Bird Use of Imperial Valley Crops [ds427]
Agriculture crops in the Imperial Valley of California provide valuable habitat for many resident and migratory birds and are a very important component of the Salton Sea Ecosystem (Patten et. al. 2003), but detailed information regarding avian species use, distribution and abundance is lacking. In 2006 the California Department of Fish and Game, the Salton Sea Authority, and the USGS initiated a monthly survey of birds using Imperial Valley agriculture fields to provide information regarding avian species composition and use. Driving transects were originally delineated on a map as one continuous transect projected to cover a good representation of a subset of the entire Imperial Valley agriculture area. The original transect included mostly highways with heavy traffic and high speeds. It was decided for safety reasons that slower speed limits and lighter traffic areas would be more suitable to this type of survey. The result is two transects of roughly the same distance. The west transect is west of Highway 111, beginning at the junction of Highway 111 and Sinclair Road and ending on Harris Road and Butters Road. The east transect is east of Highway 111, beginning at the junction of Highway 111 and Sinclair Road. and ending on Harris Road and Butters Road. See map for details http://nrm.dfg.ca.gov/FileHandler.ashx?DocumentID=4964 . Surveys began 30 minutes after sunrise on the west transect. The east transect was completed on the next consecutive day commencing at 30 minutes after sunrise. Driving speed was limited to 25 miles per hour where possible. When available, the survey was conducted with a driver and an observer. Many times the driver was alone and made all of the observations. As the vehicle followed the transect, fields on either side of the road were scanned for birds. When birds were observed in a field the vehicle was stopped and a GPS location was taken using a Magellan Sport Trac Topo. All bird species were counted using Steiner Merlin 10 x 42 binoculars and/or Nikon Fieldscope ED 20x45 zoom. Passerines were excluded from counts. Bird numbers were estimated by first counting all members of a species within one view of the binoculars or the spotting scope. This was done three to five times and an average number per optic view determined. The average value was then multiplied by the number of optic views required to cover the concentration of birds at that site. The resulting number was recorded as the estimated number of birds present. All members of a species present at an observation site were totaled to provide an estimated number of that species present at that site. Crop information, time of observation, and notes about the condition of the field were recorded for the field in which birds were detected The maximum distance birds were observed varied based on weather, angle of the sun with respect to the observer, or distance to the edge of the field birds were observed in. As a general estimate of mean maximum distance I used 400M. This distance is based on a visual estimate of the distance from a road to the far edge of a field in the Imperial Valley. Many fields had late stage crops that were tall and dense and subsequently could not be accurately surveyed from a vehicle. No effort was used to count and remove these crops from the survey area. If birds were observed flying out of or into these late stage crops information was recorded on only the birds seen in flight.