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The Red Rock, IA CBC had 4 Northern Shrikes. There have already been many
reports in Missouri. NOSHs have obviously invaded for a second year in a
row. Why?

I can think of several scenarios for the presence of a northern species
south of its expected winter range:

1. It's northern food source has crashed. (E.g. poor cone crop; crossbills
come south. These birds usually do not repeat).

2. It built up a large population during a time of food abundance, so
surplus birds have come south during a season of relative food scarcity --
i.e. winter. (E.g. lemmings build up. Snowy Owls lay 3-11 eggs. In years of
lemming abundance 6-7 owlets may fledge. Suddenly there is an overpopulation
of owls; the lemming supply declines or crashes, and the surplus owls come
south. Many of them starve, so they usually do not repeat.)

3. A large food supply in its northern summer range causes a large
population to build up. The species' summer diet includes important items
not available in its breeding area in winter (e.g. insects), so it usually
comes south. More are seen in the winter range because the overall
population has become larger. (E.g. during the seventies and eighties, Pine
Siskins were common in Missouri every winter).

4. Favorable breeding conditions cause a large population to build up. The
species maintains winter territories. There are not enough territories in
its usual winter range to accommodate the whole population, so some birds
come farther south. These birds survive the winter, return north, survive
the following summer and come south again. (This scenario is suggested by a
post speculating that the NOSH seen in the area near Smithville Lake where
one wintered last year is the same bird returning to its winter territory).

5. The species is really regular in winter, but birders don't know it until
an increasing number of birders learn how to ID the species and start
looking for it. (E.g. I can remember when Ross' Goose was #2 on the
Burroughs Audubon Society's 10 Best List. Now, we expect them in every
sizeable flock of Snow Geese.)

I wonder if the repeat NOSH invasion is scenario number 4 or 5.


Bob Fisher
Independence, Missouri
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