Each 500m resolution cell used for the model is a composite of habitat
types in which the sparrow is known to breed. These composite cells are taken from
a 32m resolution map of vegetation types constructed by The Nature Conservancy. To
construct this map of breeding habitat, a grid of
500m cells is placed over the 32m resolution map. This provides a habitat layer of 500m
resolution cells that contain an overall percentage of suitable nesting habitat.
To determine if a cell contained 'enough' suitable habitat for the sparrow to breed, the amount of breeding and nesting habitat as well as non-breeding habitat (i.e., marsh) for each cell was calculated. A cell only provides suitable breeding habitat for the sparrow if a) the percentage of nesting habitat exceeds 55% and b) the combined coverage of the marsh habitat and the nesting habitat exceeds 90%. These percentages correspond to habitat coverage values calculated from breeding bird surveys.
To determine the maximum number of territories available within each cell, an upper quartile regression technique was used. This technique allows for an estimation of the upper limit of breeding sparrows within a particular cell. A simple linear regression technique would only estimate the average number of breeding sparrows in a cell. This average would be influenced by surveys when conditions were highly unfavorable for singing (e.g., rainy days). Thus by using an upper quartile regression, the 500m resolution map accurately assess habitat quality since it uses data from periods in which the sparrows were more active.
The 500m scale of the vegetation layer was determined to be ecologically appropriate for the sparrow. Sparrows are known to have discrete territories of approximately 100m and these territories are nearly touching or may even overlap. So, a model with a scale under a 100m is ecologically inappropriate. Additionally, during the non-breeding season sparrows will only move a few kilometers from their natal or breeding site. Thus, 500m is a logical choice to represent the area in which a group of breeding sparrows interact and it also corresponds to the demographic surveys currently being conducted for the population.