First of all, thank you for the helpful replies to my first question: "Is laying eggs painful?" (The poor wonder the mother leaves right after laying the egg!) I have a few more egg-related questions - not homework questions, I'm just curious, haha.

So we've sort of established that, though it's very hard to tell when a chicken (and, I presume, any other bird) is in pain, because they don't show it the same way we do, most likely normal-sized eggs don't hurt. However, does whatever comes before laying the egg - cramps or whatever it's called - hurt? Or is it just discomfort? Does anyone know?

Second, I know that in many species there is a size difference between the genders - in falcons, for example, the female is quite a bit bigger than the male. So, would an egg containing a female chick be larger than an egg for a male chick? Or are they roughly the same size when they're hatched, and then hit growth spurts? (I know that many birds' eggs hatch a day or two apart from each other, so often they've already begun growing before their siblings are hatched.)

And last, do female birds go through a cycle, in which they are more fertile or less fertile? Does it happen every month, every season, every time they lay an egg, at all? Does it happen for reptiles? And what about platypuses and echidnas?

Wow, that was a lot. Thank you for taking the time to read all this and answer!

I think questions about animal "pain" can be a bit leading. I don't think that what animals experience is really the same thing that humans due: pain is a complex thing that involves self-awareness. I don't mean to minimise the seriousness of animal discomfort, and certainly not to excuse cruelty towards animals, but I do want to warn against over-anthropomorphising.

As far as I know, bird eggs are the same size irrespective of the sex of the bird inside.

I don't know about bird/reptile/monotreme fertility cycles. Hopefully someone else will answer that part.

birds do not have labour in the sense we think of it nor a monthly menstrual cycle. Their ovarian/uterine system is rather different from mammals. see … W7x59KjNcY