For the past 22 years, audio recordings of male songs in the Forest have been systematically studied, following up Margaret Rebbeck’s 1999 paper that suggested male Nightjar are individually distinct in the pulse rates and phrase lengths of their churring.
The components of Nightjar song are described here and analysis of a sample looks at three measures
Major Pulse rate
Minor Pulse rate
Minor phrase length
Rebbeck et al found that the major pulse rate was the strongest measure – the predominant rapid pulsing part of their churring sound. This varies between birds, in a range of around 24 – 29 pulses per second. As one might expect, most birds’ pulse rates lie in the central part of this distribution and only occasionally are birds found at either extreme.
At a very conservative estimate, I have analysed around 250 examples of Nightjar churring. It’s the ideal way to spend the winter evenings, counting 20-pulse blocks of pulses, and recording results in a spreadsheet. In these recordings, I’ve only come across five birds with pulse rates greater than 28/second and only one that measured at fractionally over 29/second.
So, noteworthy that amongst the recordings from 2024, was a bird at site M with a pulse rate of 28.5/sec and another at site DD with a pulse rate on one evening of 28.8/sec. But these were not the only curiosities in the data; at the other end of the scale, a bird with a pulse rate of 23.9/sec, the first sub-24/sec pulse rate I have come across.
You might think that such a difference would be detectable in the field, but I have to confess that I am not able particularly to distinguish between major pulse rates where there are two birds singing. It might be possible if the pulse rates were widely separated;
In this constructed example, are two such birds, the first one with a pulse of 24.6/sec and the second one of 28.5. I think that the faster pulse of the second example is obvious to the ear.
But presumably, Nightjar themselves do perceive these differences, leading to the questions as to whether an extremely high pulse rate confers any advantage, or is an honest signal of male quality.
One of the other song components, the minor phrase, produced during inhalation, can have value in distinguishing between two males in the field. When the churring ‘dips down’ in the minor phrase, this may sometimes be noticeably brief…
or long…
Once a distinctive minor phrase is noticed, the contrast with a second bird may be sufficient to allow them to be distinguished as they move between song posts around a site.
At site L in 2019, 2 males were present at each visit throughout the season. One had a distinctive longer minor phrase than the other and as it turned out from song analysis, was known to us, as he had been previously ringed and radio tagged. We were keen to re-trap him, to obtain between season comparisons recordings of a known individual. He resisted our attempts, always eluding capture, though the other male was caught and radio tagged. As with our other radio tagged birds, attempts were made to obtain a series of recordings of a known bird.
During catching attempts, we were often able to determine that the bird showing interest in the lure and flying near the net was not our target, as he was singing his characteristic song from favoured song posts. He also appeared to be the dominant bird, in that he sang more frequently and for longer than the tagged bird, which largely frustrated efforts to record the bird we had caught and radio tagged.
So returning to 2024, it was season of outliers, revealed not during fieldwork, but in the long winter nights while we await their return.