THE SECRETS IN A SWIFT’S SCREAM
Swift by Frank Jarvis
Bird species where the male and female have identical plumages are known as monomorphic. If the plumages are identical the birds clearly need some other cues to be able to identify each other, at a minimum to be able to identify the opposite sex, and ideally the individual. We know that birds can detect ultra-violet light, but this doesn’t help if the plumages are identical.
Swifts are interesting birds, apart from the nesting period, or during extreme events, they live their life on the wing - eating, sleeping and migrating thousands of kilometres twice a year. I recorded activities at a Swift colony about 12 years ago. Using a parabola I could pick up individual birds as they entered and departed the roof space of an old farmhouse in which they nested. I also recorded those events called “screaming parties” where a flock of Swifts careen through the air at high speed and close proximity making their screaming calls - you mostly see this at dusk. I recorded all these screams, and then went on my way thinking that was all there was to it.
Recently I came across a book that made reference to males and females having different calls. This surprised me as I had heard very little in the way of variation in their voices. So I dug into it, found the relevant references, and re-visited my old recordings to see if I could pick out what the authors described.
Lo and behold !! There it was clear as day in my old recordings, and I had been in sublime ignorance thinking that Swifts, whilst having a fascinating lifestyle, were really quite boring when it came to voice - not true at all……
