All
birds replace worn and damaged feathers through the annual moulting
process. For chimney swifts in Manitoba, the summer moult overlaps with
breeding activities. Feather replacement is an energy consuming affair
for all birds, but it further taxes breeding swifts.
For general information, Steve Howell (Peterson
Reference Guide to Molt in North American Birds; 2010) has an
interesting chapter on moulting in various species of the swift family
(Apodidae). For detailed technical information, Peter Pyle wrote
the bible on plumage and moult characteristics which enables
ageing/sexing of birds (Identification Guide to North American Birds.
Part 1. Slate Creek Press, Bolinas, California; 1997). Chimney swifts
are a challenge to study - they are fast fliers, both sexes are similar
in appearance, and the birds roost/nest in remote locations where we
cannot see them. However, we can get glimpses of the birds as
they enter/exit chimney sites and dash about feeding.
As
obligate aerial insectivores - chimney swifts MUST feed on insect prey
collected in the air. It is essential for swifts to remain capable of
flying, so they have a gradual, protracted moult. Some birds such as
ducks, swans, and grebes, replace their feathers synchronously over a
short period e.g., two week period, which renders them flightless.
As
chimney swifts have one moult per year they have one definitive adult
plumage which is called the "definitive basic". Birds which have
two moults per year e.g., Rose-breasted Grosbeaks, have two definitive
plumages per year - the definitive basic (adult winter) and
definitive alternate (adult breeding) (http://ofo.ca/site/page/view/
For
monitors in Manitoba, moult characteristics become an important way to
distinguish fledglings from adults. Most fledging takes place toward the
end of July and into the first week of August, and it is useful to
identify juveniles to help estimate nesting success. The flight of newly
fledged chimney swifts is characterized by: flying at low altitudes,
rapid wing beats for slow air speed, large banking turns, and,
occasionally, difficult entries to chimneys (yes, they do miss the
opening and tumble down the outside face!). However, within several days
post-fledging, young swifts are proficient fliers and they are not easy
to distinguish from adults based on flight competency alone. So how do
you distinguish juveniles vs. adults? The best way is to look for
evidence of moulting on the trailing wing margins. Juveniles will have
intact margins while adults will have discontinuities and gaps due to
worn and missing feathers.
To
illustrate how the trailing wing margin changes over time, Dave Lavigne
has once again generously provided very informative photos of adult
chimney swifts (see Dave's new site: http://www.flickr.com/photos/
Many thanks Dave for sharing your images with us all! The trio of photos, taken in Goderich, ON, clearly shows the change in appearance of the trailing wing margins from May through mid-July. For each image, the typical stage of nesting for Manitoba breeding adults, is indicated after the date on which Dave took the photo: If you start watching the swift's trailing wing margins, you will be ready to identify and greet newly fledged juveniles in a couple of weeks. Enjoy the challenge! |
15 July 2015
JULY UPDATE: FIELD IDENTIFICATION OF ADULT CHIMNEY SWIFTS IN THE SUMMER RANGE
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