Why do hummingbirds hum? Sometimes they don't. Sometimes they scream!
The first time I spent a whole day in Anza-Borrego Desert State Park in California, I was surprised and delighted to see so many hummingbirds. On that one day, I saw more hummingbirds than I had previously seen in my entire life.
I had arrived at the perfect time. There had been good rains recently, and the desert was just beginning to bloom. But the bloom was not so well established that the hummingbirds were scattered all over the desert. Only the earliest bloomers had started producing nectar, and the hummingbirds were concentrated where the flowers were.
And since it was early May, it was the beginning of breeding season for the Anna's hummingbirds (Calypte anna), which are rather common in Anza-Borrego Desert State Park.
I was thoroughly out of my element in the western desert, but I had done as much reading as my schedule would allow. I did wonder whether I'd be able to distinguish between the two common hummingbirds in the area. My little pamphlet on desert birds said the Anna's hummingbird (Calypte anna) was much larger than the Costa's hummingbird (C. costae), and their colors were somewhat different. But a hummingbird is such a tiny thing and moves so fast, I was afraid I would not be able to see the markings, or to determine that some were less tiny than others.
But the pamphlet did make some mention, without going into details, of the peculiar and conspicuous territorial displays of male Anna's hummingbirds in breeding season.
I had hiked out to Yaqui Well a couple of hours before sunrise, hoping I might see the desert bighorn sheep (Ovis canadensis nelsoni) coming down from Pinyon Ridge for an early-morning drink. I didn't see any sheep, but I had a fascinating time watching the last bats of the night catching the last moths of the night, observing the calls and activities of other crepuscular critters, and watching the creatures of the day begin stirring.
Not too long after sunrise, I began to see a few hummingbirds. And a few more. And more! I lost count somewhere above fifteen, which was more hummingbirds than I could remember seeing before in my whole life back East.
I wasn't too sure about the difference between a reddish throat and a purplish throat, but I could clearly see a difference in size. Even among the females, very similarly colored in both species, the Anna's were much larger than the Costa's hummingbirds.
On the walk back to the Tamarisk Grove Campground, I would occasionally hear a strange sound. It was an odd combination of squeaking and buzzing that rose rapidly in pitch and amplitude for a second or so, then rapidly fell off and stopped. The change in pitch sounded like Doppler shifting of a sound source that was moving rapidly past me, but I never saw any birds near me. Surely, nothing was running along the ground that fast.
On the rocky ridge about three quarters of a mile east of Yaqui Well, I stopped to see if I could see what was making this weird sound.
In the desert below me, I soon saw a male Anna's hummingbird flying in a climbing spiral. His spiral was maybe sixty feet in diameter, and he climbed about twenty feet on each loop. Five times around, he was nearly a hundred feet above the ground. He hovered there, maybe two seconds or so, then dove at a steep angle toward the desert floor. As he descended, he emitted that surprisingly loud squeaking buzz. At the bottom of his dive, when he was below the level of the tops of the ocotillo and "buzzing" the tops of the teddy bear cholla, he pulled up rather sharply and stopped screaming.
From this distant observation, I could tell that the change in pitch and amplitude were not entirely due to the bird getting closer to me as he dove. He actually was screaming louder and at a higher pitch as he approached the ground, then decreasing the pitch and amplitude as he finished.
I watched a few cycles of this behavior, then moved on. It was getting hot.
As I descended off the ridge, I got to observe this remarkable display from underneath again, this time from a different male hummingbird. Hot or not, I stayed there for a couple more cycles. Climbing spiral, brief hover, screaming descent to just above my eye level.
There was something oddly familiar about this phenomenon. A bird whizzing through the "treetops" accompanied by a scream that rose slowly then fell off rapidly.
Eventually, it came to me. A few years before, my children had greatly enjoyed the movie, "Willow," and watched it on tape often enough for parts of it to get stuck in my mind At one point, a Brownie riding an eagle rescues the baby princess from the agents of evil. As he flies through the forest with the baby hanging from the eagle's talons, he celebrates his victory by shouting, "I ... stole ... the ... babeeeee!"
That's kind of what the territorial display of the Anna's hummingbird sounds like.