I've noticed that most vertebrates have stumbled, independently, on live birth as a way of bringing their offspring into the world. Birds seem to be the exception. Why is this? Is it because of the hard-shelled egg that they lay? Is it because they don't have a uterus equipped to carry offspring for longer duration? You would think that such a development would be benificial to some flightless birds like penguins at least. Also (bonus question) Why aren't a lot of bird species active at night? (Owls being a notible exception) Is it because mammals have that particular niche occupied? Have any bird species developed a type of sonar the way bats and dolphins have?

PS thank you for the input on some of the other questions I've had, this sight rocks!

Well susan, there are a few things to deal with here! I'll start with one I do know.

Yes, some birds have evolved a type of 'sonar' - the famous cave swiftlets (those used for bird's nest soup) in Asia do have a primitive form of sonar (using sound and echoes to generate an impression of their surroundings). My understanding is that it is not actually that god. Certainly compared to a typical microbat (the megabats - fruitbats, don't have sonar) it is not that efficient, but clkearly it is good enough for basic navigation in large caverns.

As for nocturnal birds there are quite a few - there are owls, nightjars, whipporwhills and some others. However, you are right that in the grand scheme of things noctural birds are pretty rare. The obvious conclusion is that they are suffereing from ecological exclusion - essentially the bats occupy the night and the birds the day (there are also very few diurnal bats).

However, the situation is obviously more complex than this. Birds had a pretty good head-start on the bats in terms of evolution, so it seems oodd that either a) birds did not occupy nocturnal niches for a very long time, or b)  they did occupy them, but bats were abble to supplant them very quickly. Neither is hardly impossible and there is certainly an absesence of obviously noctural birds in the fossil record (one with say very large eyes or good ears), but then the fossil record of bats is also very poor. It's very hard to say, but doubtless a few of the other bird people on AAB will add a bit more!

It remains a good and frequently asked question: why haven't birds (and/or crocodilians and/or turtles) evolved viviparity? (viviparity = the ability to retain the egg within the reproductive tract until embryonic development is complete). As Susan notes, viviparity has evolved in just about everything else, in fact in fishes, amphibians and reptiles it has evolved at least 150 times! Several possible answers have been put forward (see Blackburn & Evans 1986): most recently, Andrews & Mathies (2000) argued that the eggshells and oviducts of turtles and birds - unlike those of other reptiles - do not allow much oxygen exchange to occur, and that this might be a constraint that has prevented extended embryonic development in utero, and hence also prevented the evolution of viviparity. Their argument is actually rather more complex than that, but I hope that's an adequate summary. Incidentally, there are no good reasons to think that non-avian dinosaurs (or other fossil archosaurs) were viviparous, so it is likely that this physiological constraint applied to them too.

On the nocturnal birds question... essentially I'm with Dave on this one - there are, in fact, quite a few nocturnal bird groups. Furthermore, it turns out that birds are better at night than we've long assumed, and that members of many groups - including petrels, gulls and waders - do a lot more at night than just sleep and may do as much foraging and feeding at night as they do in the day. Many passerines migrate at night. Having said all that, it may be that, in being predominantly day-active (aka diurnal), birds mostly retain the 'primitive' behavioural habit of their ancestors, itself a consequence of early reptiles being active in sunlight (for metabolic reasons), and in having excellent colour vision (and colour vision works better in the day than at night of course).

Incidentally, the fossil record indicates that birds aren't diurnal to avoid bats, but that bats are nocturnal to avoid birds.. I hope that makes sense!

Refs - -

Andrews, R. M. & Mathies, T. 2000. Natural history of reptilian development: constraints on the evolution of viviparity. BioScience 50, 227-238.

Blackburn, D. G. & Evans, H. E. 1986. Why are there no viviparous birds? The American Naturalist 128, 165-190.

Last edited by Darren Naish (17th Nov 2007 18:46:56)

Although there maybe physiological limitations, I wouldn't expect a bird that can fly to bother with viviparity due to the obvious disadvantage of the weight being carried. That leaves flightless birds. Most flightless birds have lost the ability to fly because they evolved on predator-free islands. Such places tend to be safer for eggs as well as adult birds, so there would be little selective pressure to become viviparous. The remaining birds that have been flightless for a very long time, like ostriches and penguins, have quite complex egg-care behaviours that involve the males. Viviparity requires much greater female contribution, so it seems reasonable to assume that viviparous females would be less likely to breed as often or as successfully as their egg-laying counterparts. If there was a flightless bird on a landmass inhabited by egg predators where there was no male input into maintaining the eggs, I expect there would be selection for viviparity.

As to birds being active at night - Dave and Darren have covered much of this, but I would add that mammals have the advantage of very good hearing, due to having 3 ear-bones and an external ear that provide a very effective mechanism for detecting the intensity and directionality of sound. This is a primitive feature in mammals, providing a preadaptation for being nocturnal. Birds are less good, the owls being an exception to this rule. Owls have various features that enable them to hear very well compared to other birds. Most regularly night-active birds are actually quite closely related to each other (owls, swiftlets, goatsuckers, etc.) and it seems plausible that they share enhanced auditory apperatus, relative to other birds.

On a pedantic note, there is one species of megabat that can echolocate, albeit in a rather crude way - the Egyptian Fruitbat (Rosettus aegyptiacus).