hello tweets,

every year, i notice this phenomenon of passerines who sing in the
autumn months, and every year i promise myself that i will find the
answer to why they do this. and, of course, every year, i poke around
in the literature and eventually get distracted by my dissertation
work and forget about this question.

this year, i asked my advisor this question and here is a paraphrase
of what he said.

european starlings actually experience an increase in plasma levels
of testosterone. this is associated with singing and territorial
behavior, and sure enough, they are singing and claiming next year's
nest site! winter weather suppresses the european starlings' breeding
in the seattle area, however, they do actually breed during the winter
months down south where it is warmer. this is also true for english
sparrows/house sparrows.

white-crowned sparrows are photorefractory at this time of the year.
this means that they are incapable of responding to increased daylengths
with a corresponding increase in gonadal steroid hormones. singing in
white-crowned sparrows appears to be independent of testosterone,
although high testosterone levels that are typical of spring and
early summer do increase singing behavior. additionally, testosterone
has long been known to "crystallize" bird song, and it also seems to
contribute to singing of the full song, rather than small scratchy
pieces of the song.

white-crowned sparrows use song in other interactions beyond breeding.
the females also sing quite nicely, and the youngsters sing as well.
song serves to preserve social structure in this species, which spends
the winters in large flocks that roam around in search of food.
song is important in establishing and maintaining the dominance-
subordinant structure of white-crowned sparrow flocks.

song sparrows, a close relative of white-crowned sparrows, have a
very different life history than white-crowned sparrows and their
physiology reflects that well. song sparrows do not migrate at all,
but instead, they stablish and defend winter territories which may
or may not be the same as their summer breeding territories. song
sparrows are kind of interesting because their winter territories are
maintained and defended by either two or three birds, and these
groups can be any combination of males and females, although it is
my observation that there is always at least one male in the group.
the males and females who form these winter-time partnerships may or
may not breed with each other the following year (or the previous
year). the exact relationships between these individuals has never
been investigated, although we do currently possess the technology to
learn more about the genetic relationships of these birds.

as i alluded to in my previous paragraph, song sparrows are
territorial in the winter, and they use song to establish and
maintain their territories. there is a paper in press right now,
which is part the dissertation of one of my lab-mates (and my advisor)
that partially elucidates the hormonal mileu that underlies these
behaviors. i have not yet seen this paper, but i will post the
reference when it is finally published for those who might be
interested.

you should know that song sparrows are photorefractory at this
time of year, as are their cousins, the whote-crowned sparrows.
this means that their gonadal steroid hormones are at "barely
detectable" levels at this time.

in my colleagues' upcoming paper, it was found that there is
another hormone present in significant levels at this time of
the year. this hormone, dihydroepiandrostenedion (DHEA) is
probably produced by the adrenal glands and probably also by
the testes (in the case of male birds) and released into the
bloodstream. the levels of DHEA are higher at this time of the
year than in the spring, when the gonadal steroids -- estrogens,
androgens and progesterones -- are highest.

normally, DHEA is an inert steroid hormone in birds, except at
certain times of the year when it can be converted into estrogen.
at this time of the year, the song sparrow's brain contains the
necessary enzymes needed to make this conversion, which then causes
the levels of estrogen to rise in very specific regions of the brain.
those regions are associtated with song. providing extra DHEA or
estrogen to song sparrows caused them to respond by singing. blocking
the action of DHEA caused the birds to stop singing.

there other important physiological events that are dependent upon
this DHEA-to-estrogen conversion, so song is not the only result of
this, but song is the most obvious result for those who go into the
field and look for birds.

there are no autumnal peaks in testosterone or estrogen in the plasma
of these song sparrows. if these peaks occurred, the bird would then
undergo gonal growth and other physiological changes that are consistent
with breeding. (these changes are the same as those seen in starlings
and english sparrows). however, we discovered that DHEA is relatively
abundant at this time of the year, and its product, estrogen,
apparently functions in feedback mechanisms in the brain and also
contributes to autumnal singing in song sparrows.

it is also possible that the same phenomenon that i described
for song sparrows might also be in effect in american robins, who
also undergo an autumnal singing phase, accompanied by increased
aggressive behaviors.

to summarize, there are at least three different mechanisms that
seem to trigger autumnal singing. first, there is an actual beginining
of the breeding season in autumn which is suppressed by the inhospitable
weather seen in northern latitudes. this is seen in european starlings
and english sparrows. second, there is the as-yet mechanistically
not understood singing that is used to maintain social status in
migratory and seasonally flocking species. this is typical of the
white-crowned sparrows, where females and juveniles also sing. third,
but certainly NOT last, there is the DHEA-dependent autumnal singing
that has been partially investigated in song sparrows and which may
also be shared with american robins.

i hope this has helped a little bit.

regards,

Deborah Wisti-Peterson, PhD Candidate nyneve at u.washington.edu
Department of Zoology, University of Washington, Seattle, Wash, USA
Visit me on the web: http://students.washington.edu/~nyneve/
Life is better when you are reading a good book -- Author Tim Green