Census Techniques Flashcards
Sources of Bias
Observers Habitat Birds Weather Technique
Observers Bias
eyesight, hearing (especially males
over 40), id skills, equipment
Habitat Bias
dense, noise from roads, rivers, wind,
terrain
Birds Bias
behavior, sensitivity to disturbance,
mobility, flocking, detectable, territories, diurnal,
time of day
Weather Bias
wind, rain, fog, extreme temperature
Technique Bias
number of points, sampling plan
Techniques
Mapping Transects Point Counts/Circular Plots Scan/ Look See Indirect Counts Banding/ Mark Recapture Flyway Counts Juvenile/Adult Ratios
Total Mapping
mark everyone- home range or territory use, very expensive, get accurate count, endangered species, behavior studies
Spot Mapping
Visit plot at least 8 times, systematic search, record all
birds heard/seen
Each cluster (at least 2 sightings/cluster) = 1 breeding pair
Edge problems
Very sensitive to plot size
Measure # pairs/area = density
Line Transects
More efficient than mapping techniques
Best suited for homogenous areas where
vegetation and terrain do not limit travel, linear
habitats
No Distance Line Transects
Travel transect- count all birds seen and
heard
Easy
No density estimate- only presence/absence
or relative abundance
Strip Transects
Count all birds that occur in the strip, ignore birds
outside of strip
Strip is usually 25 m on each side of line
Only need to estimate 1 distance
Technique gives density measure
Detection can be a problem
Emlen Transect- Variable
Distance Line Transect
Estimate distance perpendicular from
observer to each bird seen or heard
Can be difficult to estimate distance
Variable Distance Line Transect
Often few birds seen close to line
Use Coefficient of Detection (CD) for each
species to correct for detection close to or
far away from line
*Density = (n(pairs)/length x width)CD
Point Counts or Circular Plots
Newest method- replacing line transects
Good for linking habitat and birds, uneven terrain, dense
vegetation, fragmented habitat
Errors in distance measurement compound geometrically
No Distance Point Counts
Count all birds seen and heard during count period Easy Relative abundance and presence/absence data- not density
Fixed Radius Point Count
Like a strip transect- count all birds within circle
Only measure 1 distance
- Density = n (pairs)/pi r2
Variable Distance Point Counts
Record distance from observer to bird
Don’t lose birds outside of circle
For each species calculate CD (coefficient of detection)
Density = (n(pairs)/pi r2
) CD
Most Common Techniques
Transects and Point Counts
Scan
Scan area and record birds If area very large or many birds- divide into sectors Spend set amount of time scanning or on each sector Good for open habitats where movements would disturb birds Work quickly to avoid counting birds moving from one area to another
Indirect Counts
No density estimates Nest counts – Count apparently occupied nest Roost counts Fecal counts Track counts (in mud, etc)
