Hummingbirds that winter in Las Vegas NV – why they choose to stay

                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.

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.

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.