On May 25th we visited Site K, where there were reports of 2 males. At around 21:20 one of these birds started churring, some 100 metres away from us. About a minute later, a second male started up, much closer to our position. In the field, we immediately noticed an unusual effect in the second bird’s song, which we both remarked on at the time. One might describe it as a hiccup, which produced a slight yodelling effect. This continued for around 50 seconds, after which it settled into a normal churring pattern.
On examination, the sonogram displayed what might be termed a stuttering effect, where individual or groups of the pulses that make up the churring component of Nightjar song, are not formed properly. Nightjar churring often contains an isolated pulse that doesn’t ‘fire’ properly, but not, in my experience of close examination of hundreds of sonograms, in this way. Looking in detail at the song components, it can be seen that, where an average pulse in the churring occupies a space between 1.04 KHz and 2.8 KHz, the abnormal pulses here are truncated at around 1.5 KHz. Some have ghostly upper portions, extending fractionally higher than typical, to 2.96 KHz. It’s these upper parts that give this bird’s song its peculiar hollow squeaky effect.
This was early on in the season and the two males were counter-singing strongly. After a couple of minutes, the first bird, further away from us, gave a run-down and wing claps, followed by prolonged purring (indicating some interaction, most likely with a female). He started churring again and then both males sang at length. As the two birds sang, we noticed the familiar ‘beating’ effect, as the frequency of their pulse rates, which were close, but not identical, fell in and out of synchrony. This is an often heard effect, when two or more males are churring simultaneously, but is generally a more random and transient effect, with the pulse rates drifting apart again. However these two birds produced a more regular sustained pulsing effect, as if falling into an entrainment. This can be seen in the sonogram, where the darker bands show the pattern.
Zooming in to the detail of this shows the two strands of song coming in and out of synchrony:
The boxed areas show moments where the pulses coincide, between which they drift apart. One of the two birds was near the microphone, the other further away; hence the fainter marks between the stronger pulses of the nearer bird.
When a quick analysis of the major pulse rate was done, their average pulse rates per second were clearly distinguishable. The first bird showed a pulse rate around 28.5/s (at the very high end of the normal range) and the second around 26.8/s, much more towards the general average of the range. So the effect not the simple result of near identical pulse rates, but probably caused by the ‘elasticity’ of the pulse rate, as each male’s pulse rate drifts up and down through the length of the song.