Interesting question Richard. I agree with your examples (eggsamples!!) of specific types of bird's eggs being in the singular - chicken egg, duck egg etc. but my natural inclination when not being specific is to say 'bird's egg'- can't give you a solid reason why - just 60 odd years of English-speaking instinct!

I'm a U.S. native. This is another one of those amazing "I-know-but-I-can't-say-why" questions. To me "bird egg" sounds incorrect and "bird's egg" sounds correct, _despite_ the fact that "chicken egg" sounds correct. I think the reason is that to me, "chicken" is acceptable as a mass noun; we're having chicken for dinner. What kind of meat is that? it's chicken. They have chicken breasts on sale at the supermarket today. What do frogs' legs taste like? They taste like chicken. However, we do not use "bird" that way. Can you say "we're having bird for dinner?" Can you say "rattlesnake meat tastes like bird?" I don't think so. You cannot use "bird" as a synonym for "poultry." "He raises poultry?" Fine. "He raises bird?" No. All that said, the apostrophe-s possessive is disappearing rapidly. I live in a state where my driver's license says "driver's license" on it, but I think in most states, they say "driver license." And, oh dear... forgive the vulgarity... although "bird egg" sounds wrong to me, "bird [poop]" sounds correct, and the two words can even be joined to form a compound word. I guess I don't know.

I would write "bird egg", but agree that "a bird's egg" also works. The Chinese make a delicious soup out of bird's nest (which is the birds' saliva that glues the twigs together), and not bird nest, but that is a different thing.

Arabic is the hardest I think Any way it was very useful confersation .I didn't everything but still usefull

It is interesting to note that while Wikipedia lists the eggs of a quail as "quail eggs", the BBC writes it as "quail's eggs". See these links. https://en.m.wikipedia.org/wiki/Quail_eggs http://www.bbc.co.uk/food/quails_egg