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Breeding Population Inventories and Measures of Recruitment

I. Introduction

A. Definitions

1. Breeding Population

In general terms, the breeding population of a species is the number of birds of the species that are alive at that period of the year during which breeding activity takes place. This definition is reasonable for closed populations and is used in estimating total breeding populations, regardless of sex, for North American waterfowl. The primary use of estimates of population defined in this manner is for continental-level management such as setting waterfowl seasons and for determining trends in population size.

To address specific aspects of population dynamics, such as modeling change in population size as a function of survival and recruitment, it is preferable to define breeding population in terms of either females or breeding pairs. Many waterfowl species have sex ratios that are unbalanced in favor of males that do not contribute to recruitment in any obvious way. A definition based on breeding pairs, therefore, requires that the sex ratio be known or that females be counted to estimate breeding population (Dzubin 1969a).

The problem of defining breeding population becomes more difficult for estimates in smaller geographic regions or local study areas. Waterfowl are highly mobile and during the breeding season a particular area may contain birds that are breeding, birds that will later breed on the area, nonbreeders, and migrating birds that are staging prior to continuing migration to breeding areas elsewhere. For a local area, we define the breeding population as the number of pairs that have selected that area for breeding, even though they may not breed because of weather or other environmental factors. This definition, though necessary for evaluating population dynamics of local populations, causes severe problems in measurement because it demands that breeding birds be distinguished from migrants either by timing of census or by observation of behavioral clues such as defense of territory or mate.

2. Recruitment

Recruitment is the process by which young are added to the fall population by reproduction from adults in the spring population. It includes elements of both natality and mortality. The term recruitment, first used in fisheries science to describe the addition of young of a given age to the population (Ricker 1975), is well suited to waterfowl because recruitment is generally evaluated just prior to the hunting season. Recruits are birds of the year entering the population soon after fledging. Cooke et al. (1984) define recruitment differently for snow geese. They considered a bird a recruit when it enters the population at breeding age. We define recruitment rate as number of young females in the fall population divided by the number of adult females in the spring population. Recruitment rate is independent of the size of the spring population, unless there is density dependence. The number of recruits produced is the product of recruitment rate and size of the breeding population.

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