Dead Leafers

by John Sterling

Yellow-backed Oriole probing for insects in dead, curled leaf in Panama.

Yellow-backed Oriole probing for insects in a dead, curled leaf in Panama.

In the tropical rain forests of lowland Central and South America, there are often large flocks of insectivorous birds that can comprise of dozens of species.  Within these flocks, there are birds with specialized foraging behaviors that help reduce competition for insect prey.  One such behavior is dead-leafing.  Pioneering research by Judy Gradwohl in Panama, her late husband Russ Greenberg from Mexico through Central America, and Ken Rosenberg in the Amazon region of southern Peru, found that this behavioral adaption is prominent in a few species of antwrens, antshrikes, orioles and wrens.  In North America, we have dead-leafing Bewick’s and Carolina wrens, but also four warbler species that are dead-leafing specialists only during the winter: Orange-crowned, Blue-winged, Golden-winged and Worm-eating.  In 1988 I collected foraging behavior data on Orange-crowned Warblers in Humboldt County, California.  I found that during the winter, they fed almost exclusively by probing into dead leaves, flowers or emergent leaf buds–highly stereotypic behavior for dead-leafers.  But during the summer, they were foraging behavior generalists and lacked the propensity to probe.  March was a transition period where about half of the foraging was dead-leafing and the other half was typical warbler behavior of gleaning insects off flat, green leaves.   So….what is it about dead leaves that attracts these birds?  These leaves are often curled, forming excellent hiding places for insects.  Dead leaves often form clusters that also provide shelter for insects.  Furthermore, many  remain caught by other vegetation or spider webs in the sub canopy and understory for weeks or months thereby providing regular feeding stations that can be recolonized by insects and revisited by hungry birds.

One can  find these birds by listening for crunching or movement of suspended dead leaves–often a sign that dead-leafing birds are feeding there.  Many years ago a friend of mine, Steve Rovell, was looking for a vagrant Golden-winged Warbler in California in dense riparian vegetation with a thick blackberry understory.  I suggested that he listen for the bird rustling in dead leaf clusters in the blackberry thicket.   He did and was the only one to see that bird that morning while others searched in vain.  So knowing a bit about foraging behavior can help one find and/or identify birds.

Blue-winged Warbler reaching for a dead leaf in late fall.

Blue-winged Warbler reaching for a curled, dead leaf in late fall.