[Mnbird] ecological trap

[Raymond Faber]

For many years, I have studied herring gulls on islands in upper Green Bay,
Lake Michigan.  Part of this work involves banding 500-1000 young each
year.  Last week, two students very ably assisted me in the banding.  We
search through dense vegetation with hockey sticks to find the young.
While doing this, one of the students yelled out that he found a duck
nest.  It had 2-3 eggs in it which indicated a hatched nest with inviable
eggs left behind.  Ducks, including red-breasted merganser, common
merganser, mallard, black duck, and gadwall, commonly nest among the
gulls.  I told the students that this represents what we call an
"ecological trap" because the ducks get protection during incubation from
the very aggressive gulls (we wear hard hats), but I had never seen a
duckling leave the island.  They are attacked and eaten when entering the
water.  Just as I said that, one of the banders yelled "Oh, no!"  A
red-breasted merganser duckling just offshore was attacked and eaten by a
herring gull.  And then another.  And then another.  At least 5 ducklings
were killed.  A gruesome sight, but somehow the ducks persist--they
continue to nest here after many years.  Quite an ecological lesson in real
life!

By the way, the banding yields interesting results.  Though few bands are
reported, it appears that the age of the gulls at death is increasing,
although we do not have enough data yet to confirm this.  This is occurring
as the levels of contaminants that have ravaged these birds for many years
are decreasing significantly.  In 2015 a herring gull wearing one of my
bands was identified by an interested observer reading the number with a
spotting scope (quite a feat).  It was banded by me in 1986 and now stands
as the North American record for herring gull longevity (29 years, 3
months).

Ray Faber

-- 
Raymond A. Faber, Ph.D.
Professor of Biology
Saint Mary's University of Minnesota
700 Terrace Heights #1524
Winona MN 55987

Telephone: 507-457-1540
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