Egg Strategies

From: crpntr@ix.netcom.com (Kenneth Carpenter )

>I do not know of any bird that leaves
>its eggs exposed for more than a few minutes.

Sorry, not true. Birds (e.g. ostriches) control incubation by not brooding until all the eggs are laid (>23 days). That is to insure that eggs hatch within a relatively short time (hours) of each other. E.g., see Bertram, B., 1992. The ostrich communal nesting system. Princeton U. Press

From: ornstn@inforamp.net (Ronald Orenstein)

Actually this is true for a number of precocial bird species; it is a quite necessary adaptation for birds whose young leave the nest on hatching.

From: mickey@lepomis.psych.upenn.edu (Mickey P. Rowe)

I presume Ken meant only that some birds do leave their eggs exposed for more than a few minutes (since I think we can all agree that Greg knows what he knows better than Ken knows what Greg knows ;-) Sorry, Ken, I'd have let that go, but Ron Orenstein (ornstn@inforamp.net) gave me the opportunity to jump in when he came back with:

>Actually this is true for a number of precocial bird species;

I don't think Ken is arguing that it's never true -- he's only arguing that it sometimes isn't. Out of curiosity, Ron, how long does a bird have to be away from its nest in order for a parasitic layer (e.g. a cuckoo or a cowbird) to sneak in and lay one of its own? And how long does a bird have to be away from its nest for an African egg eating snake to get in and devour the eggs? I suspect that Greg's "a few minutes" is too restrictive since the above-mentioned events occur frequently enough for the animals in question to rely upon them. Several minutes might be more accurate, don't you think? Even using your argument:

>it is a quite necessary adaptation for birds whose young leave the
>nest on hatching.

I think the birds could be away from the nest for several minutes. Precocial birds don't just poof out of their eggs like Athena from Zeus' head... Anyway, if I'm reading you right, I suspect that most such birds have some level of awareness as to when their eggs are going to hatch and can modify their nest sitting behavior accordingly (i.e. they don't have to monitor the eggs as closely just after the eggs have been laid since at that time they're in no danger of the eggs hatching unwatched).

From: Robert.J.Meyerson@uwrf.edu (Rob Meyerson)

AFAIK, it could actually be more beneficial for a ground nesting bird to be away from the nest occasionally, because then the nest would not be as likely to gain the scent of the mother, thus leading a predator to an easy meal. However, the parents rarely have the nest out of sight for too long a period.

Few birds ever leave the eggs completely unguarded for any length of time. The only reason that the egg-eating snake can get the time to devour a whole egg is because the snake is hard to spot in the grass, and because the parents are away hunting/feeding. In fact, there are a few ecological studies that suggest an inverse relationship between the amount of parental observation and the amount of nest destruction. Those species that have a higher degree of nest destruction tend to be able to lay a larger number of clutches per breeding season to compensate.

How all this relates to dinos is anyone's guess.

From: John Obis <jbois@umd5.umd.edu>

Because it once did relate, now it doesn't.

Nearly all surviving egg layers are extremely surreptitious about the location of their eggs. There are other studies that suggest parental investment has a high cost in egg-predation because more trips to the nest mean higher chance of discovery. Also, birds choose nest sites on the basis of the quality of camouflage - there is competition for sites that have more leaves to cover them.

Dinosaurs were extravagantly above any threshold of discovery. Also, isn't it true that large herbivorous dinosaurs at least, needing to feed constantly, would have to be constantly away from the nest. This would be especially true in times of environmental stress, i.e., the dinosaur would have to range further for a meal leaving its nest untended. How much better to be a mammal and take your babies along for the meal.

From: ornstn@inforamp.net (Ronald Orenstein)

>Out of curiosity, Ron, how long does a bird
>have to be away from its nest in order for a parasitic layer (e.g. a
>cuckoo or a cowbird) to sneak in and lay one of its own? And how long
>does a bird have to be away from its nest for an African egg eating
>snake to get in and devour the eggs?

Good questions, and while I'm away from home I can't check the answers. However, I do know that brood parasites will often mount watch over suitable nests waiting for an opportunity to slip in and deposit an egg, so a few minutes may well do the trick. As for egg-eating snakes, many snakes will attack a nest with the parents still there and simply ignore their attempts to drive it off. Certainly egg-swallowing is not a rapid process and the snake cannot carry the egg away first, to my knowledge.

>I think the birds could be away from the nest for several minutes.
>Precocial birds don't just poof out of their eggs like Athena from
>Zeus' head...

The point I was making is that such birds actually avoid brooding so that the eggs will hatch simultaneously - this does not mean that they ignore the nest altogether or do not guard it against predators. Some birds can disguise the nest quite well (e.g. by pulling water weeds over it as grebes do), or rely on superb camouflage (many shorebird nests are extremely hard to spot because the eggs match the background so well). If this fails the birds may rely on "broken-wing" acts to lure predators away.

>Anyway, if I'm reading you right, I suspect that most
>such birds have some level of awareness as to when their eggs are
>going to hatch and can modify their nest sitting behavior accordingly
>(i.e. they don't have to monitor the eggs as closely just after the
>eggs have been laid since at that time they're in no danger of the
>eggs hatching unwatched).

Possibly, but it could simply be that the brooding stimulus does not kick in until there are no more fully formed eggs left in the oviduct.

From: crpntr@ix.netcom.com (Kenneth Carpenter )

The following on egg laying and incubation is from Bertram, B. 19992, The Ostrich communal nesting system. Princeton University Press. 196p.

Ostriches lay at 2-3 day intervals, as does the Rhea (no data for other ratites). Incubation does not begin until after ~16 eggs are laid by the ostrich (i.e., 32-38 days), but sometimes later if potential predation is low. As a result, the eggs undergo considerable daily temperature fluctuation (20C-40C) until incubation begins (stable temperature of 36C). Egg hatching is synchronized by controlling the temperature of individual eggs, i.e., slowed by pushing them away from the parent (heat source) and sped up by keeping them closer to the parent. Thus, eggs laid 32+ days apart will still hatch within two days of one another.