Hi,
I have spent considerable time solving this issue of, "What are the early
bluebirds that hover over the prairie getting?". The specific site I observed
most was the Pawnee Grasslands just west of Crow Valley. My conclusion is that
these are some type of "tiger moth" (Family Noctuidae, Subfamily Arctiinae
(which used to be considered the Family Arctiinae), Tribe Artiini , Genus
Grammia. We have more than one species in this genus on the eastern plains of
CO. The larvae eat various lower plants, including some grasses, and
overwinter in this stage. On warm days they become active and certain ones of
these are considered the fastest moving larvae of any North American macromoth
(large moth). When I was watching the Mountain Bluebirds on the Pawnee back in
the 1980's, they would hover kestrel-style and, in so doing, eventually
discover and obtain a prey item. The caterpillars zoom from one tuft of
bunchgrass to another. It took me a long time to figure this out. I felt
quite foolish, running to an area where a bluebird had been hovering, only to
see nothing. When I finally saw a sudden movement, it was a caterpillar
streaking across an open area between grass clumps. Once I had the search
image, I started seeing this pretty frequently and the mystery was solved.
Once a bluebird had a caterpillar, on every occasion it was taken to a fence
post (usually) or to an open area of prairie. There, it was unceremoniously
thropped until flaccid and nearly hairless. I think the majority of what looks
like excessive force during this whole process is that which removed the hairs.
The hairs have to be formidable impediments to easy ingestion and must be
removed to a considerable degree before the caterpillar body can be consumed.
But since insect food is tough to come by during the weeks when northward
movements in February and March take place, I guess an armed caterpillar is
worth the effort it takes to de-arm it.
So, that's what I think is going on. It is interesting that Mark said
something about a "pine tree", as there is a tiger moth that occasionally
infests various foothills conifers, makes conspicuous tents in the very tops of
infested trees, and the caterpillars ARE present in the winter months. But the
ones in Mark's photos do NOT look like that species (Lophocampa ingens). Thus,
I think my "pretty sure" ID of Grammia sp. is correct.
You could do a search for "Grammia tiger moth images" and see examples of the
adults. They all sort of look similar in general color scheme and pattern
arrangement, and I am sure most astute birders have seen similar creatures in
the field during the warm months.
I enjoyed your observations here.
Dave Leatherman
Fort Collins
From: mamers...@msn.com
To: daleather...@msn.com
CC: zar...@comcast.net; lindab_...@hotmail.com; tkwidj...@gmail.com
Subject: Mountain Bluebirds - Eating
Date: Tue, 10 Mar 2015 10:10:15 -0600
Dave -
Attached please find a couple of photos of Mountain
Bluebirds taken last Saturday – north of DIA – 03/07/15. Is there any way
that you could assist in identifying the caterpillars (grubs) that the
birds are eating. I believe that they were getting them from a pine tree –
but not sure of that fact.
Your assistance in this inquiry is greatly
appreciated. Inquiring minds would like to know...
Have a great week – and enjoy the improved
weather.
Mark Amershek
Denver
303.329.8646
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