On Thu, 14 Nov 1996, Brian Bell wrote:

> On Mon. 11 Nov Christopher Hill in responding regarding whether
> Short-eared Owls are diurnal said "definitely not". That they will hunt
> during low-light overcast days. He is probably right in general.
> However, I have seen them on several occasions exhibiting typical
> Short-eared hunting behavior during the middle of the day over the
> Arcata Marsh in California. The conditions varied from bright overcast
> to clear and sunny. Guess it just goes to show.
>

Herb Curl then wrote:

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.

[end quote]

I'm glad this came up again. When I originally said that Short-ears and
Snowies were "definitely not diurnal" I meant that they don't restrict
their hunting to the daylight hours. Brian Bell is right, Short-ears can
certainly be found abroad at all hours of the day. Nonetheless they hunt
primarily in the dark, as do Snowies.

I think we have to be careful of our own observational biases. Where owls
are concerned, especially, it's easy to fall into mistakes if we assume
that our daytime observations are representative. Some common widely
published mistakes along these lines are:

1. Snowy Owls are diurnal, not nocturnal.

Although Snowies are visible and conspicuous during the day, they
spend most of the daylight hours sleeping. They are more active at night,
as is shown by at least one study of transmittered birds.

2. The preferred habitat of Saw-whet Owls is dense conifer groves.

Saw-whets often choose dense conifers for daytime roosts, but they
avoid that habitat entirely when active. Saw-whets hunt in open wet
fields and marshes, and the presence of open habitat is a better predictor
of Saw-whet presence than the presence of dense conifers. Saw-whets avoid
dense conifers for a good reason - they are almost impossible to fly
through in the dark without crashing every few feet.

3. Hollow trees are what makes an area attractive to Screech Owls.

This one has a grain of truth in that if hollow trees are
completely absent, Screech Owls can't nest. However, in predicting what
areas Eastern Screech Owls use, presence of hollow trees is not nearly so
important a criterion as are presence of edge and nearness to water.

4. Short-ears hunt mostly in the day. Or, Short-ears are crepuscular,
hunting mostly during twilight.

All the observations I have made suggest that this is simply not
true. Where I have lived in Short-eared habitat for months at a time (in
Massachusetts and in Puerto Rico), and during casual observations of
short-ears elsewhere, I have consistently seen Short-ears emerging at
dark, and heard them calling in the middle of the night, had them follow
me curiously around in the dark. In all those situations they were MUCH
more rarely seen abroad in daylight. I timed the first emergence of
wintering short-ears from a Massachusetts roost on five evenings, and all
five times they emerged between 35 and 50 minutes after sunset. I'm not
saying that Short-ears don't hunt a lot during the day, because of course
they do. But they are much more active at night. I would bet that
spending a night on the Samish Flats with a night vision scope would
change a lot of peoples ideas about Short-ears and Snowies being diurnal.

[dismount from hobby horse]

Chris Hill
Everett, WA
cehill at u.washington.edu