BFL Analyzes Early Results
Veeries show a marked sensitivity to habitat fragmentation
The 1998 BFL season is already upon us, so we thought we would share insights from our preliminary 1997 results. In some cases, these early results reflect the patterns of response to forest habitat fragmentation that we had expected to see in thrush and accipiter species. In others, the results differ from our expectations, ranging from the subtle to the surprising, with some species showing high sensitivity to fragmentation, others showing sensitivity to vegetation structure in combination with fragmentation, and others showing no significant sensitivity to our analysis variables. Finally, these data suggest even closely related species with similar ranges react differently to habitat fragmentation.
Take the Veery. Based on our preliminary results, this relatively widespread species of the northern United States and southern Canada shows a marked sensitivity to habitat fragmentation. Figure 1, based on data from 449 study sites, plots on the vertical (z) axis the probability of finding a singing Veery on one or more of the two required site visits (this is scored as 'Possible Breeding' on the Visit Form). As shown in Figure 1, the probability of finding a 'Possible' breeding Veery increases from a low of near zero in small, isolated, high-altitude forest patches with an open canopy located in a fragmented landscape, to a high near 75 percent for large, low-altitude forest patches with a closed canopy that are located in a landscape with a high proportion of contiguous forest. These results are very much in line with our expectations and agree well with the habitat description included with the BFL Participants' Kit (see Reference Section 3.2.7 of the BFL Instruction Manual). With other species, however, the results are not so straightforward.
Based on our preliminary analyses, the Varied Thrush shows no sensitivity to fragmentation. Instead, as shown in Figure 2, the probability of finding a Possible Varied Thrush increases slowly as Factor 4 (a measure of the density of the forest understory or low vegetation) increases. Conversely, the probability of detection increases sharply with increases in Factor 5 (a measure of canopy height). This result seems to indicate a preference on the part of this thrush for forests with large trees and a well-developed understory, such as the coniferous forests of the Northwest, where this species is found (see Reference Section 3.2.19 of the BFL Instruction Manual). This validation of our expectations is heartening, but our conclusion is based on data from only 42 study sites, less than a tenth of the sample size for our analysis of the Veery's habitat preferences. Because of the small sample size, we are not as confident about this conclusion; it is entirely possible that this result would change if data from more study sites were available.
Another species for which we lack data is the Cooper's Hawk. This elusive forest raptor occurs in low densities all across the United States and southern Canada. This wide distribution makes it an excellent choice for BFL, but the fact that it occurs in low densities means that the probability of detecting this hawk at any given point is much lower than that of detecting one of the thrush species. Hence, we need a larger number of point counts to find the number of 'Possible' breeders we need for our analysis. For example, Figure 3 is based on only 34 detections out of a total of 437 study sites. It is clear that the probability of finding a calling Cooper's Hawk does not vary with fragmentation (Factor 1) or with elevation/canopy closure (Factor 2). Our preliminary analysis suggests that none of the included variables (Factors 1-5) have a significant effect on the Cooper's Hawk.
Several possible explanations exist for the apparent lack of effect. One could be the effects of habitat fragmentation and vegetation structure, but our current sample size is too small to detect these effects. Another is the 1,000-hectare scale at which participants calculate measures, such as percentage of forest, amount of linear edge, and isolation, is too small for a wide-ranging predator such as the Cooper's Hawk, although it is adequate for species with smaller home ranges, such as the Veery. A third is that fragmentation has no effect on the Cooper's Hawk.
We hope to address the question of sample size by ensuring that participants receive instruction materials in plenty of time to allow them to be in the field prior to the beginning of the Cooper's Hawk's incubation period, when the hawks are most easily detected. We are also working to discover the appropriate scale for measuring the effect of habitat fragmentation on the Cooper's Hawk. Lab scientists are developing methods to measure fragmentation with computers using the Lab's Geographic Information System facilities, which will enable us to make these measurements on a number of scales. These measurements, coupled with data from BFL participants, will help us pinpoint the size of the landscape we need to consider when studying raptors such as the Cooper's Hawk.
One exciting area of investigation is to determine whether two closely related, co-occurring species differ in their responses to fragmentation. By surveying for multiple species at a single study site, BFL participants help us make the direct comparisons we need to answer this question. For example, the Swainson's and Hermit thrushes appear to respond differently to fragmentation, with the Swainson's Thrush showing a significantly greater response to Factor 1, which is an overall measure of fragmentation. This intriguing interim result stresses the importance of censusing for more than one species at the same location.
Our early look at the data from the 1997 BFL field season is exciting, indicating the project's great potential. Although we need to improve (by increasing the number of participants, increasing the completeness of data forms submitted by participants, and increasing the number of sites at which multiple species are censused), this partnership between the thousands of volunteers and the Lab's scientists is off to a great start. We hope for even greater things from the 1998 field season.