Avian life histories: is extended parental care the southern secret?

Abstract

Most theories of life history evolution have been based on demographic studies from cool temperate regions of the northern hemisphere, where many birds either migrate to avoid severe winters or suffer high mortality if they stay. Life histories of species living in tropical and southern temperate regions are typified by smaller clutches, multiple nesting attempts, high levels of nest predation, and high adult survival. Hypotheses explaining the evolution of these different patterns invoke differences in food availability, nest predation or mortality. The difficulty in deciding between different hypotheses lies in the lack of information about the breeding biology of species living in the southern hemisphere. One aspect of life history variation that has received little attention is the survival of juveniles, which requires intensive study of known individuals, difficult in all but sedentary species. Extended parental care may promote the survival of juveniles, and thus counteract the effects of smaller clutches. I concentrated on the Passeriformes and reviewed the Time to Independence and the length of time for which juveniles were allowed to stay in the family group or territory after fledging. Species of the northern temperate regions show patterns in which young associate with their parents for only a short time before having to fend for themselves, less than one month in a majority of species. This contrasts with patterns of parental care in species of the tropics and southern temperate regions, in which Time to Independence is generally at least one month, and a majority of species remained with their parents for at least three months. I suggest that by increased investment in a few young, total reproductive effort is reduced, which may contribute to higher adult survival. The need for management to take account of life history attributes is emphasised.