Does a robin lay an egg a day? If so, won't one baby bird be larger than the next?

When birds mate, more than one sperm is transferred from male to female. So, the female has enough sperm to fertilize a number of eggs. It is true that eggs are laid over a period of several days, but many birds do not start brooding the eggs until most of the eggs have been laid, so the nestiling(s) that hatches from last egg(s) laid may be smaller than the others (because they begin feeding later) and they may have a lesser chance of survival when there is a food shortage. It may be one way to make sure every nestling has enough food by reducing the brood size when food abundance is less than optimal. If food is abundant then all nestlingsm even the smallest ones, may survive.

They don't start incubating until the entire clutch is laid (some birds start incubation with the penultimate egg) so the eggs all usually hatch within a 24 hour period of each other..so the first hatched gets a little bit of a head start, but not by much.

All birds copulate cloaca to cloaca.

Each semen deposit contains tens of thousands of sperm cells-only one fertilizes one egg.

Yes,each egg laid is older(thus bigger-duh)than the next.

Do *your* research before blathering to the ignorant,thanks.