Sunday, 11 January 2015

Cowbirds and Cars; Gulls and Golf Balls

I had a quick look at a paper* in Proc Roy Soc which shows that cowbirds fly away from moving vehicles at a certain distance rather than responding to the speed of approach. At sufficiently high speeds that strategy is fatal, ending in a splat. The birds do not have time to get away and become instant roadkill. Anecdotal observations on the local beaches and in the fields suggest that gulls and pigeons respond similarly. Dogwalkers upset local birdwatchers as their hounds put to flight the large flocks of gulls and waders that gather near the mouth of the Doon. It seems to me that no matter what the speed of the dog is, the birds take flight at a set distance, leaving dogs to the frustration of having missed again. In short, the ‘flight distance’ is just that and the speed of approach does not come into the reckoning. Do they respond differently to aerial predators?

View image |      Brown-headed Cowbirds and Ford Pickup Trucks were used in the experiments to see how the birds responded to the approach of vehicles at different speeds

A hazard which does not seem to be recognised as such by gulls is the golf ball. I have seen at least three gulls hit by golf balls in circumstances where they seemed to have every opportunity to take avoiding action. The first time was over 30 years ago. My friend and former colleague, an ornithologist of note, hit a less than perfect but long shot straight down the middle of the fairway towards a flock of Black-headed Gulls. The ball flying about a foot above the ground hit one the birds in the nape of the neck. The bird turned head over heels several times and ended up with its feet in the air. We though it must be dead but as we approached it turned over, shook itself and took off to join the rest of the flock. Then again, a couple of years ago, my playing partner, hit a juvenile Lesser Black-backed Gull with a low-flying ball. This one was hit side on and bowled over onto its back. This time we approached to within a few yards until, once again, the bird opened it eyes, got to its feet and flew away. Finally, last year, another partner with a very long but, it has to be said, sometimes inaccurate drive pulled his ball into an area of rough well beyond the fairway. Those of us watching saw the ball hit a gull side on. As we hurried to see what had happened to the bird and to see if we could find the ball, we were unsighted from the site of the collision. Thinking the bird would still be on the ground injured if not dead, we searched the whole area. There was no sign of a bird, alive or dead; it must have flown away.

Two of these birds ended up on their backs and appeared dead for a short time. Whether this was ‘tonic immobility’ or ‘playing dead’ or whether the birds were stunned by the impact I do not know but that state is extremely easy to induce in a number of birds. Just turning domestic geese and ducks over while keeping their heads in the shade is often sufficient to induce total quiescence for long periods and stories of bird hypnotists go back hundreds of years.

So not only do the birds not ‘see’ an approaching golf ball, they seem to absorb the impact surprisingly well and live on to face the foxes which, judging by the bones and feathers, kill and eat a good number each year.

*DeVault TL, Blackwell BF, Seamans TW, Lima SL, Fern├índez-Juricic E. 2015 Speed kills: ineffective avian escape responses to oncoming vehicles. Proc. R. Soc. B 282: 20142188.