The landscape images used in the simulation model were produced on a raster-based GIS (Idrisi, Clark University, MA) from a digital air photo mosaic made from 1: 25 000 aerial photos taken in 1994, provided by the Western Australian Department of Land Administration. Aerial photos were used in preference to Landsat TM images, because the initial resolution of a Landsat TM image is too coarse to accurately define small vegetation elements that may be important to dispersing animals (e.g., individual shrubs and trees). For the Blue-breasted Fairy-wren study landscape, (23 000 ha) we used 28 aerial photos to produce the mosaic; for the White-browed Babbler landscape (14 000 ha), 21 were needed.
Because no two consecutive aerial photos are taken from exactly the same angle, they are subject to individual distortion. To rectify this problem, we used a rubber-sheet transformation (first-order polynomial fit, nearest neighbor resampling) to register each image against a Landsat TM scene of the entire study area. (If a Landsat scene is not available, any digital representation of the entire area would be suitable). An aerial photo mosaic was then produced at 5- m resolution (i.e., each pixel depicts a 5-m square).
The final landscape image was produced by reclassifying individual pixel values of the mosaic into one of two categories: vegetation and other. This 5-m image was then contracted by pixel aggregation to a 30-m image and the pixel values were further reclassified into agricultural land (no 5-m pixels with vegetation per 30-m pixel), sparse vegetation (1-18 5-m pixels with vegetation per 30-m pixel), and thick vegetation (19-36 5-m pixels with vegetation per 30-m pixel).
Separate images (of the same 30-m resolution and overall dimensions) representing the bird territories (patches) and their psuedo-centroids (points centrally located within each patch) were digitized by hand, using the landscape image as a backdrop.
All image files were saved as raw single-byte binary files with no file header.
Computer code
The simulation software was written in C programming language as a stand-alone executable file for IBM-compatible computers. The advantage of this approach is that the software can access raw image files made using any raster-based GIS software and is therefore not platform specific.