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Bird - Wikipedia, the free encyclopedia |
 | | Most passerines, in contrast, lay coloured eggs, even if, like the tits they are hole-nesters. |
 | | The brown or red protoporphyrin markings on passerine eggs reduce brittleness and are a substitute for calcium when that element is in short supply. |
 | | The colour of individual eggs is genetically influenced, and appears to be inherited through the mother only, suggesting that the gene responsible for pigmentation is on the sex determining W chromosome (female birds are WZ, males ZZ). |
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