On Monday, May 27th, birders reported seeing up to 150 Swallow-tailed
Kites in foraging flocks north of Dade City, Florida. Bob Stalnaker, of
the Seminole Audubon Society, contacted Dr. Meyer of ARCI to ask why
they would be seeing possible pre-migration feeding flocks in May
instead of June when they usually form.
ARCI's satellite tracking
study of 13 Swallow-tailed Kites revealed unusually deleterious wind
patterns this year during the kites' 2013 northbound migration. This
resulted in several mortalities, late arrivals, and below average
nesting effort, which could lead to larger and earlier foraging
aggregations.
Following is Dr. Meyer's full statement regarding
the large number of Swallow-tailed Kites observed foraging in west
central Florida.
"For many
years, large flocks of Swallow-tailed Kites have been reported by
birders and wildlife managers from mid-June through July. These
previously-observed flocks, consisting of both adult and hatch-year
kites preying mainly on insects, have regularly gathered in the same
open areas within and between years. Some (including Harry Robinson and
Brian Ahern) have suggested that this year’s May appearance of flocking
adults might indicate high local nesting concentrations, unusually
early nesting, unusually high nest-failure rates, or some combination
thereof. I believe these explanations are unlikely.
As with
other Swallow-tailed Kite breeding-season behaviors, these feeding flocks have always
occurred during the same period each year, between fledging (which peaks in
early June) and the formation of pre-migration communal roosts (which peak in
late July). In addition, ARCI’s monitoring of nests and pre-migration roosts
since 1988 has not revealed any changes in the year-to-year timing of fledging
and roost development (consistent with predictions of others for long-distance
migrants facing climate change), nor have we documented early nesting or
unusually high failure rates this year.
One
possible explanation for this year’s early feeding aggregations is the pattern
of winds during March 2013 when Swallow-tailed Kites (and many other species,
obviously) were trying to migrate northward over the Gulf of Mexico. Up to that
point, ARCI (with colleague Jennifer Coulson) had been satellite-tracking 13
adult Swallow-tailed Kites tagged in Florida, Georgia, South Carolina,
Mississippi, and Louisiana (www.arcinst.org; Research; Tracking studies). As always since this study
began in 1996, we were collecting wind data (NOAA) during the kites’ trans-Gulf
flights. In March, however, we noted a very unusual three-week series of large
high-pressure systems that produced nearly continuous northerly winds from the southeastern
U.S. to the northern Yucatan Peninsula, where the kites stage before their
challenging over-water flight. A few of the kites made it safely to the U.S.
Gulf coast before this began, but most waited for the infrequent spells when
the strongest headwinds briefly abated, only to encounter stronger, more
persistent northerlies once over the Gulf. Given that we were monitoring 13
birds, there are many stories within this story, but the net effect was that
three tagged kites (two from Florida, one from South Carolina) did not survive
because their lengthy, convoluted flight paths failed to reach the U.S. coast in
time (we have learned that this species’ over-water limit is about four days). Of the 10 that survived, three have not
attempted to nest – a higher proportion than usual. Their late arrival may have
resulted in loss of their previous territory or mate, or their preferred mate
may not have survived migration and there was not sufficient time to pair with
another. Of the seven that did attempt
to nest, at least three started unusually late.
In all,
based on our sample of 13 marked birds and the uncommon weather, it seems likely
that nesting effort would have been below average this year. This could help explain the early feeding
aggregations of adults. A possible contributing
factor is that summer insect abundance might be peaking earlier due to climate
change. However, this in itself would
not hasten formation of post-breeding feeding aggregations without earlier
nesting and/or higher failure rates, neither of which is occurring. This is only speculation; I do not know if
climate-related shifts in regional insect abundance have been documented.
In a
related post, Patrick Leary suggested that post-fledging counts of long- versus
short-tailed kites (presumably adult versus hatch year) might be useful for
monitoring productivity. This method, along with guidance on aging based on
plumage (complicated by some molting adults having symmetrically short tails) and
a caution about the timing of southbound migration relative to age (adults
leave earlier, causing the age ratio to shift continuously over time), was
presented in a report to Florida Fish and Wildlife: Meyer, K. D. 1998. Communal
roosts of American Swallow-tailed Kite in Florida… (available at ARCI’s website:
www.arcinst.org; Publications).
While systematic and repeated photographic counts at large pre-migration roosts
that control for gradually decreasing ratios of adults to juveniles might
support statistically valid estimates of annual productivity, less rigorous
efforts likely will not.
Please
feel free to share this information with attribution to Avian Research and
Conservation Institute and its website, www.arcinst.org.
* * * * * * * * * * * * * *
Kenneth D. Meyer, PhDExecutive Director, Avian Research and
Conservation Institute
Associate Professor (adjunct), Wildlife
Ecology and Conservation, University of Florida"
http://birding.aba.org/message.php?mesid=457198&MLID=&MLNM=Florida%20Birdbrains
