[cobirds] Likely food of recent Black-throated Blue Warbler in Fort Collins (Larimer)

The male Black-throated Blue Warbler first reported by Lori Brummer was apparently a first-fall bird based on greenish overtones on its upper parts, brownish wings and white flecks on some of the black throat feathers.  It stayed in a fairly small area of the Northern CO Environmental Learning Center just north of the Suspension Bridge for at least four days (November 14-17).  Any bird that stays in one spot that long is probably under the influence of a food resource.  Loving both warblers and food issues, of course, I tried to see the bird and figure out the attraction.

 

The bird spent a great deal of its time while I watched on the 17^th going back and forth between two large Siberian elms that still held green leaves and a third elm also retaining green leaves.  Siberian elm has a number of insects associated with its leaves that would be good fare for an insectivore like a warbler (elm leaf beetle, elm leafminer and European elm flea weevil being the primary ones).  However, none of these should be accessible up in the crown in mid-November, but rather should be under loose back or down in the leaf litter.  During the few hours I watched the bird, it was almost always up in the crowns of the three elms foraging fairly far out on the branches, but sometimes nitpicking within the crown interior from trunk bark.  On a few occasions it went down low in bushes and disappeared from sight along the banks of a small drainage.  On the 17^th I got absolutely no indications of what it was getting except on occasion it made brief “flycatching” jaunts that indicated at least during those movements the prey was flying.

 

I went back on the 18^th with my insect net.  The bird was apparently gone, all the birders still wanting to see it had given up and I took the liberty of sweeping the green elm leaves on the lower branches of all three elms with my net.  All I got were fairly good numbers of midges of at least two species.  Going from left to right, the first three images below of a gray species show a female (short antennae) and two males (feather duster antennae). The second species at right below was black and much smaller. 

 

                                   

 

Since midges are always present near the open water that spawns them, and the site has one small water-filled drainage plus the nearby main channel of the Poudre River, my conclusion is that the warbler was using chironomid midges to fuel its late, off-course, 1900-miles still to go, first trip to the Bahamas/Greater Antilles.  Maybe the adult midges, which seem to land on anything once out of the water, preferred roosting in the interior crowns of trees still holding leaves because such micro-sites are a bit warmer and shielded from the wind?  Not sure.  Whatever was going on, the bird was resourceful in finding them and I wish him well.

 

Dave Leatherman

Fort Collins