That’s the burning question many feeder-watchers pose each fall. When “our” birds don’t show up at the buffet some folks fear the worst: widespread population decline. Fortunately, the actual cause of these shifts in feeder visits is usually not catastrophic. Often what’s happening is not a population change at all, but rather a behavioral change.

As the seasons of the year cycle around, many bird species adjust their feeding activities to exploit different natural food sources. This is true for the birds that visit feeders as well as those we’d rarely see anywhere near our homes. Take the American Goldfinch, for example. Goldfinches feed exclusively on weed seeds (coneflowers, joe-pye weed, thistle, etc.) and tree seeds (birch, spruce, and so forth). These kinds of seeds are overall most abundant in fall. So chances are good that the Goldfinches you’ve been missing at your thistle sock are simply exploiting these natural windfalls. As preferred seeds become harder to find and temperatures drop they’ll probably return to your yard. Unless they’ve moved south, that is, in which case the individuals you see in winter could be different from those present in summer…a topic for another time…

Another example of these seasonal shifts in feeder usage is the Blue Jay. Here in Maine many of us are seeing higher than usual numbers of Blue Jays at our feeders, chowing voraciously. The situation for them is the opposite of the Goldfinches. While tree and weed seeds are super-abundant, the Jay’s preferred natural food -- nutritious, fat-rich acorns -- are in short supply in these parts this fall.

The reason is that, by and large, oaks produce strong acorn crops every third year. This banner year is followed by a year of very poor acorn production, then a year of average production, then another year of strong production, then another crash, and so on. This strategy enables the trees to periodically “overwhelm” acorn eaters (Wild Turkeys, squirrels, jays) so that more acorns survive to germinate in the spring. In the year following high acorn production the trees need to rest and rebuild energy and nutrients; hence the boom-bust acorn cycle.

Other factors, such as rainfall and temperature, can create further variation within the oaks’ three-year cycle. For example, this year’s poor acorn crop might be even smaller due to poor pollination owing to a chilly, wet spring. If our winter is harsh and snowy, the deer, turkeys and jays will be scrounging and possibly more likely to visit feeders. Fortunately for them, there seem to be good supplies of “soft mast” fruits and berries like crab apple and winterberry, in the Maine woods this year. “Hard mast” food other than acorns, such as the seeds of ash, maple and cherry trees, can help to offset the dearth of acorns to some extent as well.

And like American Goldfinches, Blue Jays are partial migrants and can move to areas where there are more acorns. We can track their movements to some extent through our Christmas Bird Count and Project Feeder Watch data.

Other possibilities

Besides changes in food availability, there are other reasons why you might be seeing more or fewer individuals of particular species at your feeders or in your yard. One possibility that I’ve alluded to is partial migration, especially in late fall and winter, in response to local weather conditions. Red-tailed Hawks, for instance, might stay on their breeding territory through thick and thin. Or they might choose to move to an area with less snow cover or a heartier concentration of small rodents. Ever walk through a field and see mice and voles scurrying for cover left and right? Wintering raptors will be looking to share your experience, but for very different reasons. When rodents are scarce, or as they move around during their spring and fall migratory cycles, raptors might undertake their own version of Project Feeder Watch: another factor that can temporarily cause your feeder birds to dine elsewhere for a short period.

Another possibility is that local conditions have changed in an area for the long-term. When trees are cut down and houses built, some birds will have to move on. Natural changes like old fields succeeding to forest once again will also change avian populations.

So when your favorite birds go missing, don’t be dismayed. Appreciate that they’re out doing what wild birds do best: living in harmony with their environment. If you’re interested in statistics from feeders in your area and elsewhere, visit the Project Feeder Watch website to explore regional trends.

Peace and good birding,

Scott Cronenweth

www.naturalpathwalks.com