As I
cleaned up that backyard just after dawn on the first day of winter, I heard an
Anna’s hummingbird scolding me in the tree in the yard to the north of me,
while a male Costa’s hummingbird whistled at me from the bottle-brush bush in
the yard to the south of me. Both were anxious for me to finish so they could
drink sweet water in peace. After the cold night they needed the calories to
refuel their bodies.
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| A female Costa's that chose to winter in Las Vegas.She is sitting on the Texas Sage in my front yard. |
Not too far south of Las Vegas NV
winter weather is much warmer. Even just 100 miles or so makes a difference of
10 degrees in daytime highs and nighttime lows. In Yuma, 305 miles south of Las
Vegas, the December highs are in the high sixties, and lows are in the high
forties. So why would any hummingbird choose to stay all winter here in Las
Vegas with our normal lows in the mid-thirties and normal highs in the mid-fifties,
while extreme lows drop well into the teens?
When I
looked on the Cornell University ornithological site I found that even the
experts don’t really know what makes birds decide to go or stay. Since science
moves forward by a combination of observation and inference, I will happily go
out onto a limb and expound on my observations of bird migration and the
inferences I have formed from those observations. Since I spend several days going
over the definitions of those two terms with my science students I will refresh
your memory on the subject as well.
Observation
is what we can detect with our five senses. If you use the same ‘equipment’ as
another person you will observe the same thing. Inference, on the other hand,
is an explanation of what we observe. Inference is always open to argument.
Scientific law is based on observation; it merely tells you what will happen
without any attempt to tell you why. All science theory is based on inference,
an attempt to explain how and why things happen in nature. Whenever we venture
into the realm of inference we are putting ourselves in danger of being wrong,
but that is the price we pay for satisfying our very human desire to know why
things happen. It is no accident that a toddler’s favorite word is ‘Why?’
I lived
in Ketchum, Idaho one summer. I spent a great deal of my time walking in the
area of the Wood River. During August I watched the young Robins flocking in
preparation to leave on migration. The adults were still busy with new
fledglings so the older juveniles were on their own to find their way south.
They soon left the area while the parents still had adequate food to feed their
last charges before they too left for the winter. My inference then was that
what drove the migration was the shortening days. Certainly food didn’t enter
into the picture.
I lived
in southern Idaho for several decades. There I watched flocks coming and going
every spring and fall. Yet we had many year round residents. We would have
forty of more robins on every Christmas count. They were living down by the
Snake River feeding in the Russian olive trees. Yet these were not the same robins
I had watched throughout the valley during the summer months. These Christmas
robins were migrants from further north. There was plenty of food to satisfy
their winter needs, so why didn’t our summer resident birds stay around and
avoid the rigors of migration?
When
you look at the range maps for robins you see that there is an area farther
north where they summer and an area farther south where they winter. Between
them is a large area where they are year round residents. My inference is that
as the northern birds move south the resident summer birds also move further
south to leave room for the new comers. So there is a general shift south and
then in the spring a general shift north. In this case the migration for those
in the middle is based not on length of day or food supply, but on population
pressure. One area simply doesn’t have enough food supply to feed that many
robins.
When I
lived in southern Idaho I had feeders out for the hummingbirds. Our summer resident
hummingbird was the Black-chinned. In the mountains surrounding us, the
Calliope hummingbird was also a summer resident. Hummingbirds do not flock to
migrate, it’s a solitary activity for them, so I couldn’t watch them prepare to
leave they just suddenly were gone.
For the
Black-chinned hummingbird this happened in late July. There was still plenty of
food for them, many flowers were at their peak of blossom and small flying insects
abounded. But the shortening days announced it was time to go and so they did.
The Calliopes would come down from the hills and be on their way as well. In
August the Rufus hummingbirds, which nest as far north as Alaska, would come
through on their way south to Mexico. I would see the last Rufus on September
15. Unless we had had an unusually early killing frost, there were still plenty
of flowers and insects for them to feed on. This was necessary because it takes
a lot of energy to migrate. A half-starved bird will die enroute. They found plenty to eat, but they knew the
days were getting short, cold weather was on its way and they had better be
getting to their wintering grounds.
So how
does all this apply to our Costa’s and Anna’s? Chances are very good that our
wintering Costa’s are not the same individuals that we had in the summer. If
you look at the range map, they nest up into southwestern Utah and the Spring
Mountains but winter here in Las Vegas. Our summer birds left and then our
winter birds came. That would account for the days that I neither heard nor saw
a Costa’s at my feeder, while I’ve both seen and heard them every day since.
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| A juvenile male on the first day of winter. This is probably the same individual who had his early morning picture taken December 17. |
Our Anna’s, on the other hand, are
probably the same individuals we had in the summer. If you check out their
range map, the only place where they don’t live all year round is in the Sierra
Nevada Mountains. Those individuals probably move into central California for
the winter.
Enjoy our wintering hummingbirds
and keep feeding. It’s such a joy to see those energetic birds flitting back
and forth here in Las Vegas NV.
