Thursday, 16 June 2011

Noctules at Shortlands

Summer chafer, Amphimallon solstitiale, with wings partly extended.  They were swarming around poplar trees and some were on the ground.  Warren Road playing fields, just after sunset on 14 June 2011.
Summer chafer, Amphimallon solstitiale, with wings partly extended.
Warren Road playing fields, just after sunset on 14 June 2011.
Ishpi Blatchley, a local bat expert, let me know that noctule bats had been seen feeding in recreation grounds in Shortlands. This only happens at this time of year so I went out in the evening to see what was there.

I parked at an entrance to Beckenham Place Park where there were several open fields, and a stream, the river Ravensbourne. I also found a fenced-in playing field, and across from that an unfenced one, where I saw someone entering from the far side; it was Ishpi. Two other enthusiasts arrived not long after that.

The photo on the right is once again not a bat. It is one of the insects on which they were feeding. There were hundreds of these summer chafers swarming around poplar trees along the edge of an area of rough, but mown, grass, which from the remains of white line markings is used for cricket and football and perhaps other sports. Some of the chafers were on the ground, which is where we photographed this one. The flash makes it look a light brown; it was darker to the eye in the late evening light.

Only a few minutes after sunset I heard bat sounds on my detector and saw a noctule flying straight across the field. A few minutes after that, more appeared. Soon there were four of them flying at around treetop height, moving around the open area but mostly keeping near to the trees. Their sound on the detector was different from any of the other four bat species I have seen (common and soprano pipistrelles, Daubenton's, and serotines). There were loud, low clopping sounds and at the same time a series of higher chirps, all at about 20 khz. Most of the time it wasn't easy to distinguish the sounds made by individual bats. Sometimes one would swoop low, and occasionally one would fly just overhead, making it easy to see against the sky. They seemed to move faster and flap more often than the serotines I have seen recently.

Every now and then I heard one of them make a feeding buzz, a rapid series of clicks that echolocating bats use as they close in on their prey. Twice, I saw something small fall away from a bat after it made a buzz. Were the chafers getting away, or were the bats just biting off the juicy parts and discarding the rest? They were too far away to tell.

A chafer must be quite a substantial catch for a bat. It would probably be too big for one of the little pipistrelles — I saw one of those too — but the bigger noctules had no trouble. After about half an hour there was only one noctule left, so perhaps the others had had their fill, or they might have gone on to feed somewhere else.

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