The female may be motivated to seek extra pair copulations to ensure that all her eggs are fertilized but the male guards her to make it less likely that she can mate with another male because his interest is to ensure that all the chicks in the nest are his- that his efforts towards parental care are not given to some other male's offspring. The male and the female have conflicting interests. I am confused on proximate and ultimate causes of behavior. Proximate cause refers to mechanism or development. So if the cause of behavior is a genetic effect, or due to the action of a hormone, or muscles, or a change in development due to an environmental cue, this is proximate.
Ultimate cause refers to evolution and history.
For us, this usually means the adaptive significance or function of behavior. Explanations involving selection, or costs and benefits, or strategies, or effects of behavior, are ultimate explanations.