Birders,

Several of you, in both public and private postings, picked up on
my comment that "if most of the Rufous Hummingbirds arrived here
[Vancouver] in late March, they'd probably starve" and asked, if this
is true for Rufous Hummingbird, why Anna's Hummingbirds seem to
survive the winter quite well.

First of all, my statement was predicated on the assumption that
Rufous Hummingbirds are relying on natural food sources. There aren't
enough hummingbird feeders in the Northwest to provide food for more
than a tiny fraction of the Rufous Hummer population that migrates
through every spring, and most of the native foods aren't available
until after March. On the other hand, the small resident population of
Anna's in the Northwest is PRIMARILY dependent for winter survival on
hummingbird feeders and on introduced shrub species that flower in the
winter, when few if any native shrubs are flowering. Sure, they eat
insects when these are available, and they need the protein from
insects for many bodily functions. However, the carbohydrates (sugar)
supplied in feeders are critical in maintaining body temperature--
probably the most immediate need on cold winter days. Out of dozens of
Anna's Hummingbirds I have seen in B.C., only one was more than a few
hundred yards away from an active feeder.

Anna's and Rufous Hummingbirds are very different animals in terms
of their ecology and migratory patterns. Anna's Hummingbirds are
primarily non-migratory throughout their range. If they couldn't
survive the winter here, they wouldn't be here at any time of year.
In fact, that was pretty much the case until the early 1970s, when
they expanded their range north from California and became a regular,
but uncommon, year-round resident in Northwest urban areas. This range
expansion followed a veritable population explosion of Anna's in
California, and a vast increase in the abundance of winter-flowering
shrubs, both in California and the Northwest. The range expansion,
and its causes, were well documented by Dale Zimmerman in an article
in "American Birds" in 1973.

Rufous Hummers, on the other hand, are strongly migratory, and
winter almost exclusively south of the U.S. They might well be able to
survive the winter if they lingered around feeders beyond September,
but they almost never try to. Migratory versus non-migratory behavior
is the result of a long evolutionary history, and is not likely to
change in response to recent changes in winter food availability.

Another feature which applies to both Anna's and Rufous Hummers is
their ability to go into torpor or semi-torpor overnight-- that is, to
lower their body temperature, and hence their metabolic rate and rate
of heat loss. Hummingbirds and nightjars, and I believe some swifts,
are the only birds known to have this ability to lower their body
temperature, which is akin to hibernation in mammals. I'm not sure if
it has been conclusively shown in the field, but I suspect that both
Anna's and Rufous Hummingbirds drop their body temperature
significantly every night when the temperature is near freezing (as it
often is in late February or March when the first Rufous arrive). If
not, there is a good chance that these tiny birds would burn so much
energy trying to stay warm that they would expire before dawn.

Incidentally, Anna's may have an advantage over Rufous
Hummingbirds in surviving cold weather because of their larger body
size. Even though Anna's are not much longer than Rufous, they are
significantly heavier. Heat loss in birds is proportional to the
volume (or body weight) divided by the surface area. Thus, the
smallest birds have the toughest time maintaining body heat. If you
DID have both Anna's and Rufous at your feeder in a January freeze,
the Rufous would probably croak first.

Hummingbirds are among the most intriguing of birds to birders and
ornithologists alike. I hope, for some of you, that these comments
have helped to increase both your knowledge and your sense of
fascination with these amazing creatures.

Wayne C. Weber
114-525 Dalgleish Drive
Kamloops, B.C. V2C 6E4
Phone: (604) 377-8865
wayne_weber at bc.sympatico.ca