>Speaking of using Skyrats as food: In my novicehood as a birder
>I've wonder several times why, with the large numbers of rock doves,
>house sparrows, and starlings around, why the populations of the
>appropriate raptor preditors haven't increased accordingly, so as
>to keep the numbers of skyrats, skymice, and skydodos down. I've
>assumed there must be some built-in limitation. In the "wild",
>don't preditor population increases automatically follow increases
>in prey populations?

The maximum can be driven by limited breeding sites. Ian Newton,
in his 1970s classic "The Population Ecology of Diurnal Raptors"
details a two-decades long study he did of northern sparrowhawk (a
eurasian accipiter slightly larger than a sharpie) in his village.
In almost every year, nearly all breeding territories were occupied,
with the fluctuation in the overall population levels resulting
in dramatic swings in the number of non-breeders who were unable
to occupy one of the territories.

So the potential for annual production was capped by available
territories. Prey availability would in part control how much of
that potential was realized, but a gigantic increase in prey
just leads to all territories full of happy, fat chicks and
can't lead to them exceeding that potential.

This is just one population, of course. But at Malheur about three
years ago, 32 of 35 known historical nesting territories were
occupied by golden eagles. Since some of those sites hadn't been
used in many, many years this probably represented saturation of
available territories. A year later only a couple of territories
were occupied, due apparently to lack of prey.

The above is all kind of simplistic because birds are very mobile
and can move around in search of other territories when food's
not available at choice #1. He (Newton) also mentioned studies
that showed that a vast majority of peregrines that lost their
mate early in the season, but after territories were established,
got new mates very quickly. Apparently unattached birds float
around, always willing to comfort the bereaved :)

Numbers of breeding raptors in Portland, at least, is pretty low in
comparison to "sky rats". Most of our raptors are seasonal, coming
here for winter. While all the available food maybe makes it less
likely they'll starve, they also have to deal with cars and other
hazards here so who knows if they've actually found raptor nirvana
or not? Whatever, I think their population is probably controlled
more by conditions on their breeding grounds than conditions they find
here in winter.