Spring 2006 Bird and Bat Migration Surveys at the Proposed Deerfield Wind Project in Searsburg and Readsboro, Vermont


Title: Spring 2006 Bird and Bat Migration Surveys at the Proposed Deerfield Wind Project in Searsburg and Readsboro, Vermont
Publication Date:
December 01, 2006
Pages: 40
Sponsoring Organization:

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Stantec Consulting (2006). Spring 2006 Bird and Bat Migration Surveys at the Proposed Deerfield Wind Project in Searsburg and Readsboro, Vermont. Report by Stantec Consulting. pp 40.

During spring 2006, Woodlot Alternatives, Inc. (Woodlot) conducted field surveys of bird and bat migration activity at the Deerfield Wind Project areas in Searsburg and Readsboro, Vermont. The surveys are part of the planning process by PPM Energy Inc. (PPM) for a proposed wind project, which will include the erection of 15 to 24 wind turbines along two ridge lines. Field investigations included nighttime surveys of birds and bats using radar and bat echolocation detectors. These surveys represent the latest of the four (over three years) radar migration surveys undertaken at the Deerfield Wind Project area.


The overall goals of the investigations were to:

  • document nocturnal migration activity in the vicinity of the project area, including the number of migrants, their flight direction, and their flight altitude; and
  • document the presence of bats in the area, including the rate of occurrence and, when possible, species present during the spring and summer migration period.


The results of the field surveys provide useful information about site-specific migration activity and patterns in the vicinity of the Deerfield Wind Project area, especially when reviewed along with previous results of the 2004 and 2005 surveys. This analysis is a valuable tool for the assessment of risk to birds and bats during migration through the area.


Radar Survey


The spring field survey targeted 30 nights of radar surveys to collect and record video samples of the radar during horizontal operation, which documents the abundance, flight path, and speed of targets moving through the project area, and vertical operation, which documents the altitude of targets. While 30 nights of sampling were targeted, a total of 26 were sampled due to inclement weather creating conditions in which the radar could not adequately document bird movements or the available data was unsuitable for analysis.


Nightly passage rates varied from 5+2 targets per kilometer per hour (t/km/hr) to 934+120 t/km/hr, with the overall passage rate for the entire survey period at 263+45 t/km/hr. Mean flight direction through the project area was 58 degrees +  54 degrees. Seven percent of all radar targets were classified as insects.


Flight direction varied between nights and was probably due to variation in the weather (particularly wind direction and speed). The mean flight height of targets was 435 meters (m) + 36 m (1,427' + 118') above the radar site. The average nightly flight height ranged from 43 m + 20 m (141' + 66') to 913 m + 92 m (2,995' +  302'). The percent of targets observed flying below 125 m (410') also varied by night, from 0 percent to 94 percent. The seasonal average percentage of targets flying below 125 m was 11 percent. Nights with the lowest mean flight heights were typically associated with passage rates well below the seasonal mean.


The results from the spring 2006 surveys at the Deerfield Wind Power Project were generally similar to those documented during the 2004 and 2005 surveys, which were conducted at different locations within the proposed development. In general, most of the reported survey metrics (i.e., passage rate, flight height, and percentage of targets below turbine height) were within the range of those results from the three previous seasons of surveys.


The mean flight direction, qualitative analysis of the surrounding topography and landscape, and mean flight altitude of targets passing over the project area indicates that avian migration in this area involves a broad front type of movement over the landscape. Although migration paths through the project area would bring migrants across ridgelines of the proposed wind farm, the high flight height of targets indicates that the vast majority of bird migration in the area occurs well above the height of the proposed wind turbines. This is consistent with the three previous seasons of data.


Analysis of NEXRAD weather data was examined to identify the proportion of the migration season during which the radar survey at the Deerfield Wind Project occurred. In general, approximately 50 percent of the nights with regional migration activity were sampled with the on-site radar. Nights with light and heavy regional migration activity sampled with the on-site radar occurred in proportion to how they occurred throughout the full migration season. However, nights with moderate regional migration activity were sampled on-site in greater proportion to how they occurred throughout the migration season, while nights with no regional migration activity were under sampled.


Bat Activity-Spring 2006


Five bat detectors were deployed in the project area: two in the Eastern Project area and three in the Western Project Area. Detectors were deployed from April 14 to June 13.


A total of 194 detector-nights of data were recorded during the sampling period, during which only 15 call sequences were recorded. The overall detection rate was 0.1 call sequences per detector night, which is nearly identical to spring 2005 surveys at this and other sites in the region.


Of the recorded call sequences, four were classified as unknown due to poor file quality or too few call pulses on which to make identifications. Five call sequences were identified as within the big brown bat guild-which includes the big brown bat (Eptesicus fuscus), silver-haired bat (Lasionycteris noctivagans), and hoary bat (Lasiurus cinereus)-and five were identified as myotids. One call was identified as either an eastern red bat (L. borealis) or eastern pipistrelle (Pipistrellus subflavus).

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