Sunday, October 5, 2008
Why are Yellow-Rumped Warblers so common?
Thanks for the question, Lori V. in Menlo Park, CA.
(UPDATE: The answer below is outdated, given recent name classification changes. Thus, see the 3/7/11 article here at this blog for an update in relation to the Yellow-rumped Warbler split into various subspecies.)
Here's why you see Yellow-Rumped Warbler in many different habitats and, especially, en masse during a prolonged spring and autumn migration (in the Midwest and East):
1. Among the six subspecies in the Yellow-Rumped Warbler (YRWA) species, four are Audubon subspecies (including the male Audubon YRWA seen in the above photo) and two are Myrtle subspecies.
2. YRWA exploit multiple feeding niches within the profile of the forest. That is, you are as apt to see them in the interior branches near the main trunk as you are to notice them near the ground. As a “hover and gleaner” feeder (like many Dendroica genus wood-warbler members of which the YRWA is enrolled), you can expect to see YRWA in a forest profile, but they also might be at the seashore (finding invertebrates near the surf) or, even, on grassy lawns (where insects may provide food resources).
3. In turn, given YRWA is able to exploit multiple environments and feed on diverse food resources – from insects, to fruit (such as poison oak and poison ivy berries, in addition to privet and wax myrtle berries – the species is well-adapted to sustaining its populations in times of food scarcity. That is to say, the YRWA has a diversified portfolio – a perfect tactic to ensure that the boom and bust of food resources does not impact the species’ numbers.
4. The hydrochloric acid content within YRWA individuals appears to be more potent than the digesting apparatus contained within most other wood-warblers. In fact, the wax myrtle berries that help YRWA survive harsh conditions in winter along the mid-Atlantic are not digestible by most birds, including the 51 other wood-warbler family members typically found annually in North America north of Mexico. Given the abundance of wax myrtle, YRWA are able to survive into the deep fall (and sometimes throughout the winter) in states that often encounter harsh winter conditions: WI, MI, OH and, in the East, MD, VI, NY (and in some New England states during some winters).
Seeing YRWA in November and December, thus, is not uncommon in the states mentioned immediately above. Increasingly warmer winter seasons, too, provide succor to YRWA that do not need to vacate northern latitudes as is the custom in the vast majority of other songbirds.