Wednesday, October 7, 2009

Black-throated Green Warbler foraging

Text and photographs (all copyrighted) by Tony Leukering

A nicely cooperative immature male Black-throated Green Warbler in a cherry on the north side of the Hawkwatch platform on the 6th provided a great opportunity to study (and photograph) the typical foraging behavior of the species. As the genus Dendroica is so diverse but is of a recent and closely inter-related radiation of species, it is a great group in which to study how foraging behaviors differ among the species of such groups. Many birders pay little attention to such, but foraging style and behavior are useful clues to identification. Though not of the genus Dendroica, Worm-eating Warbler is a great example of a distinctive foraging style, as it is a dead-leaf specialist -- foraging for arthropods primarily among clusters of dead-but-still-attached leaves.

The various members of Dendroica have varying specialializations -- or not -- in types of trees, particularly in the conifer vs. hardwood dichotomy, but some also prefer to forage in species of trees with smaller leaves vs. those with larger leaves (as in the useful foraging-strategy difference between Philadelphia and Red-eyed vireos). Some are more aerial, capturing prey flushed into the air, and some are more restricted to gleaning from leaf surfaces. Among the gleaners some forage primarily on the upper surfaces of leaves, some on the lower surfaces.

If one watches a Black-throated Green Warbler for any length of time, one should note that it spends a lot of time perched and looking upward. That is because the species has a strong tendency to glean the lower surfaces of leaves and there's no better way to scan such than to look up from below those leaves. Yesterday's B-t Green did just that. I captured pictures of that behavior a number of times, and provide a couple of those pictures below.

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