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What Do
We Mean By "Invading"? |
The BirdSource North American Winter Finch Survey is Tracking the 1997-98 "Superflight" The BirdSource North American Winter Finch Survey Helped Track the 1997-98
"Superflight"
During certain years, several winter finch species simultaneously irrupt
into southern Canada and the United States from their normal wintering
areas in the far North. This phenomenon is called a "superflight" (Bock and
Lepthien, 1976, "Synchronous Eruptions of Boreal Seed-eating Birds,"
American Naturalist Volume 110: 559-571). Through BirdSource, the North
American Winter Finch Survey tracked a major invasion of winter finches
into the United States, and the 1997-98 winter season revealed itself to
be a superflight year. As BirdSource mapped the movements of all nine
species on the survey form, six species in particular--Red-breasted
Nuthatch, Red Crossbill, White-winged Crossbill, Pine Grosbeak, Common
Redpoll, and Hoary Redpoll--were most dramatically associated with this
superflight.
It is unknown precisely why superflights happen some years and not others,
though they seem to be associated with food production on the wintering
grounds of these finches. Crossbills and nuthatches extract seeds from
cones; redpolls forage on catkins (the seed-bearing structures of birches
and alders), and Pine Grosbeaks specialize in fruit seeds. Typically, the
fruiting cycles of these different plants vary from species to species, so
that not all finch species are affected at the same time. (There is
speculation that this variation in food production is an evolutionary
strategy that forces these birds south every few years, thereby reducing
their long-term impact on the plants.) When the seed production of all or
some of these plants are affected simultaneously, the birds' southward
search for food may result in a superflight.
Although weather does not seem to directly trigger winter finch movements,
it is certainly a factor in food seed production. Therefore, a broadscale
weather event that has taken place months or even years prior to an
irruption impacts the seed crop, which then may lead to a superflight.
Since Evening Grosbeaks, Pine Siskins, and Purple Finches forage on many of
the same plant species as the other winter finches, it is still unclear why
the numbers of these birds were not as strong during as they were for the
other species.
The 1993-94 winter season saw an irruption that involved primarily
redpolls. This is likely the result of the catkin production cycle of
birches (Betula sp.) on the wintering grounds. This two-year cycle has been
directly correlated to redpoll invasions. During years of low catkin
production, redpoll numbers increase in the U.S; likewise, a good year for
catkins usually means fewer redpolls coming into the country. The 1993-94
redpoll irruption was almost certainly due to a major decline in catkins.
This same year, crossbills and Pine Grosbeaks were scarcely reported,
suggesting that cone and berry crops were plentiful enough to sustain them
on their wintering grounds.
As indicated on the animated maps, some parts of the U.S. have experienced
only brief visits from winter finches during the 1997-98 superflight. This
may be due to a lack of sufficient food to keep the birds in those areas.
For example, relatively few reports of White-winged Crossbill came in in
from Central New York, where the cone supply was negligible. On the other
hand, the White Pines (Pinus strobus) and Eastern Hemlocks (Tsuga
canadensis) in portions of Pennsylvania, New Jersey, Maryland, and northern
Washington offered a robust quantity of food seeds for winter finches,
according to reports from observers in these states. An abundance of Pitch
Pine (Pinus rigida) and Japanese Black Pine (Pinus thunbergiana) cones in
Massachusetts, Rhode Island, and the Long Island shoreline, and high tree
and plant seed availability in much of the northeast seemed to have kept
winter finches in these regions. Fruits and berries in much of New York
and New England sustained the superflight in this part of the country.
The last time a superflight occurred was in 1982-83. There was also a
superflight in 1972-73, and before that, in 1968-69.
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