Through behavioral observations of a color-banded population of northern mockingbirds, I measured parental defense of offspring against potential predators and feeding of fledglings while renesting. Observations of defense against human intruders acting as potential predators of young showed (1) both males and females defended eggs and nestlings, (2) males defended young more strongly as measured by distance of approach to intruders, proximity of attack dives at intruders, and following intruders to the edge of the territory, (3) male, but not female, defense increased with age of nestlings, (4) levels of defense by males and females were correlated within pairs, and (5) pairs in which males strongly attacked intruders had higher seasonal nesting success than other pairs.Observations on fledgling care showed (1) both parents fed fledglings, but males fed more, (2) there was no brood division, and (3) an offspring received more food from its parents as a fledgling than it did as a nestling. There was a temporal division of labor in feeding fledglings. Both parents fed newly fledged young, then the male reduced his feeding rate for one to several days during nest building. When the male returned to feeding, the female ceased feeding, finished nest-building, laid the next clutch, and incubated. Pairs cooperated in this way in caring for fledglings while renesting.A qualitative model is developed that explains high male parental investment in monogamous birds as resulting from sexual differences in juvenile mortality with females becoming the limiting sex. Female scarcity allows surviving females to favor high male parental investment under the threat of desertion. Supporting evidence for this model is presented
