Thanks Jack for a really educational post. From your description I could
almost visualize the whole process happening.

>1) Detected birds fly below 4 km (Which birds? Likely mostly warblers and
>thrushes as you mentioned). 2) The migrations are nocturnal phenomena. 3)
>The migrations happen across a wide front (the WPDN network is national in
>scope); apparently they can even be nationwide! This has likely been
>suggested before by banding station data. 4) Flight speed of migrants can
>reach 15 m/s or about 30 miles an hour (I have seen other references stating
>the 12-15 m/s speed).

Neat info. Based on the 15 m/s given, a bird with 10 hours of nocturnal
flying should be able to travel 540km in a night! That means that it would
take less than four days for a bird to get from Vancouver to where I live
now (nr San Francisco) given adequate fat stores and weather. Wow!
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. Does anyone know how much of a difference
this will make? How much shorter is the night of May 15 as opposed to Sept 7
around the latitude of Vancouver, Washington (All American city)? This
differential will increase at high latitudes, so does that mean that a
Blackpoll or Grey-cheeked Thrush heading to Fairbanks, Alaska makes shorter
and shorter nightly flights as they get closer to its breeding grounds? I
imagine that they just get forced to fly in the day, who knows. This brings
up another question, why are there so few diurnally migrating passerines? Is
it just predation pressure, or something else?



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



Alvaro Jaramillo
Half Moon Bay, CA

alvaro at quake.net