A) In birds, both males and females can provide a great deal of parental investment for the young, because both can incubate the egg(s) and both can bring food to the chick(s).
B) Birds are not as highly evolved as primates, so they have simple undifferentiated mating systems mostly involving male songs and calls.
C) With their larger brains, mammals can form a wider range of relationships than can birds and therefore can form more productive mating systems.
D) Because they can fly, birds do not experience limitations on food availability and therefore undergo mainly sexual selection and almost no natural selection.

A) the period of most intensive competition among females for access to potential male mates
B) the sum of male and female parental investment
C) the period of time during which a female is receptive to mating
D) the length of time between births, or the "interbirth interval"

A) resource competition is high for females.
B) the cost of acquiring additional mates is low for males.
C) the cost of acquiring additional mates is high for males.
D) the fitness of offspring raised by two parents is much higher than the fitness of offspring raised by one parent.