[cobirds] Warbler food

Going thru the eBird alert that entered my email this morning, I was particularly interested in a pic attached to Richard Bunn's checklist from Pueblo showing a worm-eating warbler probing a red, male cottonwood catkin.  I am almost certain the target of such foraging are the larvae of catkin weevils in the genus Dorytomus.  This was the subject of "The Hungry Bird" column in "Colorado Birds" in issue 45(2) back in April 2011.  I think this is archived on the CFO website.

I was in Lamar last week and obtained the following photo of catkin weevils south of Granada.  The larger female appears to be multi-tasking (feeding just prior to, or just after, mating).  Its chewing jaws are at the distal end of the long black snout.  These weevils overwinter in leaf litter, emerge, feed on catkins inside the flower buds, mate, lay eggs in the catkins, and the rather nondescript white larvae feed among the pollen-producing flowers of cottonwood during primetime for spring warbler migration.  They drop to the leaf litter to pupate in late spring/early summer, turn into adults and repeat the process the following spring.  The other photo below shows the red male catkins that will appear after the yellowish flower buds (like the one the weevils in the photo are on) expand.

           

While there was not a lot of landbird activity in the Lamar area while I was there during three different April visits, what I did see was mostly in cottonwoods (white-crowned sparrows, house finches, red-winged blackbirds, northern cardinals, yellow-rumped warblers).  All but the warblers were indiscriminately eating the nutrient-rich catkins themselves.  The yellow-rumps engaged in more careful examinations consistent with hunting for weevil larvae. 

I also noted a fair number of yellow-rumps feeding on bibionid flies (also called March flies, lovebugs).  These black, midgelike, slow-flying flies are common this time of year at Lamar Community College Woods and many other near-water locations, usually on understory vegetation.  Their habit of copulating on the wing led to their being called "lovebugs".  I have seen bibionids be the subject of warbler foraging on many April-May occasions, particularly on windy days when the flies can be found down low on vegetation in calm microsites within the larger wind-whipped habitat situation.   Day before yesterday, as quite strong winds blew for the second straight day from the north, joining the warblers getting these flies were r-c kinglets, robins and a hermit thrush in the soapberry thicket at the calm south end of LCC.  The following poor photos show a yellow-rump surrounded by several airborn March flies (taken a few years ago at LCC) and a cropped telephoto of a March fly.

                        

Gall-making psyllids have emerged from overwintering sites in tree trunk bark (or are in the process of emerging) and are busy laying eggs on expanding hackberry leaf buds, so hackberry is also a good place to check for neotropical migrants at present.  I saw both yellow-rumps and ruby-crowned kinglets in Lamar in hackberry.

Dave Leatherman

Fort Collins