[cobirds] Rose-breasted Grosbeak pair, Loveland (Larimer) on 6June2022, etc.

On my way back from Lake Estes to see and try to confirm the diet of the beautiful Canada Warbler found days ago by David Wade, I stopped in Loveland to further observe the bird response to the cankerworm infestation discovered by Derek Hill.

The feeding phase of the fall cankerworm (Alsophila pometaria) infestation of boxelder and a few associated willows is winding down but yesterday the bird response was still strong.  This area is in Loveland along the Big Thompson River Trail east of Wilson a few hundred yards (the quarter mile marker "09" is just e of where I saw the most action).  The heaviest defoliation of boxelders is south of the trail.  The Loveland Recycle Center Pond and the Loveland Recycle Center are due south of this area.  My guess is that a week from now, all caterpillars will either have been eaten or dropped to the ground to pupate until emerging as moths late next fall.  By mid-June the easy pickings possible during this rather unusual event will be over and the birds will have to switch to their normal summer schemes, including opportunism, to feed themselves and their young.

I was most interested in seeing both a male and female Rose-breasted Grosbeak feeding together on the caterpillars and wondered if they might attempt a rare Colorado nesting.  The male sings a lot and I think I heard the female do so, as well.

Other birds for sure observed feeding on the caterpillars were Bullock's orioles, common grackles, red-winged blackbirds, blue jays, black-capped chickadees, a white-breasted nuthatch, house wrens and even western kingbirds (the latter looked sort of strange down at lower levels of a riparian woodland).  Other birds heard in the area that could be getting in on the fun are song sparrow, downy woodpecker, yellow warbler and northern flicker.

My FOY red crossbill, which seemed small and did not sound like a typical Type 5, flew over sw to ne as a bonus.  Too quick to get a sound recording (even if I knew how to use my phone).

As for the diet of the Canada Warbler, it appears certain that when in pines it is mostly foraging on adult midges (at least three species) emerged from Lake Estes, and when in river birch (and probably the few thinleaf alders also present) it is eating birch catkin bugs (Kleidocerys resedae).  These items were confirmed as being the two most common insects in the areas frequented by the bird by sweeping with an insect net when the bird was busy elsewhere.

Dave Leatherman

Fort Collins