Question: There have been a large number of Yellow-rumped Warblers at my feeders this fall and winter. Over the past month I’ve noticed that when they land on a bird feeder or on a tree limb they will flutter and open their wings and flash their yellow rump. I don’t believe that they nest here so I was wondering what might be the reason for this behavior. I don’t remember seeing this behavior in years past. – Joleen Ardaiolo
Answer: The Yellow-rumped Warbler is a remarkably understudied bird particularly when not on breeding territory. Therefore, this response is somewhat speculative. The only reference I can find as to this behavior on the web site Birds of the World reads: “Yellow rump-patch often displayed during foraging and hawking; may be passive display (perhaps territorial); spreads tail as threat (Morse 1989a).” Behavior – Yellow-rumped Warbler – Setophaga coronata – Birds of the World
What we learn from this scant information is the birds perform the behavior you see often—no big surprise there. The purpose is speculated as being a “passive territorial display.” This may indicate that the Yellow-rumped Warblers seen at feeders use the rump patch flash as a mechanism to tell the other nearby Yellow-rumped Warblers to stay away.
While researching this question, I had confirmed what I had heard in the past. The reason the Yellow-rumped Warblers spend the winter in in our area results from their ability to digest waxes and lipids. This enables Yellow-rumped Warblers to survive on a diet of exclusively Bayberry (Myrica pennsylvanica) if necessary. They will also eat Wax Myrtle (M. cerifera), Junipers, Poison Ivy and insects to name just a few of the winter foods. During the breeding season, they eat almost exclusively meats in the form of insects and other arthropods. So, these birds can frequently be seen between the beach and the forest.
While out exploring the environs around Seabrook Island, you may see a number of Yellow-rumped Warblers flitting about, but notice how they do not cluster together like American Goldfinches or Cedar Waxwings. The Yellow-rumped Warbler is not that social, though they do pay attention to their nearby neighbors as to may help them find new food sources. Yet, watch long enough and you will see birds chasing each other away for where they feed. I will now need to pay attention to see if that rump flash accompanies these interactions.
Yellow-rumped Warblers are very “pishable”. I don’t know why…maybe they are just curious.
-Bob Mercer
SIB 