[Tweeters] irruptions

Arch McCallum archmcc at qwest.net
Sun Nov 21 12:25:11 PST 2004

Hello Tweeters and Obol,

It has been interesting seeing all the reports of mountain chickadees and
other species outside their breeding range. It looks like a major irruption
is on, for a number of species. Someone on Tweeters asked what the cause of
this might be. The usual answer is food. Irruptive species tend to have a
cyclical food supply. The predators build up a large population while the
food supply is abundant, then when the food supply crashes, the predators
emigrate/erupt from their breeding areas. That makes for an "irruption"
into the receiving areas. Voles and conifer cone crops are notoriously
cyclical and their avian predators, e.g, boreal (small b) owls and finches,
montane corvids, parids, and sittids, are some of our best known
irruptives. Interestingly, this year both boreal and montane irruptions
seem to be under way.

I studied this phenomenon with Mountain Chickadees in New Mexico in the
1980s. If you'd like more information, read on.
The following was written in August in response to a report of a Mountain
Chickadee on Mary's Peak, Oregon coast range.:

Having color-banded and followed a couple of thousand Mountain Chickadees
in New Mexico during the previous century, I find this observation very
interesting. First, because it is at high elevation in, I assume, conifer
forest that resembles breeding habitat. Second, because it is in early August.

My studies in NM suggested to me that there are two distinct pulses of
movement away from "home" in this species. First, 2-3 weeks after fledging,
all juveniles (every last one of them in my study) leave their natal
territory, mill around for a few days to a few weeks, then settle down in a
new home. Some settle as close to home as one territory away. Others
disappear, to be replaced by birds from elsewhere. The place a MOCH settles
a few weeks after fledging can be its home for the rest of its life.
Second, in some years (at least in that study in the 80s), none of them go
anywhere after that. In other years, almost all [juveniles] leave. Those
are the years that high numbers appear on Christmas counts in the lowlands.
In other years, most of the juveniles (and all the adults) would stay put,
and additional birds would arrive in October and Novermber, be banded, and
stay around several months before departing suddenly in March. Also in
March, in departure years, some of the birds that had been missing since
September would re-appear, set up territories exactly where they had last
been seen in the previous fall, and nest.

Got all that? In other words, there seemed to me to be a clear distinction
between natal dispersal, which happened in July and August every year, and
migration, which happened in September and October, with a return flight in
March and April, in some years. Dispersal was not explained by any
environmental variable. Migration intensity (% leaving) was highly
correlated (negatively) with the size of the pine seed crop. Technically,
this migration would be called conditional, because it's conditioned on
environmental conditions, and partial, because only some individuals do it.

So, what does this have to do with the bird on Mary's Peak? Because of the
date and the elevation/habitat, I suspect it is a disperser. That means
there was either a successful Mountain Chickadee nest in the Coast Range a
few weeks ago or the bird came all the way across the Willamette Valley
from the Cascades. Both are possible. (It's also possible it is an adult,
but that only pushes the question back into the past.) That's not an
amazingly long distance for a MOCH to go, on migration (as some show up in
sw Kansas, many miles from nesting habitat, in flight years). We don't know
much about the longer dispersal distances, but a few long ones is
consistent with what's known from better studied species. The Coast Range
is a bit moist for this species, but an occasional successful nest isn't
that far-fetched.

I would predict that this bird tells us nothing about the likelihood of a
big Mountain Chickadee flight this fall and winter. On the other hand, Noah
Strycker writes in BOGR, "During peak movement years, may expand to w.
Oregon as early as late summer or fall, sometimes later." That statement is
undoubtedly based on observation, so I could well be wrong.

[note added in November: and I was wrong, there's nothing like data for
testing a prediction.]

Arch McCallum
Eugene, OR

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