The bird survey method refers to a set of techniques used to estimate bird populations and diversity in a given area. It involves standardized counts of birds along set routes or at fixed points during set time periods. The data collected from bird surveys allow researchers to monitor populations and track changes over time. This provides an important tool for assessing the impacts of habitat change, climate change, and other factors on bird communities. Bird surveys are a critical component of avian conservation and management programs.
What are the main types of bird survey methods?
There are several common bird survey methods used by ornithologists and conservation agencies:
Point counts
In point counts, an observer stands at a fixed spot and records all birds seen or heard within a set time period, usually 5-10 minutes. The points are distributed across the survey area at regular intervals along transects. This method provides a snapshot of the bird community composition in the vicinity of each point. The radius for observations is usually limited to 100-200 meters to reduce double-counting.
Line transects
With line transects, an observer walks a set path through the habitat, recording all birds detected within a fixed distance of the line. Perpendicular distance to each bird is recorded to allow density estimates to be calculated. Transects should be randomly placed or systematically spaced to get a representative sample.
Territory mapping
Territory mapping involves intensively surveying an area to delineate the boundaries of breeding territories for each species. This provides detailed information on the density and distribution of territorial species. It requires multiple site visits through the breeding season to observe displays and movements.
Area search
In an area search, observers explorer a designated area thoroughly to document all birds present. This method provides a species list and total number of individuals for the site but no density estimates. It is useful for surveying small areas or fragmented habitat.
When are bird surveys conducted?
Most bird surveys are conducted during the breeding season, which allows detection of breeding residents as well as spring migrants. This coincides with peak vocal activity for many species. Surveys generally begin shortly after dawn when birds are most actively singing and calling. Conducting surveys at the same time each season and under suitable weather conditions provides comparable data across years. Some specialized surveys are also done outside of the breeding season, such as winter waterfowl counts. Migration surveys may focus on documenting all migrant species passing through an area.
How are survey locations and timing determined?
Survey locations
Locations for bird surveys depend on the specific goals but generally aim to get representative coverage of the habitats present across the area of interest. Points or transects are usually stratified by habitat type and placed randomly or systematically within those habitats. This ensures adequate sampling of each habitat. Replicating transects or points in the same habitat improves the precision of results. Locations may also be chosen to monitor specific sites over time.
Survey timing
Most breeding season surveys are conducted within a standardized window, usually from late April to early July in temperate regions. This corresponds to the peak breeding period for most species. Exact timing varies by latitude and elevation. Multiple surveys spaced a week or two apart provide the best detection probabilities for species that arrive at different times. Migration surveys may be timed for specific periods based on patterns of migratory passage. Winter surveys are often scheduled to coincide with annual waterfowl counts.
What data is collected on bird surveys?
Bird surveys record several key pieces of data:
- Species – all species detected are recorded
- Count – the number of individuals of each species
- Location – precise location for point counts or distance along transects
- Time – time period for point counts or time of detection on transects
- Distance – radial distance to each bird for point counts or perpendicular distance for transects
- Breeding status – observed evidence of breeding such as nests, territorial behavior, adults carrying food, etc.
- Sex and age – distinguishing features noted if possible
Additional data such as breeding habitat characteristics, behavior, movements may also be recorded depending on the survey objectives. Standardized data forms are used to record all observations.
What equipment is used?
Bird surveys require minimal specialized equipment:
- Binoculars – essential for detecting and identifying birds
- Rangefinder – used to measure distance to birds on point counts or transects
- Flagging tape – to mark transect routes and point count locations
- GPS unit – records locations of routes, points, or bird detections
- Field guide – for confirming identifications
- Clipboard and data forms
- Watch or stopwatch – to time survey durations and intervals
High-quality binoculars and a good bird field guide are the most important equipment needs.
What are the limitations of bird surveys?
While bird surveys provide extremely useful data, there are some inherent limitations and challenges:
Imperfect detection
Surveys will inevitably miss some birds that are present but undetected. Detection rates vary by species, habitat, season, observer, weather, and other factors. Statistical methods can help estimate and account for imperfect detection.
Large sample effort required
To obtain reasonably precise estimates of populations, a large number of survey points or transects covering the area of interest are required. This level of effort may be infeasible for very large areas.
Snapshot in time
Surveys only provide a snapshot of the bird community at the time they are conducted. Multiple surveys are needed to characterize bird use throughout the seasons and years.
Observer variability
Different observers have varying ability to detect and identify birds by sight and sound. Consistent observer training and testing helps reduce this source of variability.
Access restrictions
Ability to conduct surveys may be limited by lack of access to private or remote lands. This can bias results, particularly if habitat differ from accessible areas.
Resource intensive
Implementing high-quality bird surveys requires significant time, personnel, and funding resources. Analysis and interpretation also requires technical expertise.
How are survey data analyzed?
There are a variety of analytical methods used to derive estimates of population size, density, and trends from bird survey data:
Species richness
The total number of species detected in an area. Comparison of richness between sites or over time can provide simple diversity indexes.
Relative abundance
The total number of individual birds detected per unit effort (e.g. per survey point). May be a simple index of population size.
