3.4 Relative Abundance

Recommended method(s): See Tables 2 and 3 for species-specific recommendations.

Observation Stations - Relative Abundance for all stages of life cycles (see Tables 2 and 3). It is the preferred method for counting breeding pairs, and broods for all but the most conspicuous species (e.g., swans and geese).

Helicopter Surveys - Relative Abundance for breeding pair counts brood counts, or incubating adults counts for the most conspicuous species (e.g., loons, swans and geese), or breeding pair and broods counts for dabbling ducks in remote areas.

Boat Transects - Relative Abundance for non-breeding birds on the ocean and in large wetlands (e.g., lakes or rivers) or along shorelines. Boat transects are suitable for surveying flocks of moulting and wintering sea birds in defined areas (Savard 1982).

Aerial Transects - Relative Abundance for detecting for breeding birds (swans, geese and Sandhill Crane), brooding ducks, and non-breeding bird counts on a large geographic scale.

Ground Transects - Relative Abundance for detecting breeding pairs and broods of Sandhill Cranes, and for detecting non-breeding birds in wetlands for some species.

Call Playbacks - Relative Abundance for breeding Pied-billed Grebes, and likely for other secretive waterbirds vocalizing during the breeding season. Other vocal and secretive, or at least sometimes hard to detect, waterfowl and allied species for which call playback might work include loons, Canada geese, American Coots, and Sandhill Cranes.

3.4.1 Overview of Methods

Observation Stations

Birds are detected by a stationary observer at each of several wetlands distributed throughout the project area. Wetlands may be chosen systematically, randomly, or randomly throughout stratified parts of the area. Usually the entire wetland is surveyed (this will vary with openness of habitat). If it is not possible to survey the whole wetland, then an adequate number of randomly or systematically placed, independent observation stations can be used to sample the wetland.

Advantages:

• Easier to locate wetlands randomly than lay out transect routes randomly;
• A well spaced sample of wetlands will provide more representative data than a few transects in fine-grained habitats;
• A stationary observer has more time to detect and identify birds that are cryptic; and
• Habitat data can be collected at the same time and then is easily associated with the occurrence of individual birds.

Disadvantages:

• Density of moving birds is overestimated in surveys where moving birds are included;
• Birds sensitive to observer will be flushed before they can be counted;
• More time-consuming than transects;
• If only part of the wetland is surveyed, relative density estimates of absolute abundance are susceptible to error arising from inaccurate distance estimation, although this can sometimes be corrected by plotting bird locations on airphotos or drawings of the wetland; and
• Relative abundance estimates can be influenced by the effect of differences in vegetation structure between sites on bird detectability.

Transects

Birds are detected along a continuous route through the study area. Transects are selected systematically, or randomly throughout stratified portions of the area. The amount of stratification necessary, if any, depends on the habitat diversity and the likelihood that birds will occur in different densities in different habitats. Transects are spaced to avoid recounting the same birds. An observer moves at a speed that enables detection of birds. The transect width must be measured to generate relative estimates.

The ability to see birds must be approximately equal for all portions of a transect and between transects. If this is not the case, then adjustments must be made for the varying ability to see birds in different parts of the transects. Width of sampling area may be divided into two or more zones. There is a trade-off between the number of transects and the length of the transect - numerous short transects offer a more diversified sample and few long transects are easier and more efficient to conduct. There is also a trade-off when considering the width of the transect. Wide transects cover more area and allow for a higher sampling intensity. However, this is offset by the fact that the probability of seeing a bird decreases as one approaches the edge of a wide transect.

Mode of transport along the transect depends on visibility and density of the birds as well as accessibility of the route. For example, an aerial transect is most effective and efficient when conspicuous birds are widely dispersed at low densities in areas that are inaccessible by road or boat.

Advantages:

• Most efficient way to gather data in large areas that are relatively uniform;
• Covers lots of ground in a fixed time;
• Generates large samples;
• Able to detect flushing birds as observer approaches;
• Less likely to double count individuals on aerial transects than by other survey methods because you move faster than the birds; and
• Area surveyed is linearly proportional to the lateral distance from the transect. Thus relative density estimates are less susceptible to error arising from inaccurate distance estimation than those calculated from point counts where the area surveyed is proportional to the square of the distance from the observation to the centre of the plot.

Disadvantages:

• Poor in small areas or fine-grained habitats;
• Constrained by accessibility depending on mode of transport (e.g., difficult to lay out transects randomly for ground surveys where accessibility is limited);
• Some birds conceal themselves from a moving observer; and
• Some birds in some habitats are difficult to detect when observer is moving.
• The visibility of a bird will tend to decrease with its distance from the transect. To compensate for this, complex estimates involving theoretical or empirical visibility rates can be used.

Data Analysis for Relative Abundance

The main assumptions of relative abundance surveys are:

1. Identical or statistically comparable methods are used when comparison between areas or monitoring trends in one area over time is an objective of inventory effort.

2. Environmental, biological, and sampling factors are kept as constant as possible to minimize differences in survey bias and precision between surveys.

3. Surveys are independent; one survey does not influence another.

If these assumptions are met then each replicate survey should show (on average) the same relative bias allowing calculation of trends and comparison between areas.

However, each assumption should be scrutinized carefully when investigating the applicability of count-based methods, like point counts. Of particular importance is the assumption of equal bias between surveys. Factors such as variable weather and changes in observers can influence whether this assumption can be met.

The quantification of sampling intensity and effort is fundamental to the use of indices and relative abundance measures. This way the assumption of equally bias surveys between areas and over time can be met. In addition, the usefulness of indices depends on the precision of estimates. It is strongly recommend that power analysis procedures be integrated into the study design of all these techniques. As described in Species Inventory Fundamentals, manual No.1, Appendix G, programs such as MONITOR, POWER AND PRECISION, and NQUERY are user friendly, and can be easily used in an adaptive fashion to calculate sample sizes needed for the ultimate analysis questions.

If studies are designed appropriately the following general analysis methods can be used (Table 7).

Table 7. RIC objectives and analysis methods for relative abundance data.

Objective	Analysis method1	Programs2
Trends in abundance over time	Sample methods Regression techniques Power analysis	Generic statistical packages MONITOR
Comparison in abundance between areas	ANOVA, method Power analysis	Generic statistical packages Power analysis software
Determine whether habitat modifications have altered population size	T-test method Power analysis	Generic statistical packages Power analysis software

¹See manual No.1, Section 5, for more details on analysis techniques.
²See manual No.1, Appendix G, for more detail on software packages.

Difficulties with count data: One inherent problem with count data is that they are rarely normally distributed, which makes the application of parametric statistical methods risky, especially if sample sizes are low. Before data are used in parametric tests, the assumption of normality should be tested. Transformations may make frequencies nearly normal in some cases. For a detailed discussion of analysis of count data, see manual No. 1, Species Inventory Fundamentals, Section 5. White and Bennets (1996) introduce an alternative method for point count analysis and use songbird counts as an example of this analysis technique.

Trend analysis: The basic method for determination of trends is linear regression and associated techniques. There are a variety of refinements to linear regression that can be used with data depending on sampling assumptions and other characteristics of the data. Manual No.1, Section 5, provides a detailed discussion of these techniques.

Comparison between areas: Parametric tests and other methods can possibly be used to compare areas if surveys are conducted concurrently. If surveys are conducted non-concurrently (such as in different years), then the results might be biased by population fluctuations. See manual No.1, Section 5, for a thorough discussion of analysis of count data.

Habitat based inference: Logistic regression or similar methods can be used to test for habitat associations, but this approach requires that habitat units be the primary sample unit as opposed to population units.