Friday, October 04, 2002

Ruby-throated Hummingbird Fall Departure Dates

Intrigued by recent discussions on the WV-Bird and other bird listservs about the influence of artificial nectar sources (i.e., hummingbird feeders) on fall migration of hummingbirds, I decided to pull together some real data on Ruby-throated Hummingbird fall departure dates. To that end, I extracted reported late fall dates for West Virginia and the greater Appalachian Region from the seasonal reports published in the journals Redstart and North American Birds (and its predecessors) for the period 1949-2000. I located 52 fall departure dates (from 33 different years) for which the exact day was reported. Reported dates ranged from September 17 to November 8. The median reported departure date was October 7 (in other words, half of the reported departure dates were before, and half after, October 7). Fewer than 10 percent of reported last departure dates fall after October 24; the five extreme dates are:

November 8 (1961): Marmet, Kanawha County, WV; “extremely late”
October 27 (1997): Hamilton County, TN
October 27 (1964): Lexington, VA
October 27 (1949): Morgantown, Monongalia County, WV; “still visiting flowers in garden”
October 25 (1997): Shady Valley, TN

So how does this tie in with hummingbird feeders? Hummingbirds are highly evolved to feed on the nectar of flowering plants. I propose that fall departure of hummingbirds is more closely linked to changes in the availability of natural sources of nectar than it is to either decreased photoperiods or air temperatures. In our region, nectar sources probably peak in late summer/early fall (mid- to late September). After that, the availability of nectar starts to gradually wane. There is some threshold of nectar availability below which it is no longer economical for a hummingbird to remain in an area. At that point, a bird has two choices: (1) remain in the area and gradually starve, or (2) seek out other, more abundant, sources of nectar. Fifty-two years worth of field data indicate that, under natural conditions, very few Ruby-throated Hummingbirds linger in West Virginia and the Appalachian Region past October 24.

Despite what the experts say, I am not convinced that artificial feeders do not entice some Ruby-throated Hummingbirds to linger in our area beyond their “normal” departure dates. Why take a chance? My recommendation would be to not, under normal circumstances, leave hummingbird feeders up past mid-October. If you still have a few hummingbirds lingering about at time, then gradually decrease the amount of nectar (i.e., stimulate natural conditions). If the hummingbirds are smart, they will journey southward in searth of more abundance amd dependable nectar sources (plus milder climates). If not, then we can reason that they were simply not meant to contribute further to the gene pool. Who was it that said, “Nature is a cruel mistress” (is that a real quote or something I made up?).

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