How is an egg created inside a bird's body?

[punkmuppet]

How is an egg created inside a bird's body? And how does it get it's shape?

[New Forester]

Female birds have only one active ovary, the left.  It's tucked up next to the kidney, between the guts and the ribs.  Between this and the vent is the oviduct, a thing like a floppy sausage skin.

 

The ovary looks like a small bunch of grapes, except that the grapes are yolks, and the yolks are all different sizes.  If the hen is laying there will be one full-sized yolk for tomorrow, a smaller one for the day after and so on, down to hundreds of little ones the size of lentils.  I think that (as in mammals) all the hens eggs for her lifetime are present from youth.  It takes about ten days for the yolk to grow to full size.

Each day one yolk drops off the ovary, loose into the body cavity.  The end of the oviduct is funnel-shaped, and it sucks up the yolk, which then sets off down the oviduct.  If the hen has mated it is fertilised, then it's coated in turn with white, membrane, shell and base colour if any.  The shape is from the shape of the oviduct when full of egg.

 

Then in many species (including some domestic fowl breeds) spots are applied from many little nozzles -- moving the egg if streaks are their thing.  On specked hen or turkey eggs you can often see the same spot pattern repeated with slight movements between.  Finally a thin protective layer of  protein goes on top.

 

Then the egg passes down the oviduct ready for laying.  Up to now it's been pointed end first, but just before laying it's usually turned round sideways and is laid round end first.

 

Most species have eggs which are, how can I put this, egg shaped.  However some (such as owls) have almost round ones, while others (such as waders and guillemots) have quite pointed ones.  Colour also varies a great deal too.

[clarabel82]

what an excellent answer New Forester! I was wondering this myself.

[punkmuppet]

Yeah great answer, thanks! I'm wondering now how you know birds' insides so well hehe.

[Lindy Loo]

Just to fill you in...New Forester is very involved in ecology, conservation, farming, books, web sites.....in short he is brilliant and as you use AB you'll recognise his posts before you read his name at the bottom!!