 Breeding takes place during the spring and summer . Following copulation , the female lays eggs on a daily basis over a period of several days . If an egg is lost during this time , she will lay another to replace it . There are normally four or five eggs that are <unk> in shape and pale blue or occasionally white , and they commonly have a glossy appearance . The colour of the eggs seems to have evolved through the relatively good visibility of blue at low light levels . The egg size is 26 @.@ 5 – 34 @.@ 5 mm ( 1 @.@ 04 – 1 @.@ 36 in ) in length and 20 @.@ 0 – 22 @.@ 5 mm ( 0 @.@ 79 – 0 @.@ 89 in ) in maximum diameter . 
