Fw: [cobirds] Rare warbler diets

I intended this to go to everyone, not just Nathan.  Sorry.

Dave

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From: DAVID A LEATHERMAN <daleatherman@msn.com>
Sent: Saturday, April 22, 2023 10:23 AM
To: Nathan Pieplow <npieplow@gmail.com>
Subject: Re: [cobirds] Rare warbler diets

 

Nathan et al,

Short answer is "I don't know".

Long guess is as follows.

When multiple species of birds are foraging on multiple types of plants in multiple styles, and the situation is near water, my default guess is always an aquatic insect hatch.  And the most likely aquatic insect group is chironomid midges.  We have several species, some of which are quite small.  The nymphs feed on organic plant and animal matter in the water and emerge from the surface as flying, mosquito-like adults.  As adults they often form swarms, especially in the evening.  Adults also sit around on all manner of vegetation and are not particular about which kind.  We have had real pulses of midge emergence over the last few weeks.   Flying midges and sitting midges could account for the two types of foraging (flycatching and standard twig gleaning).

Why just one stretch of the creek?  Perhaps it has just the right organic matter, or the flow rate is just right.  Or the lighting in general along this stretch is such that birds can more easily spot flying/perched prey than stretches on either side of it.  Hard to say.  The same way one 9 cubic foot section of air has a swarm of midges, it's easy to imagine a clumped distribution of ideal larval development conditions in the creek bed.  It's a clumpy world out there.

Staying with the guess of aquatic insects, I suppose the next most likely prey suspect is one of the caddisflies, but they would be darker/bigger and more conspicuous than some of the smallest midges.  If it was caddisflies, Nathan's sharp eye would have seen them.

About the only way to confirm any of this is to be present with a net and sweep it thru the air in the area with birds and the nearby inactive areas, and see if there's a difference in the catch.  I did some of that in November 2021 when "the pond" on the north side of the CU campus had the magnolia warbler.  Lots of aphids of many types in the air and on foliage, but the pond and creek had plenty of midges, too.  Late fall is more likely to be when flying adult aphids compete with midges for the crown of best staple for migrating passerines.  In early spring, 1) midges; 2) small flies, bees and weevil larvae attracted to, or in, willow/cottonwood flowers; 3) overwintering scale insect nymphs (especially elm); 4) gall-making adult psyllids emerging from hackberry trunk bark or other types of tree bark near hackberries - these are consistent common denominators in many spring rare bird events.  Aphids not so much. 

The only thing more fun than observing the mysteries is solving the mysteries.  I must say it's a great comfort to know this task will never end.

Thanks, Nathan, for your efforts and questions.

Dave Leatherman

Fort Collins

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From: cobirds@googlegroups.com <cobirds@googlegroups.com> on behalf of Nathan Pieplow <npieplow@gmail.com>
Sent: Friday, April 21, 2023 10:29 PM
To: cobirds <cobirds@googlegroups.com>
Subject: [cobirds] Rare warbler diets

 

On Wednesday afternoon, Curtis Beimborn discovered a Yellow-throated Warbler and a Northern Parula along a section of Boulder Creek just below the university, about 100 yards east of 17th Street in Boulder. Both birds were still present this evening at about 6:00 PM. With the exception of a 24-hour disappearance by the Yellow-throated, both birds have been consistently sticking to the same 50-yard stretch of creek.

This same stretch of creek has also been the center of activity for juncos, goldfinches, nuthatches, Ruby-crowned Kinglets, and Yellow-rumped Warblers. But if you walk more than a few dozen yards away from this spot in either direction, bird numbers drop precipitously. This situation has remained stable for the past three days, and so, Dave Leatherman-style, I decided that the birdy spot must have some kind of food that is keeping all these birds around.

I tried this evening to observe food habits. Interestingly, the two rare warblers are engaging almost exclusively in flycatching -- especially the parula, which sallies constantly. The Yellow-throated's sallies are less frequent and possibly less successful, and it also gleaned some branches while I was there. Whatever they are catching is too small for me to see in the air. And most of the other birds present seem to be following more typical foraging strategies for their species -- i.e. few of them are flycatching.

The rare warblers are frequenting some dead branches over the creek as well as some barely leafed-out elms, cottonwoods, and buckthorns. Dave, what do you think they are going for? Flying adult aphids?

Why would the flying insects be so faithful to this one short stretch of creek? And how long might this situation last? Can we expect other migrants to join the fun in the coming days?

Nathan Pieplow

Boulder

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