>Ah, semantics . . . . "Diurnal" means during the day. "Nocturnal" means
>during the night. "Diel" means during a 24 hour day. Thus, Short-eared
>owls are largely (only?) diurnal. Barn Owls and Burrowing Owls are diel;
>they hunt both day and night; Barn Owls, if it's overcast. Great-horned
>Owls etc. are nocturnal. There's another nice word, "crepuscular" which
>refers to dim light as in dawn and twilight. And so it goes.
>
>---Herb Curl

Even if we agree on the semantics, there's plenty of room to discuss the
facts, I think!

I'd call Barn Owls quite nocturnal. They come out typically when it's just
about too late to see them. They're supremely adapted for nocturnal
hunting and, I would say, are even more nocturnal than Great Horned, which
quite frequently come out early enough to grab the last (entirely diurnal)
squirrels before they go to bed. Food-habit studies would easily define
the hunting times for owl species, as indicated by the proportions of
diurnal and nocturnal prey. Unfortunately, voles are common prey for many
owls, and they can be active at any time.

Burrowing Owls are largely nocturnal in the tropics but probably
increasingly diurnal at high latitudes. Nevertheless, they are also
primarily nocturnal in Washington. The reason you see them out during the
day so much is that the male is out of the burrow and conspicuously
perched, more or less guarding his territory at that time.

Short-eared Owls really match up with "crepuscular" best, as that seems to
be the peak hunting time for them, although they definitely hunt at any
time of day as well. Snowy Owls probably fall in this category during the
winter. Most of their hunting is while at rest, so you don't see them
flying around looking for prey like Short-ears do.

Northern Pygmy-Owls are our only fully diurnal common owl, at least from
studies of their prey. They can be active (calling) at night, too, but I
don't know if they feed then. Hawk Owls are quite diurnal, too. Look at
the heads of owls; the more distinct the facial disk, the more sound is
probably used in hunting. Of course, sound can be used during the day too;
Great Gray Owls, with their huge facial disks, often hunt diurnally.

I've often wondered what the highest-latitude "nocturnal" species such as
Boreal Owls do when it's light all the time. I guess they must hunt in the
daytime.

And, about those semantics--I think "diel" means "having a 24-hour cycle,"
not "active both day and night." Any organism that has behaviors (i.e.,
activities) that vary regularly around 24 hours has a diel cycle. It can
be diurnal or nocturnal.

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
web site: http://www.ups.edu/biology/museum/museum.html