Harriers are large rather slender raptors (the body about 50cm long with a wingspan of over…
Bar-throated Apalis (and bird lungs)
Apalis thoracica (apalis = soft [Greek], thoracica = of the chest [Latin]
Call (duet by male and female) https://www.youtube.com/watch?v=cmFWxXq9Ygs
This is the only Apalis in the region with a whitish eye: you will have to take my word for it (as I take the word of my birdbook). There is in fact quite a wide variety of subspecies of this Bar-throated Apalis with varying plumage, ranging in colour on the breast from grey to yellowish green. The black throat-bar is pretty stable, however, though it tends to be lighter or narrower in females. Otherwise, males and females are plumaged alike.
It is useful to know that birds like this can vary quite a lot in colouring: the pictures in the books can give the impression that this is how this bird looks and all birds of this kind look like this. Sometimes it is the printing of pictures in these books which use colours that are not exactly like the bird itself. But the birds themselves can vary in colour: just like us humans, I guess, individual birds can vary in colour, size and shape. No doubt among themselves they can notice more differences between individuals of their own species than we can. (Otherwise natural selection would not work, I suppose.)
One cannot help being charmed by these game little birds, bouncing, bobbing and flitting busily about among trees. Birds, or most of them, have to work very hard for their living, constantly searching for food for themselves and their young. Small birds like this, with small wings, have to beat them at a furious rate, making a whirring or purring sound, while larger ones can afford to keep their wings still occasionally and save energy by gliding between wing-beats.
We tend to forget, perhaps, how much energy is needed for birds to fly. Indeed, it seems that a single pair of lungs is not enough to supply sufficient oxygen to power flight, and so birds have evolved a unique breathing system. Their lungs are connected to a network of air sacs throughout the body: though these do not absorb oxygen themselves, they act as a store of air and ensure a constant supply of oxygenated air into the lungs. They act, it seems, like little bellows taking it in turns to blow a continuous stream of air into the coals of a smelting furnace; or like compressors pushing air under pressure into a jet engine. Or like the bag of the bag-pipes which enables the piper to manipulate much more air, through many more reeds than her own lungs could manage. That’s enough similes: you get the picture, I imagine (I like the bag-pipes one best). These air sacs also help water birds that spend time under water looking for food. Migrating birds have a specialised form of haemoglobin that enables them to absorb oxygen even at great heights where the oxygen is thin.
– Stephen Buckland –