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II. Number of Potential Breeding Birds

The number of potential breeding birds present on an area is an important determinant of the number of young produced. Spring populations must include members of both sexes, obviously, but in the population dynamics of many species it is common to express production of young strictly as a function of number of females, assuming that there are enough males to insure fertilization of all females. We are interested in the determinants of numbers of both males and females on the breeding grounds, with emphasis on females.

The number of birds on the breeding grounds is a function of population size during the previous breeding season, reproduction, survival, emigration, and immigration. The birds in a particular breeding area include those that were located there during the previous breeding season (as either adults or young) and those that were elsewhere. The number of individuals in the first group is determined by the number of adults on the area in the previous year, the number of young they produced, the probability of surviving between breeding seasons, and the conditional probability of returning to the same breeding area given that it is alive to do so. Similarly, the number of birds in the second group is determined by numbers of adult birds on other areas in the previous year, their production of young, annual survival probability, and the conditional probability of going to the particular breeding area (which is different from the one used the previous year), given that it is alive. This section deals with annual survival and homing and pioneering.