Alvaro Jaramillo wrote:

" It occured to me that since migrants are present in the breeding grounds
for less than half a year and never symmetrically around the solstice, they
will have a longer night while going in one direction than the other.
Roughly it appears to me that more passerines migrate closer to the summer
solstice (approx June 21, minimum length of night) in the spring than in the
fall. I am basing this on warbler migration peaking about mid May in spring
and early September in the fall, of course this varies with latitude and
with species. If we assume that nocturnal migrants do not keep flying in the
day, then this means that they will be flying shorter distances in the
spring per night than in the fall."

This is interesting, as, from what I've read, spring migration in a given
species typically progresses faster than fall. This is the dogma, I think
even for adults (obviously fall migration is longer when you add the young
of the year), but I don't know how it applies to an individual bird.
Waxing hormones pull faster than waning ones push? Quite interesting
question you raised, though.

Al also wrote:

"This brings up another question, why are there so few diurnally migrating
passerines? Is it just predation pressure, or something else?"

Winds are lower at night, and this could make a big difference when they
were unfavorable. Night air is well known to be more stable. Also, their
chip calls (for contact?) may carry better at night; I'm not sure if I read
that or made it up :-) There is a lot of info on migration, and I haven't
reviewed it lately.

And, of course, there *are* quite a few diurnally migrating passerines.
Those that migrate in flocks (waxwings, blackbirds, starlings, pipits,
Turdus thrushes) and those that feed by day (swallows) come to mind. Note
these are protected from predators to some degree, either by their agile
flight or flocking behavior. Note also that lots of nocturnal migrants do
migrate by day when they cross water bodies too wide for a single night's
flight. Eleonora's Falcons couldn't make a living if there weren't lots of
passerines migrating across the Mediterranean and North Africa by day.
It's interesting we don't have an equivalent bird in this hemisphere.

And:

"There once was a bird from Seattle.
who could fly 15m/s at full throttle.
.....
I will stop before someone letter bombs me!"

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Dennis Paulson, Director phone: (206) 756-3798
Slater Museum of Natural History fax: (206) 756-3352
University of Puget Sound e-mail: dpaulson at ups.edu
Tacoma, WA 98416