geese

[John Bois <jbois@umd5.umd.edu>]

Rising to Varner's challenge

A six year study in Sweden found that red fox density increased in the
Southerly direction.  This correlated negatively with the number of broods
in grouse species. "The results support the hypothesis that higher
generalist predator densities are responsible for lower breeding success of
forest dwelling grouse moving southwards in Fennoscandia." (Ref. if needed).
I claim red fox density would also impact goose recruitment.

Density of foxes capable of preying on goose nests (e.g., arctic and red
foxes) is as follows: "Densities of active arctic fox dens are relatively
high on Herschel Island (2-7 natal dens/100 km2) and relatively low on the
Yukon coastal plain (0-0.04/100 km2). Most of the dens on the coastal plain
were located in the deltas of the Firth and Malcolm rivers, and by Clarence
Lagoon."  In Prairie pothole region red fox density fluctuated but averages
about one family per 10 sq. km.

Diversity of predators also increases with decreasing latitude.  For
example,
Greater Snow Geese suffer little predation in the extreme North of Canada.
Jaegars and arctic foxes take some eggs.  The snowy owl takes few goslings.
And that's about it.  In contrast, more southerly predators include red fox,
coyotes, and, depending on the area, grey foxes, and wolves.  Incidentally,
I haven't found mention of significant wolf predation on geese.

An interesting case study can be found at

http://www.usask.ca/biology/goose/result.html/

...where, on Banks Island, nest success is very high (many of these geese
are currently destroying local ecologies)--between 65% and 94% between 1996
and 1998 (compare this with 90+% _mortality_ in Brian Bertrand's ostrich
monograph!).

This is predator swamping, indeed:  at one colony (Egg River) there averaged
about 300,00 nesting geese

"Arctic foxes preyed mostly on eggs when foraging among nesting geese and
most of these eggs were cached for later use (83% were cached, 3% were
eaten, 1% were lost, and 13% had unknown fate). From 1996 to 1998, foxes
took 681 eggs..."
In one post I said goose colonies were unlike hadrosaur colonies in that
they served no defensive function.  Well, I was wrong.  Although foxes seem
to be able to get as many eggs as they want, they target single-parent nests
over two-parent nests.

To Varner's skepticism of birds' ability to site nests with a "mind" to
future predation, see

http://www.oikos.ekol.lu.se/jab.29.1.abstracts/2093.htm

where breeding prey birds apparently show predator-avoidance in their
nesting sites relative to kestrel nesting sites.

We can't get into the minds of geese to know what they're planning.  We also
can't (yet) look at specific genes to find out which cause specific
behaviors.  However, it seems apparent that they are (intentionally or not)
practicing an incredibly effective strategy: that of siting their nests in
locations where there are fewer predators.  They do have genes for these
behaviors.  They are selected.  Many sites possess very poor forage.  It is
unlikely the genes for migrating, then, are selected for this purpose at
least in some geese.  Moreover, the general pattern and direction of geese
nesting migrations _away_ from many very productive areas argues for some
universal factor _other_ than forage.  Competition with other geese must
come into play.  But it does so only on a very narrow geographic gradient, I
mean between one area of low predator density and another.  For example, I
would guess no geese are fighting for the rights to nest on the Serengheti,
although productivity is very high at some times.

And none of this has anything to do with David Attenborough whose shows
are wonderful.  If I disagree with him over the relative importance of one
factor over another, so what.