|Ruppell's Robin-chat: an impressive mimic. Lake Duluti|
There are two parts to the question, of course: why do birds sing? And why do they sing in the morning more than at other times of day? The first I think we've had a pretty good idea about for a long time, and there are two main reasons: to attract mates, and to claim their territory. In general the two actions aren't mutually exclusive - as a male bird you can sing to both let the other males around know that you're still in your territory, and you can at the same time let females know that you're around and looking for a mate. For some species by listening to the song you can tell what the bird is actually trying to say - Nightingales are well known in Europe for their beautiful song, but it's not uncommon to hear them singing down here during the non-breeding season too (they seem to be more territorial than many migrant species). The difference is that here, I've almost never hear the typical long whistle notes that are so frequent in the breeding areas during the early breeding season (listen to them here). They also use even more of these whistles during the middle of the night before they find a mate, and it seems that these whistles are particularly for attracting females, which obviously isn't relevant when not breeding. As females might be flying over at night, singing in the middle of the night and whistling a lot, might be a very good way to attract a female to your territory.
|Yellow-vented Bulbul - nesting all year round in my garden (March 2012), |
both sexes sing far too early in the morning for comfort...
As I said before, there's also plenty of evidence that it's not only about attracting a female: even after finding a female and her sitting on the nest many species keep singing for a while - this is clearly about territory defence. And the first example of how effective this is comes from a nice experiment where birds were removed from their territories and the time it took other birds to come and occupy the patches was recorded: it's pretty quick, but if you play a tape recording of the song of the resident male even though he's been removed, it takes a lot longer. Clearly birds looking for a territory recognise individual songs and as long as the territory holder is still around and still singing they won't bother trying to take over - an efficient system of defence if ever there was one.
|Usambiro Barbet, a typical African species with duets. Serengeti, Dec 2011.|
So. Having tried to answer why birds sing, let's tackle why we think they sing most in the morning. There are a number of (not mutually exclusive) theories here too. They all start with the assumption that singing is good (for the reasons we've covered above) - but it's costly in terms of time and energy (another reason why females might like males that sing more: they're showing honestly how strong they area). So the challenge birds face is in deciding when and how much to sing. One of the oldest ideas is that they sing in the morning because it's still too dark to be out and about finding food, so you might as well sing. That's a pretty solid idea - but it doesn't really explain why they sing more in the morning than in the evening, when the light also fades - or even in the middle of the night. Another theory is that the conditions early in the morning - often cool and with lower humidity than later in the day - might be particularly good for letting the sounds of the song carry further, though recent experiments suggest that actually the middle of the day might be the best time to sing if acoustic conditions are what's important so I don't think that's a winning idea. The third and most interesting theory is that birds sing most in the morning because that's when, most days, they've got spare energy to use up [Rob, surely there's a short snappy label for this theory? The McNamara/Mace/Houston hypothesis seems a bit of a mouthful? The stochastic environment hypothesis?].
|Many species, like this Cisticola are best identified by song. |
More drinks for the first correct ID! Nr Arusha, March 2012.
Slater, P., & Mann, N. (2004). Why do the females of many bird species sing in the tropics? Journal of Avian Biology, 35 (4), 289-294 DOI: 10.1111/j.0908-8857.2004.03392.x
Thomas, R., & Cuthill, I. (2002). Body mass regulation and the daily singing routines of European robins Animal Behaviour, 63 (2), 285-295 DOI: 10.1006/anbe.2001.1926