VA Birders,
American Goldfinches in your back yards and elsewhere in
Virginia this spring are still in small flocks and not nesting pairs like most
other local passerines. This is because, as most of you undoubtedly know, the
American Goldfinch is one of the latest breeders of all temperate zone
passerines, typically not starting to breed until late June or early July. This
results in a very short nesting season in which it is difficult to produce more
than one brood, although experienced breeders frequently produce two broods if
eggs are laid early and the first brood is successful. In order to produce
second broods, approximately 15% of females, usually those with previous
breeding experience, switch mates after producing a first brood.
The cause of the American Goldfinchs late nesting is not well understood and
has puzzled many scientists. Some have suggested that it is due to dietary
considerations. The American Goldfinch eats seeds almost exclusively and
consumes little insect matter, even when feeding young. (This, by the way, is
likely why the Brown-headed Cowbird fails to survive in nests of the American
Goldfinch. Although cowbirds hatch successfully in goldfinch nests, their
growth is retarded and almost all die before they can leave the nest.)
According to this theory, the American Goldfinch nests late because thats when
its main food - the seeds from grasses, weeds and other plants, particularly
thistle seed - is plentiful and available for the feeding of young. This
explanation for late nesting appears to be supported by the close relationship
between the flowering of thistle and the start of nest building by American
Goldfinches.
Another explanation for the late nesting of the American Goldfinch is that it
is attributable to the physiological effects of the American Goldfinchs
pattern of molting its feathers. The American Goldfinch changes from winter
plumage to breeding plumage by a complete molt of its body feathers in the
spring. According to the author of an article on the molt of the American
Goldfinch cited below, the American Goldfinch is the only member of the
Carduelis genus (which in North America also includes the Common and Hoary
Redpolls, Pine Siskin and Lesser and Lawrences Goldfinches) to acquire its
breeding plumage by a second complete molt in the spring, and all other
Cardueline species have just one complete molt each year in the fall and
acquire breeding plumage through the gradual wearing down of feathers through
use. According to the same author, the time required for the American Goldfinch
to molt is longer than for other migratory finches, which may be a necessary
adaptation
which
permits the American Goldfinch to meet the energy demands of molt on a
protein-poor seed diet. Among other things, prolongation of the body molt at a
suppressed rate also distributes the energy demands of molt over a protracted
period and thus avoids a restricted period of high energy demand which may not
be able to be satisfied with available food resources.
Perhaps due to its ability to complete a body molt before nesting, the American
Goldfinch is the most strikingly sexually dimorphic and the only seasonally
dimorphic of the Cardueline finches. Sexual and seasonal dimorphism enhance
reproductive success (through the more protective cryptic coloration of the
female) but in the case of the American Goldfinch at a cost, as the energy
demands of a complete molt may preclude the simultaneous development of the
gonads and result in a late timing of the constituent phases of the gonadal
cycle. Thus, while the greater abundance of seeds in late summer compared to
March-early June may be important in ensuring larger supplies of food for
young, completion of molt rather than food availability may determine the
timing of the American Goldfinchs breeding. See Middleton, The Molt of the
American Goldfinch, The Condor, Vol. 79, No. 4 (Winter 1977) at pages 440-444,
which is available for free on the Internet.
Perhaps because the American Goldfinch is the only Cardueline finch which has
extensive seasonally dimorphic plumage, it has been described as unique among
Cardueline finches in acquiring its breeding plumage through a complete spring
molt. See the above-cited article and the entry for the American Goldfinch in
Beadle and Rising, Tanagers, Cardinals and Finches of the United States and
Canada The Photographic Guide (2007) at page 166. The American Goldfinch is
not the only Cardueline finch which has a complete or nearly complete molt in
the spring, however, as both the less-studied Lawrences Goldfinch and the
interior subspecies of the Lesser Goldfinch have such molts. In the case of the
Lesser Goldfinch there are substantial differences between subspecies in the
extent and timing of molting and sequence of plumages. The interior subspecies
of the Lesser Goldfinch, psaltria, has two molts a year - a prebasic molt in
late autumn and a prealternate molt in spring - both i
nvolvi
ng all of the body and all or part of the wings and tail in both sexes;
westernmost members of hesperophila have a complete prebasic molt in autumn in
both sexes, and a partial prealternate body molt in spring primarily in
females. These differences in timing and extent of molt correlate with the
different breeding times of these populations, which varies geographically. The
Lesser Goldfinch breeds primarily from April through July along the West Coast
and from June through September in the interior of the West. The differences
are correlated with the western winter to early spring precipitation maximum
and eastern summer precipitation maximum, respectively. See the entries for the
Lawrences and Lesser Goldfinches on the Cornell Labs The Birds of North
America Online and in Beadle and Rising, Tanagers, Cardinals and Finches of the
United States and Canada The Photographic Guide (2007).
The late breeding of the subspecies of the Lesser Goldfinch which undergoes a
complete spring molt is consistent with the late breeding of the American
Goldfinch and suggests a relationship between the extent and timing of molt and
the availability of food sources and commencement of breeding in these species.
The presence of numerous American Goldfinches in breeding plumage in Virginia
each spring which appear to be capable of breeding earlier than they do also
suggests that the energy demands of a spring molt is not the sole cause of the
late nesting of the American Goldfinch.
Gerry Hawkins
Arlington, VA
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