Hi Tweets,

This may be one of the greatest stupid questions ever voiced in
Tweetersville's town square. Why do crows roost communally?

Late yesterday afternoon and evening, I was watching a crow messing around
in a tall, fairly dense cedar. As evening began, more and more crows went by
high overhead to their roost site, wherever it may be. Well, crows often fly
many kilometers to their roost sites and the flocks can be enormous, usually
several hundreds and often over a thousand. The pre-roost staging flock at
Still Creek along the Vancouver BC/Burnaby boundary can number well over ten
thousand birds before they head over to the roost on Burnaby Mountain.

I was idly thinking about this while watching the crow in the cedar,
thinking that if that crow wanted to save itself a long flight and still
stay safe from horned owls, it could simply stay where it was and burrow
deeper into the cedar, and then the question came up: why do they bother at
all with these long pre-roosting flights? Why don't they simply dive into a
nearby dense tree, of which there is certainly no shortage in either local
shoreline-abutting forest or city plantings?

So why do they undertake often long flights twice a day--often heading out
from the roost site in heavy pre-dawn dusk and returning to it in equally
deep evening twilight--sometimes going extensive distances across water,
such as crossing Burrard Inlet from Point Grey to a roost site in the
forests on the side of Mount Hollyburn or Black Mountain in West Vancouver,
a distance of about 10 km (6 mi), most of over open water. Why? There's a
*ton* of good cover in the mature coniferous forest of Point Grey. Why do
they seem to head up to mountainside roost-sites? There's just as many
horned owls there as the forest on Point Grey and it's pretty much the same
type of forest. Over water, they're vulnerable to falcons and eagles, in a
flock they're equally so to falcons and goshawks (there was a gyrfalcon who
wintered mostly on a high-rise apartment near the mega-busy intersection of
Kingsway and Willingdon near the Still Creek staging area, whacking a
couple-three crows a day; what a picture: the falcon snoozing on the traffic
light or telephone poles right *at* the 10-lanes-total intersection while
armies of trucks and cars roared by).

Is it useful to note that both the Black Mt./Hollyburn and Burnaby Mountain
sites are in similar mature second-growth doug-fir and redcedar forests and
both are south-facing? That, though some cross the great water (age-sorting
at roosts?) most of the crows on Point Grey fly east in large (500+) flocks
at sunset, presumably to Still Creek and on to Burnaby Mountain, a distance
of some 20 km (12 mi) from there?

So, what would be the advantages of this system of mass-roost, radiation
from that site to forage areas? If it didn't confer some advantage, a smart
animal like a crow wouldn't do it. I'm missing something obvious here. Help!
Any ideas?

Michael Price We aren't flying...we're falling with style!
Vancouver BC Canada -Buzz Lightyear, Toy Story
mprice at mindlink.net