The secret whistle of the redwing

Richard Collins investigates the ‘see-ip’ call of this visiting songbird.

WHEN gremlins got into the works last week, this column declared that “there are about 20 species of the common crossbill”. This should have read “20 races”, not “20 species”. Apologies.

Crossbills are accomplished migrants but, this week, we turn our attention to a different bird. On calm nights you may hear a quiet ‘see-ip’ sound coming from the sky. This unobtrusive call, so characteristic of late Autumn, is produced by the redwing, a visitor from Iceland and Scandinavia. Slightly smaller than a song thrush, this winter bird has distinctive red flanks and a white stripe over each eye.

Sleeping perched on a branch uses up fat so migrating at night is energy efficient. It leaves the daylight hours free for feeding and there’s a security pay-off; night migrants aren’t ambushed by peregrines or merlins. Redwings, therefore, travel at the night, navigating by the stars.

The mysterious ‘see-ip’, they say, is a ‘contact call’ used by flying birds keeping in touch with each other. But is it? The evidence, it seems to me, is far from convincing. Birds do use contact sounds; the whooper swan’s trumpeting honk and the mewing wing-beats of their mute cousins are examples. The fieldfare, another winter migrant thrush, has a characteristic ‘che-chuck’ contact call. You would expect songbirds to be noisy in flight but many of them fly in silence. Apart from the occasional ‘see-ip’, the redwing is one of them.

A bird losing touch with its peers while in flight would surely call repeatedly. Also, a contact call requires an answer from the birds to whom it is addressed; otherwise what’s the point of it? The ‘see-ip’ call, however, is given very intermittently and not repeated. Nor do other birds reply to it. A second ‘see-ip’ may be heard a minute or two after the first call but this must come from a different bird; the first caller will have travelled on by then.

The characteristics of the call, too, are interesting. Its pitch and tone are those of the warning note which songbirds produce when they spot a predator. Issuing warnings is a dangerous business; a calling bird draws attention to itself and risks becoming a victim. Why birds warn their peers, when they themselves have nothing to gain from doing so, is an evolutionary mystery; discretion being the better part of valour, it’s wiser to keep one’s trap shut when enemies are around.

The source of a low-pitched sound is easy to locate; there is a detectable ‘phase difference’ between the sound as heard in each ear. High-pitched noises don’t travel very far; lightning strikes can only be heard up close, whereas low-pitched thunder carries for miles. The drop in strength of high frequency noises is so marked that the ‘head-shadow’ between the hearer’s ears is enough to indicate the source.

However, when the pitch is neither too high nor too low, the caller is very difficult to locate. If the wavelength is about the same as the separation of the hearer’s ears, both phase difference and head shadow effects are minimised. To a medium-sized hawk, with ears a few centimetres apart, sounds at a frequency of about 7kHz are very difficult to pin down, so this is the pitch which songbirds use for their alarm notes.

Other birds can hear the call clearly but predators can’t locate the caller. Warning calls have another refinement; they begin and end very quietly which makes detection even more difficult. If a redwing wants to alert other birds to its position, the ‘see-ip’ call, almost identical to the 7kHz alarm note, would be a most ineffective choice. So what is going on?

The birds, it seems, want of give minimal information when they call at night. It’s as if they feared that a predator might detect their presence. Eagle owls can take a bird in full flight but they prefer to hunt for perched ones. The ‘see-ip’ call has the character of a general broadcast, similar to a military drumbeat or a referee’s whistle, but pitched so as not to give away the flock’s location.

Just why the referee needs to whistle at all, however, remains a mystery.