Today I was watching a buzzard soar above the Morvan National Park in Burgundy. Tonight, I'm listening to one owl--maybe more--hooting gently in the valley below me that the raptor was dominating earlier, as the scene is very gently lit by a waxing, but still small, moon. They seem mysterious, unknowable - hard to track their lives and imagine what they would be like, but I've got a bit more of an idea after reading Tim Birkhead's Bird Sense: What It's Like to Be a Bird.
I know that the raptor has a preponderance of cone cells in its retina ("like low-speed colour film - high-definition and performing best in bright light"), and the owl a majority of rod cells (which "can be thought of as working like old-fashioned high-speed black and white film--capable of detecting low levels of light"). While humans have only one fovea--a spot in the back of the eye where images are sharpest, some birds, including raptors, shrikes, hummingbirds, kingfishers and swallows, have two. So that buzzard's visual acuity, ability to see fine detail, is roughly twice my own, while I probably didn't need the science to tell me the owl can see a lot better in the dark than I. (I do sometimes go walking in the forest at night without a torch, but it's a case of walking by feel rather than sight.)
That might sound quite technical, but really Bird Sense is a highly readable book, that puts sometimes quite complicated science into terms entirely accessible to any interested lay person. The task of a mallard duck seeking food at the bottom of a muddy pond is likened to a human being given a morning bowl of muesli and milk to which has been added a handful of fine gravel:
"To understand how this is possible, first catch a duck. Then turn it over and open its beak so that you can examine its palate. The most striking feature is a series of grooves radiating around the curved tip, but you need to look beyond these at the outer edge of the bill. What you should be able to see now is a series of tiny holes or pores--some 30 of them. If you look on the lower jaw, you will find even more--about 180. Examining these pores with a magnifying glass, you will see that from each one protrudes the top of a cone-shaped structure called a 'papilla', inside which is a cluster of around 20 to 30 microscopic sensory nerve endings--these are the touch receptors--that connect to the brain."
Migration is of course one of the great mysteries of bird life, and what stands out from Birkhead's very clear explanation of the current state of knowledge is just how sketchy and uncertain it is. He begins the chapter with an account of his own work with guillemots on Skomer Island, off the Pembrokeshire coast, getting from new geolocating technology finally, in 2009, after decades of working with them, a detailed understanding of where they go when not nesting on the island. (In short south at the end of July for a few weeks in the Bay of Biscay, before flying 1,500km north to spend most of the winter off northwest Scotland.)