Just a heads-up to everybody, a common denominator I am seeing in person and in
looking at photographs taken by others: a lot of insectivorous neotropical
migrants, as this spring migration peaks, are finding Boxelder (Acer negundo)
to be a good restaurant. The reason is a small caterpillar lurking in leaflets
tied together with silk called the Boxelder Leafroller (Caloptilia negundella).
This lime green with a black head caterpillar is small and hides inside
leaflets that it pulls around itself like a sleeping bag and then ties together
with silk. Its mission is to eat itself out of its home and transform into a
small, unimpressive moth.
Boxelder is an atypical type of maple in that it has compound leaves, usually 3
leaflets (sometimes 5) comprising one leaf. The branches display leaves that
are opposite in arrangement (like other maples). They are usually found in
riparian habitats. Do a search for images of boxelder and become familiar with
it. The birds find the caterpillars by looking for deformed leaflets, knowing
that the makers of the deformities are also food. They probe inside the leaf
roll and extract the caterpillar.
Birds I have seen doing this of late (and in the wonderful photographs of
others like Tom and Mary France, Mark Chavez, etc.) are: Mourning Warbler,
Yellow-billed Cuckoo, Black-headed Grosbeak, Blackpoll Warblers, Rose-breasted
Grosbeak, Golden-winged Warbler, Bullock's Oriole. Probably many other birds
are discovering this entree this wet spring.
Dave Leatherman
Fort Collins
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