Comparison of survey methods for wintering grassland birds. Roberts, J. and Schnell, G. Journal of Field Ornithology, 77(1):46–60, Department of Zoology, Sam Noble Oklahoma Museum of Natural History, University of Oklahoma, 2401 Chautauqua Avenue, Norman, OK 73072, United States, 2006.
abstract   bibtex   
Although investigators have evaluated the efficacy of survey methods for assessing densities of breeding birds, few comparisons have been made of survey methods for wintering birds, especially in grasslands. In winter, social behavior and spatial distributions often differ from those in the breeding season. We evaluated the degree of correspondence between density estimates based on different survey methods. Surveys were conducted during two winters (2001-2002 and 2002-2003) on 16 grassland sites in southwestern Oklahoma. Line-transect (using a detection function to account for birds present but not detected) and area-search (where density was based on the total count within a given area) methods were employed. Observations on line transects were also analyzed as strip transects, where density was based on total count within a given strip width and no detection function was used. Savannah Sparrows (Passerculus sandwichensis), LeConte's Sparrows (Ammodramus leconteii), Song Sparrows (Melospiza melodia), Smith's Longspurs (Calcarius pictus), Chestnut-collared Longspurs (C. ornatus), and Eastern Meadowlarks (Sturnella magna) were sufficiently abundant to allow comparison. Area-search density estimates tended to be higher than line-transect estimates for Savannah Sparrows, Song Sparrows, and Eastern Meadowlarks, suggesting that some individuals initially located close to the transect line were not detected on line transects. The area-search and line-transect methods gave similar density estimates for Chestnut-collared and Smith's longspurs. Area-search estimates of Eastern Meadowlarks were significantly higher in the second year of the study only. For this species, area-search estimates did not differ from those of strip transects covering an equal area, so the reason for the differing meadowlark estimates is not clear. Higher density estimates using the area-search method likely resulted from: (1) birds that might escape detection by hiding were more likely detected (flushed) during area searches because of the repeated passes through the area, and (2) birds close to the line in line transects escape detection by hiding, biasing those estimates low. We also evaluated the correspondence of density rankings for the six species as determined by the different survey methods and for the same species across survey sites. Correlations among the six species of the area-search results with those of line transects and strip transects generally were high, increasing in 2002-2003 when densities of birds were greater. All three methods provided similar density rankings among species. Density rankings within species across sites for the four non-longspur species generally were concordant for the three methods, suggesting that any of them will adequately reflect among-site differences, especially when densities vary greatly across sites. Further research is needed to determine the extent to which grassland birds are missed on line transects. We suggest that workers using line transects to study these species give careful consideration and make additional efforts to satisfy the distance-sampling assumption that all birds on or near the line are detected. If density is measured as a total count in a fixed area, we recommend that observers pass within <10 m of all points in the area. ©2006 Association of Field Ornithologists.
@Article{RobSch06,
  author      = {Roberts, J.P. and Schnell, G.D.},
  title       = {Comparison of survey methods for wintering grassland birds},
  journal     = {Journal of Field Ornithology},
  year        = {2006},
  volume      = {77},
  number      = {1},
  pages       = {46--60},
  abstract    = {Although investigators have evaluated the efficacy of survey methods
	for assessing densities of breeding birds, few comparisons have been
	made of survey methods for wintering birds, especially in grasslands.
	In winter, social behavior and spatial distributions often differ
	from those in the breeding season. We evaluated the degree of correspondence
	between density estimates based on different survey methods. Surveys
	were conducted during two winters (2001-2002 and 2002-2003) on 16
	grassland sites in southwestern Oklahoma. Line-transect (using a
	detection function to account for birds present but not detected)
	and area-search (where density was based on the total count within
	a given area) methods were employed. Observations on line transects
	were also analyzed as strip transects, where density was based on
	total count within a given strip width and no detection function
	was used. Savannah Sparrows (Passerculus sandwichensis), LeConte's
	Sparrows (Ammodramus leconteii), Song Sparrows (Melospiza melodia),
	Smith's Longspurs (Calcarius pictus), Chestnut-collared Longspurs
	(C. ornatus), and Eastern Meadowlarks (Sturnella magna) were sufficiently
	abundant to allow comparison. Area-search density estimates tended
	to be higher than line-transect estimates for Savannah Sparrows,
	Song Sparrows, and Eastern Meadowlarks, suggesting that some individuals
	initially located close to the transect line were not detected on
	line transects. The area-search and line-transect methods gave similar
	density estimates for Chestnut-collared and Smith's longspurs. Area-search
	estimates of Eastern Meadowlarks were significantly higher in the
	second year of the study only. For this species, area-search estimates
	did not differ from those of strip transects covering an equal area,
	so the reason for the differing meadowlark estimates is not clear.
	Higher density estimates using the area-search method likely resulted
	from: (1) birds that might escape detection by hiding were more likely
	detected (flushed) during area searches because of the repeated passes
	through the area, and (2) birds close to the line in line transects
	escape detection by hiding, biasing those estimates low. We also
	evaluated the correspondence of density rankings for the six species
	as determined by the different survey methods and for the same species
	across survey sites. Correlations among the six species of the area-search
	results with those of line transects and strip transects generally
	were high, increasing in 2002-2003 when densities of birds were greater.
	All three methods provided similar density rankings among species.
	Density rankings within species across sites for the four non-longspur
	species generally were concordant for the three methods, suggesting
	that any of them will adequately reflect among-site differences,
	especially when densities vary greatly across sites. Further research
	is needed to determine the extent to which grassland birds are missed
	on line transects. We suggest that workers using line transects to
	study these species give careful consideration and make additional
	efforts to satisfy the distance-sampling assumption that all birds
	on or near the line are detected. If density is measured as a total
	count in a fixed area, we recommend that observers pass within <10
	m of all points in the area. ©2006 Association of Field Ornithologists.},
  address     = {Department of Zoology, Sam Noble Oklahoma Museum of Natural History, University of Oklahoma, 2401 Chautauqua Avenue, Norman, OK 73072, United States},
  comment     = {http://www.blackwell-synergy.com/doi/abs/10.1111/j.1557-9263.2006.00024.x},
  file        = {Roberts&Schnell2006.pdf:Roberts&Schnell2006.pdf:PDF},
  keywords    = {Area search, Density, Grassland bird, Line transect, Strip transect, Survey, Winter},
  owner       = {Tiago},
  subdatabase = {distance},
  timestamp   = {2006.11.19},
}
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