VA Birders,
The American Kestrel, a small, open-land falcon which often may be
seen perched on a telephone pole or wire, is perhaps the most common diurnal
raptor in North America. So it perhaps was a little surprising that the many
birders who covered still largely-rural Loudoun County, Virginia on
International Migratory Bird Day this last Saturday have to date reported
seeing only two American Kestrels. The population of the American Kestrel is
thought to be stable overall but it appears that there have been significant
decreases in populations of this species in some areas in recent years,
particularly in the Northeast. Although valid conclusions cannot be reached on
the basis of one day or even a season of birding and the American Kestrel is
still a fairly easy bird to find in appropriate habitat, some suspect that the
American Kestrel is similarly undergoing a long-term decline in northern
Virginia and elsewhere in the region.
Although the causes of the American Kestrels decline in the Northeast are not
known, it is suspected that the loss of open habitat from human development and
the natural reforestation of former agricultural areas have resulted in
increased populations of and predation by the Coopers Hawk. This factor is
suggested by the numbers of these birds recorded at the Cape May, New Jersey
Hawk Watch in recent years and anecdotal testimony regarding the predatory
activities of Coopers Hawks at American Kestrel nest boxes. As to the former,
the year 2004 produced a new all-time high for Coopers Hawk and a new all-time
low for the American Kestrel at the Cape May Hawk Watch. Numbers of the
American Kestrel increased there in 2005 and decreased again in 2006 to a
number which was significantly above the 2004 level, while numbers of the
Coopers Hawk continued to set new records in each of 2005 and 2006. Another
potential cause of the decline in the American Kestrel is pesticides. Ther
e is,
however, no evidence of a current widespread contamination threat to the
American Kestrel from pesticides, although some harm to it may result from
efforts to control insects, a principal part of its summer diet. Factors
relating to the density of small rodents, another principal source of prey,
also must be considered, although it is unlikely that this factor would result
in a long-term, as compared to cyclical, decline in numbers. Finally,
populations of the American Kestrel may be affected by a scarcity of nest
sites. The American Kestrel is a secondary cavity nester which nests in holes
in trees, rock cavities and crevices in cliffs, as well as artificial nest
boxes and even small spaces in buildings. The number of suitable breeding
cavities limits the breeding density of the American Kestrel, and the
availability of secondary nest sites thus must be considered in any evaluation
of a decline in the population of this species.
It is hoped that some information about the reason(s) for the decline of the
American Kestrel in the East will result from a study of the American Kestrel
recently started by the Delmarva Ornithological Society. This study will
attempt to estimate the current population of the American Kestrel in Delaware,
identify reasons for its declining numbers there and develop strategies, such
as increased placement of appropriate nest boxes, to stabilize or increase the
population.
If, as suspected, the decline of the American Kestrel in the Northeast is in
fact primarily due to increased predation by Coopers Hawks, it is likely that
one little-known plumage feature of the American Kestrel is not serving its
suspected purpose very well. Both sexes of the American Kestrel are of course
easily identified by the thick, vertical black streaks behind and in front of
their white cheeks, or mustache marks, but also have a much less noticeable
black spot behind each back mustache mark, toward the rear side of the rufous
nape. These black spots resemble a pair of eyes (ocelli, or false eyes) when
the bird is viewed either from the rear or from the front when the head of the
bird is bent over working on prey. (For a good depiction of this plumage
feature of the American Kestrel, see Plate 35 on page 87 of Hawks of North
America Peterson Field Guides (Second ed. 2001).) Similar neck spots are
present on the Northern Pygmy Owl and the Ferruginous Pygmy Owl, an
d ther
e are hints of such spots on the neck of the Elf Owl. These neck spots are
similar to the prominent white spots on the back of the ears of tigers, which
can similarly resemble a pair of eyes.
It has long been theorized that neck spots might function as false eyes which
deceive potential predators and prey as to which way the animal is facing. The
American Kestrels habit of bobbing its head may help make its neck spots look
even more realistic, except perhaps to a voracious Coopers Hawk which has
otherwise identified it by its prominent mustache marks or otherwise!
.
Gerry Hawkins
Arlington, VA
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