn’t stick around at the least desirable sites, however. Only 25 percent of urban feeders hosted redpolls during more than one FeederWatch observation period, compared to 40 percent of rural feeders.

A number of other site characteristics were associated with whether or not redpolls stayed at feeders. Large count areas were more likely than small count areas to have redpolls during more than one observation period. Redpolls were more likely to visit quiet feeders than feeders disturbed by squirrels, cats, and people. (Dogs didn’t make a difference, though.)

This study helped us understand the dynamics of winter redpoll invasions. Many questions remain, however. Do other northern finch invaders, such as Pine Siskins and Evening Grosbeaks, share the redpolls’ preferences for certain site characteristics? Do these species move across the landscape in the same way that redpolls do?

We hope to answer these and many other questions using the data from your Site Registration Forms. So, as you fill in your 1995-96 form, remember that it’s an important contribution to our knowledge of the ecology of the birds that winter in North America.