Abtsract Thursday: "Mystery"

Youoregon1 (many thanks once again Robert) has set yet another fab challenge for this week's Abstract Thursday - "Mystery" (or puzzle, enigma, conundrum, riddle, secret, problem, secrecy, obscurity, uncertainty, mystique! thriller, whodunit)!

So the first mystery in this blip is: "What is it?"
(It's half of a cracked, empty bird's egg I found in the garden, photographed then severely abused in Photoshop)

The second mystery is: "What kind of bird laid it?"
(I don't have much idea myself and I don't suppose anyone can tell from this pic. Judging by the bit I have, the whole egg was probably about 2-3cm long and plain white - ?anyone know? -?maybe a House Martin?)

The third mystery is: "Which came first, the adult bird or the egg...?"!!
(I don't know - do you??!)

(PS I'm very grateful to my Chief Editor [aka Mrs C] for the inspiration for this blip - a common occurrence.)