Density estimation
Using distance sampling data to mathematically model detection probabilities and derive density estimates adjusted for detectability.
Occupancy estimation
Estimating proportion of area occupied by a species using repeated survey data to account for imperfect detection.
Population estimation
Extrapolating density estimates to a defined area to calculate population size. Requires a representative sampling design.
Trend analysis
Modeling survey data over time to estimate population trends and significance of change. Regression or occupancy models may be used.
Advanced statistical software is commonly used to implement these methods.
How are bird survey data used?
Data from bird surveys have many important conservation and management applications:
- Establish baseline population levels against which to monitor future change
- Detect significant population declines that may require intervention
- Model species-habitat associations to define important bird habitats
- Assess impacts of habitat loss and other environmental changes
- Evaluate effectiveness of management strategies and response to conservation actions
- Identify locations, species, or habitats to prioritize for protection
- Inform endangered species listing decisions and recovery plans
- Contribute to scientific understanding of avian ecology and conservation needs
Results are often used to set conservation priorities, implement habitat enhancements, revise species recovery plans, focus research needs, and inform land management policies and regulations.
Breeding Bird Survey
One of the largest and most well-known bird monitoring programs is the North American Breeding Bird Survey (BBS) administered by the US Geological Survey and Canadian Wildlife Service. The BBS tracks the status and trends of hundreds of bird species across North America using thousands of roadside survey routes since 1966.
Methods
BBS routes follow secondary roads and make 50 3-minute point counts at 0.8 km intervals over a 40 km route. Surveys start 30 minutes before local sunrise and take 4-5 hours. Routes are randomly located then run annually by volunteer observers. Data is publicly available online.
Results
Analysis of BBS data has shown significant population declines in many grassland and aerial insectivore species since the 1960s. Some forest species have increased following farmland reforestation. The BBS provides essential long-term monitoring of breeding landbirds across the continent.
| Species | Population Trend 1966-2019 |
|---|---|
| Grasshopper Sparrow | -3.18% per year |
| Eastern Meadowlark | -2.67% per year |
| Northern Cardinal | +0.91% per year |
| Mourning Dove | +0.47% per year |
Limitations
Roadside sampling may under-represent species sensitive to habitat edges and fragmentation. The large geographic coverage precludes intensive sampling at specific sites. Participation varies with observer interest and effort.
Christmas Bird Counts
One of the longest-running and most famous bird surveys is the Audubon Christmas Bird Count (CBC), which began in 1900. Though initially more casual and competitive, the CBC developed into a large-scale annual bird census with over 2000 count circles across the Americas.
Methods
During a single day between December 14-January 5, volunteer observers across North America record all birds detected within a 24 km diameter circle. Tens of thousands of people now participate each year, providing a huge sample of winter bird numbers.
Results
CBC data has documented shifts in wintering ranges northward and increasing population trends in many species, likely driven by climate change effects. It provides insight into overwinter survival and habitat use patterns. The combination of volunteer effort and standardized protocol yields an unparalleled dataset.
Limitations
CBC circles are not randomly located and vary in habitat coverage and observer effort. Historically, counts were less rigorous and prone to duplicate entries. Recent statistical methods help control for some of these issues.
Project FeederWatch
Project FeederWatch is a popular citizen science bird survey organized by the Cornell Lab of Ornithology. Since 1987, thousands of participants have contributed data on birds at their feeders.
Methods
Individuals observe their feeders over 2 consecutive days each week from November to April and report highest numbers of each species seen at one time. Over 15,000 feeder watchers submit data each year.
Results
Unusual irruptions of northern finches, shifting arrival dates of migrants, and detection of disease outbreaks are some patterns revealed. Easy participation provides great public engagement.
Limitations
Biased by supplemental feeding and focused on a subset of mostly common species. Limited ability to quantify populations or trends accurately.
Breeding Bird Atlases
Many states and provinces have conducted atlasing projects to map detailed breeding distributions of all bird species. These comprehensive 5-year surveys enlist hundreds of volunteers to canvas blocks across the region.
Methods
Each block is repeatedly surveyed over multiple years during the breeding season using point counts, transects, and area searches. Evidence of confirmed, probable, or possible breeding is documented for each species. Over 10,000 priority blocks may be covered per state.
Results
Hundreds of thousands of records are compiled into maps and databases depicting occupied breeding ranges, densities, habitat use, and population locations. This guides conservation planning and provides a baseline for future resurveys.
Limitations
Full coverage of remote areas is challenging. Breeding evidence criteria may be inconsistently applied by observers. Block sizes and survey intensity vary between states.
Conclusion
Bird surveys employ standardized field methods to gather important data on avian populations, distributions, and habitat relationships. Limitations exist, but systematic surveys remain a critical tool for monitoring and conserving bird populations. Major programs such as the Breeding Bird Survey provide long-term data on population changes that guide conservation at continental scales. More localized surveys help managers track site-specific bird communities and responses to habitat management. Continuing to improve survey methods while expanding monitoring networks will be key to ensuring data adequately informs effective conservation strategies. With many bird species in decline, high-quality surveys are essential for detecting and diagnosing these troubling trends.
