[chris (OP)]

Sarah wants to know:

How do birds reproduce? Specifically, how do the sperms from the male bird get through the egg shell to fertilise the egg?

Can you help?

[SquarishTriangle]

Hi Sarah,

Birds actually undergo internal fertilisation, like mammals, even though they lay eggs externally. The egg is fertilised well before it has a shell, or a 'white' or even the majority of the 'yolk'. The male's (sometimes males') sperm cells travel from the female's cloaca (the single main opening for urine, faeces and reproduction) towards the top of her oviduct, where mature eggs are deposited from the ovary (ovulation). There in the upper oviduct, sperm can either fertilise an ovum immediately or be stored in special tubes for later. Sterm storage occurs so that the female doesn't need to mate multiple times to fertilise a whole clutch (since ova are not all released at the same time), and also because it allows some flexibility in the exact timing of the mating event (she doesn't have to find the right mate at exactly the right time to match for her ovulation).

Each fertilised eggs then continues to develop and travel down the oviduct and uterus, gaining its yolk, white (albumen) and finally shell over about 20 hours, before finally being laid as a shelled egg. Apart from fertilisation itself, the egg's development is not all that dissimilar to making an unfertilised egg, which is why the unfertilised chicken eggs you eat are almost exactly the same as the fertilised ones at the farm.

[chris (OP)]

I speculated that this was going to be the case, but it's nice to have the authority answer. Thanks!

[chris (OP)]

Thank you for explaining this - all very clear now.  Happy Easter! Sarah