Monitoring Recommendations

Recommendations for monitoring population change in nocturnally migratory owls through banding

1999. Erica H. Dunn, Canadian Wildlife Service, 100 Gamelin Blvd., Hull, Quebec, Canada, K1A 0H3

The use of migration counts for monitoring bird populations has been thoroughly reviewed by Dunn and Hussell (1995; contact for a copy), and there is a detailed set of recommendations posted on the web for banders who want their data to be useful for this purpose (Hussell and Ralph 1996). The latter recommendations are appropriate also for owl monitoring, but contain much additional material that does not apply to nocturnal operations. The recommendations below summarize the points most relevant to owl banders, followed by more detailed explanation.

1. Banding should be done on a near-nightly basis throughout the migration season, for a minimum of 5 years.

2. Banding protocols should be strictly standardized within each site.

3. Banding protocols need not be standardized between stations.

4. Habitat at the site should be relatively stable.

5. Retraps from previous nights should be recorded.

6. Characteristics of the site should be amenable to long-term monitoring (see details below).

To monitor nocturnal owl migration, a station should establish standardized protocols, and follow them for a minimum of five years. (©Gene Harris)


1. Banding should be done on a near nightly basis throughout the migration season, for a minimum of 5 years.

The North American Migration Monitoring Council recommends that stations should cover at least 75% of the days in the period during which 95% of migrants of a target species normally pass through a site. Computer simulations of migration count analyses have shown that fewer than 5 days/week of coverage greatly reduces the ability to detect significant population trends (L. Thomas unpubl.). Because daily counts vary tremendously according to date in the season and weather conditions, many days have to be sampled in order for analyses to take those factors into account. At least five years of data are needed before annual trend analysis can give meaningful results.

It may be difficult for many stations to achieve the level of coverage required by this recommendation. Limiting the hours of nightly coverage may make it easier to cover more nights (see below), as will recruitment of additional personnel to help share the workload.

2. Banding protocols should be strictly standardized within each site.

The number, size and placement of nets can affect the number of owls caught, as can the use of tape lures. For purposes of trend analysis, it is important not to make changes in protocol that will alter the number of owls captured from night to night or from year to year. For example, consider the following hypothetical example:

Site Number of owls caught in % difference

between years



Year 1 Year 2
A 20 18 -10 No
B 100 90 -10 Yes
C 50 75 +75 Yr 2 only

The difference between years at Site C is much more likely to have resulted from the change of capture methods than it is to reflect change in population size.

To ensure that the same standardized methods are used from year to year it is necessary to have a written protocol, sufficiently detailed to allow new personnel to duplicate the standard procedure without supervision. The protocol should include details on nets (size, mesh width, material, thread weight), exact location of each net, dates and hours of operation, number and qualifications of personnel required for operation, and so on.

If tape lures are used, the banding protocol should include a regular schedule for replacement of tapes, instructions specifying brand of equipment, criteria for volume, and very specific instructions on the amount of time that playback is to be used. Tape lures are difficult to standardize, because volume is hard to quantify and duplicate from night to night, and tapes wear out and need to be replaced. Moreover, equipment can be expensive, and failures can lead to losing nights of data. However, the resultant increases in capture rate (and therefore maintaining interest of personnel) may outweigh the drawbacks.

3. Banding protocols need not be standardized between stations.

This is surprising to many, because of the strong emphasis on standardized methods in breeding bird surveys. In the case of breeding season surveys, however, the number of birds counted at each site is assumed to represent breeding density at that location, and data are combined across many sites to estimate regional population density. It is therefore important that differences between sites not be attributable to differences in methodology.

In the case of migration counts, however, the number of birds captured at each site clearly does not represent population density of a region or a migration route. Some sites are known to concentrate migrating birds (e.g. because of geographical features), while others might capture far fewer individuals–even though they may be sampling birds from the identical portion of the breeding range. For these reasons, trend analyses of migration counts can only look at changes over time at single stations; then we can look for patterns in results across broad geographic regions. In other words, the density of birds at individual sites has no effect on estimates of regional population change. In the table above, for example, stations A and B both experienced 10% declines in the number of birds caught between year 1 and year 2, even though they used different methods and caught very different total numbers. Each one is an equally good index of population change. No assumption is made that station B samples a larger area or a denser breeding population.

The fact that field methods do not have to be identical at all sites allows each station to use the set-up most suitable for the particular site and circumstances (such as number of volunteers). Stations should experiment to determine what works best for them, then write their protocol and adhere to it thereafter.

4. Habitat at the site should be relatively stable.

The number of birds caught at a particular site may gradually change over a period of years because of alteration of habitat (e.g. trees growing taller than net height). Such changes may be indistinguishable from trends in population, and should be avoided to the extent possible. This is easiest if sites are selected that are near climax state of vegetation or are held in an earlier stage by natural causes. If regular cutting and trimming is needed to maintain vegetation at a certain stage, instructions for carrying this out should be part of the station protocol. At the very least there should be an annual written assessment of habitat changes (perhaps with photographic documentation) to help with interpretation of long-term trends in numbers.

5. Retraps from previous nights should be recorded.

Trend analyses assume that the numbers captured each night are independent samples — i.e. that birds captured tonight were not included in last night’s sample. If retrap data have been compiled, the nightly capture totals can be adjusted during analysis to account for birds stopping over.

6. Characteristics of the site should be amenable to long-term monitoring.

The site should be reasonably accessible and near a ready supply of potential volunteers, but far enough from homes to avoid disturbing people with tape lures. It should be reasonably secure (i.e. the above conditions should be expected to hold true indefinitely). While a minimum of 10 individuals captured per season is necessary for trend analysis for a given species, higher numbers are preferable both for analysis and to maintain interest of personnel.

Careful selection of a site is crucial to maintaining coverage at the high level necessary for long-term population monitoring. There will inevitably be turn-over of personnel, so ability to attract new volunteers is important. Habitat stability is also an important issue in site selection (see #4).