Birds of Oakhaven

Most games flap a bird's wings up and down like a toy: the same speed both ways, the wing held flat. A real pigeon does not, and we can say exactly how, because Eadweard Muybridge photographed one in 1887. His plates 755, 756 and 757 in Animal Locomotion, "Pigeon flying", catch a single bird frame by frame from the side and from the front. They are in the public domain. Here is plate 756.

What the photographs show is not what you would guess. At the bottom of every downstroke the wingtips finish below the bird and ahead of its head: the wing sweeps forward all the way down. At the top the wings are up and behind the shoulders, and on the way back up the wing folds in tight before it opens again.

We also read the Harvard study of pigeon takeoff and landing by Berg and Biewener, which measured how the angle of the beat changes from a takeoff to a landing. Our first reading of it was wrong. Its angles are measured against the ground, not against the bird, so we had the takeoff sweeping the wings backwards. Laid against Muybridge's photographs, the answer was simpler. Relative to its own body, a pigeon beats down and forward when it takes off and when it lands. What changes is the angle of the body, nose up to climb and tilted back to brake.

So the town pigeons now have three animations of their own, built from those references:

• Flight: six wingbeats a second, a long downstroke sweeping forward and a quick folded upstroke, with the body bobbing slightly on each beat.
• Takeoff: a crouch and a jump, nose up, then three hard beats at about ten a second, the wings clapping together over the back on each upstroke.
• Landing: the flare, wings forward and legs reaching out in front, two braking flutters, touchdown, a flick of the tail, and the wings folding away.

Walk at a pigeon in the game now and that is the order you see them in: takeoff, flight, landing, then settling down to rest.

The next eight birds to join the hedgerows are the house sparrow, chaffinch, goldfinch, greenfinch, bullfinch, coal tit, yellowhammer and collared dove. Each one gets its own voice: its song, its call and, where it has one, its alarm. No borrowed chirps, and no one generic "small bird" noise played for all of them.

The voices come from real field recordings, released free on Freesound under CC0 or CC-BY, chosen from Britain wherever we could find them. A field recording is a minute or two of a hedge, not a clean sample. It has other birds, wind, a car going past and long stretches of nothing. So before cutting anything we drew every recording as a spectrogram, a picture of sound with time running left to right and pitch running bottom to top. On a spectrogram a chaffinch and a passing van look nothing alike, even when they overlap.

For each recording a script marked every moment where the bird's own pitch band was at least ten decibels louder than the rest of the file. One minute of house sparrow gave twenty-five candidates. We then looked at each candidate on the picture and picked the cuts where the bird is alone and close: two different chaffinch phrases from one long recording, for example, so the song does not repeat identically, and three different sparrow sounds for its song, its alarm chirp and a whole bush of them chattering.

Most of these birds were recorded beside a road, and traffic sits far below anything a small bird makes. Left in, every chirp would come with a car underneath it. So each cut has the low end filtered off just under the bird, and the dashed blue line on each panel below marks where. That line sits at 2 kHz for the coal tit and yellowhammer and at only 200 Hz for the collared dove, whose coo is the lowest voice of the eight. Every clip is mono, so the game can place it in the hedge it comes from. Last of all we drew the thirteen finished clips again as one sheet, below, to check that every cut lands on the bird and not on the gap beside it.

Read the panels and you can see the field guides' descriptions. The chaffinch's song runs down the scale as a rattle and ends in a flourish. The yellowhammer's "a little bit of bread and no cheese" is a quick run of short notes and then the long "cheese". The greenfinch's wheeze is one long slide downwards, and the collared dove is three low notes repeated, far below everything else on the page.

A bird that sounds right but behaves like every other bird still gives the game away. So for each new species we read its RSPB and Wikipedia pages and pulled out every line about how it moves, feeds, flies, flocks and calls, and how wary of people it is. Each species' behaviour comes from those notes, not from a template.

• How close you can get. The house sparrow has lived alongside people for thousands of years and is the tamest small bird here, so it lets you nearest. The bullfinch is shy and unobtrusive, so it notices you from furthest away and slips into thick cover instead of flying off in the open.
• How it gets away. Goldfinches feed together on thistle and teasel heads and lift off as one, twittering, so they flush as a group. The yellowhammer feeds in the field margin and flies up to the top of the hedge, where it sings. The rest dive into the hedge.
• On the ground. Sparrows hop and never walk. The chaffinch shuffles under the hedge for seed. The coal tit darts in, grabs a seed and darts off to hide it, so it is rarely still. The collared dove walks under bird feeders with the same head-bob as a pigeon.
• Company. Sparrows are sociable all year and gather in bushes to chatter together. A goldfinch flock can reach forty. Bullfinches are seen in pairs, almost never in a flock, and collared doves sit in pairs on aerials, roofs and wires.
• Where, and how many. The yellowhammer is a farmland bird, not a garden one. The greenfinch has been hit hard by a disease of finches, so it is kept rarer in the game than the chaffinch and goldfinch, as it is in the wild.

The birds already in the game were built the same way: the blackbird stops, cocks its head and listens, the blackbird and magpie head for cover in rain and wind, and the song thrush flushes out across the road before it climbs, which is how thrushes really get hit by cars.