The Family Life of Birds.

FEATURES More than 90 percent of birds appear to be monogamous, in that males and females form pair-bonds and raise chicks together. Blue tit chicks sired by extra-pair males survive better than their half siblings, and extra-pair bluethroat (Luscinia sve-cica) chicks have stronger immune systems. An equal number of male and female chicks hatch out of eggs, but, for reasons that are not entirely clear, more male offspring make into adulthood, resulting in an excess of males, all of whom find it relatively hard to find a new mate. Black-legged kittiwakes (Rissa tridactyla) that have to find a new partner have higher stress levels and raise fewer chicks than those that stick with a long-term mate, even if some of their chicks are the products of infidelity. [Extracted from the article]