The dinosaurs are still on top of the food chain, in a way. I am sitting on my terrace, drinking my morning coffee and observing the swifts speeding along above me. They are descendants of theropod dinosaurs, so in a sense, they are dinosaurs. The mosquitos suck our blood and the swifts eat the mosquitos. So they are above us in the food chain, in some sense. And they still rule while the mammals are taking a little niche. Yesterday evening, I observed a small bat flying around in the garden, hunting mosquitoes as well. While the swifts are ruling the sky all day, in large numbers, that single bat comes out only for a short while at dusk, hiding away the rest of the day under the rim of the roof. That old pattern of pre-Chicxulub times, when the dinosaurs ruled and the mammals where confined to little niches, still survives here.
On hot days when barometric pressure is high, the swifts fly high up, obviously because the mosquitos do. The mosquitos cannot go up there to feed, so probably they do this to mate. If they would scatter all over three dimensions, their chance of meeting each other would become small, so probably they just stay at a height with a certain air pressure to be in one plane or relatively narrow slice or the air. That would increase their chances of finding each other. However, it also increases the chances of them being found by hunters like swifts and swallows. So there should be an evolutionary pressure to squeeze them together into one layer and another to let them spread Maybe several sub-populations could be separated into different layers. If one small group collects at a different level, they would have a higher chance not to be found by the swifts. They could then become genetically isolated from the rest of the population, eventually leading to another species.
Of course, these are just hypotheses. I don’t know if there is any research about these things.
A bunch of swifts dashes through the garden, with their characteristic calls.
What is the function of these sounds? To keep the bunch together, or to avoid collisions? Or both? What is the advantage for them to fly together? High up, where they hunt, they disperse over large areas. Maybe the presence of other swifts in a place is an indication of the presence of mosquitos, so if a swift is not inside a mosquito-rich part of space, it will fly towards other swifts, if it can see some; something that could be thought of as an attractive force whose strength depends on the density of the mosquito field. The sounds might then be warnings to avoid collisions, a repulsive force transmitted by the sounds of the calls. One would have to check if the birds only shout when they are closely together or approaching each other.
In any case, they are fascinating animals, the masters of the air. Up there, the dinosaurs are still ruling.
(The picture is from
The sound file is from http://commons.wikimedia.org/wiki/File:Mauersegler.ogg