RECOMMENDATIONS FOR BANDING OF NOCTURNALLY MIGRATING OWLS FOR THE PURPOSE OF MONITORING POPULATION CHANGE - Erica H. Dunn, Canadian Wildlife Service - Project Owlnet - Northern saw-whet owl

RECOMMENDATIONS FOR BANDING OF NOCTURNALLY MIGRATING OWLS FOR THE PURPOSE OF MONITORING POPULATION CHANGE

Saw-whet Owl population monitoring. Changes in the number of owls migrating through a site each year are indicators of change in population size. As long as banding effort is standardized, banding totals can be used to detect long-term trends in owl numbers. For Saw-whet Owls, migration counts may be the best source of trend information available, because these birds are poorly sampled by other population surveys, such as the Breeding Bird Survey.

Following the recommendations below will ensure that your data set will be useful for population trend analysis. Not all owl banders will be willing or able to follow these recommendations, and indeed, such standardization is not needed to accomplish many of the other goals of owl banding. However, if you already do intensive banding and could easily modify your methods to make your data set suitable for future trend analysis, we hope you will consider doing so.

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 Erica.Dunn@ec.gc.ca 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, http://www.rsl.psw.fs.fed.us/pif/int_prot.html). 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.

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.

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:

	Number of owls caught in:
Site	Year 1	Year 2	% difference between years	Tape lures used?
A	20	18	-10	No
B	100	90	-10	Yes
C	50	75	+50	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.

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.

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.

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.

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 turnover of personnel, so ability to attract new volunteers is important. Habitat stability is also an important issue in site selection (see #4).

Dunn, E.H. and D.J.T. Hussell. 1995. Using migration counts to monitor landbird populations: review and evaluation of current status. Pp. 43-88 In D.M. Power (Ed), Current Ornithology, Vol. 12. Plenum Press, New York.

Hussell, D.J.T. and C.J. Ralph. 1996. Recommended methods for monitoring bird populations by counting and capture of migrants. Unpubl. report of the North American Migration Monitoring Council.

MOCK-UP OF A FIELD PROTOCOL FOR MIGRATION MONITORING OF NORTHERN SAW-WHET OWLS

This example is meant to illustrate the type of information that should be included in a good protocol and the amount of detail necessary to ensure long-term standardization. It is not meant to recommend particular practices over others. Most of it has been made up, but certain sections are quoted from Granlund, J., 1996, "Methodology for monitoring owls during the fall migration at Whitefish Point Bird Observatory," and from Wojnowski, J.K., 1997, "Migration monitoring at Thunder Cape Bird Observatory in 1997."

GENERAL METHODS: [example only--modify to briefly give overview of the program at your own site]

The owl banding program is run from ___ to ___ [name range of dates that covers migration at the site], for X hours each night starting one-half hour after sunset. Two arrays of 4 nets are set up, each array in the form of a square with one net on each side. A speaker is placed in the center of each square, connected to a tape player that continuously broadcasts calls of Saw-whet Owls (the principal target species). Eight additional nets are scattered through the netting area. All nets are raised on extra-long poles with a pulley system to put them beyond the reach of ground predators. All owls captured are banded, and complete banding data is recorded on forms provided, along with netting effort. Safety of the birds is paramount, and the protocol should be temporarily abandoned if safety is compromised.

At the peak of migration, if interest and number of personnel warrant, banding may continue for more than 4 hr, but all records for continued banding are kept separate from those for the standardized coverage period.

DETAILED PROTOCOL FOR STANDARD COVERAGE PERIOD (See end for special instructions regarding start and end of season. General instructions for care of nets, extracting birds, handling and measuring birds, etc. can be found in [give reference: e.g. http://www.psw.fs.fed.us/publications/Documents/gtr-144/04-constant.html; or, prepare a separate manual of your own on this topic].

In general, trapping should occur on all suitable nights between ___ and ___ [name dates of migration at the particular site].

A minimum of two qualified people per night is recommended (one a licensed bander and both experienced in extracting owls from nets). If no one else is available, the station can be run with a single fully qualified person, but nets must be closed promptly if bird volume is too much for one person to handle. [Set criteria appropriate to your own site.]

Trapping should not be undertaken in rain, wet snow (light dry snow is acceptable) or very strong winds, or in the presence of a predator. It is difficult to set absolute rules for wind interference as the effect depends on wind direction and density of leaves on the surrounding trees, but force 6 or more is too strong at any time.

[Give directions to site.] A map of the site is shown in Figure 1, and the placement of net poles is shown in greater detail in Figure 2. [Maps should give sufficient detail so that net poles removed between seasons can be placed in exactly the same locations the next year by a person who has not been to the site before. Show the location of any sanitary facilities.] All banding is done in the garage, about 200 meters west of the net site [or whatever is done at your site].

3. Pre-banding set-up [example only--modify to describe system at your own site]

Plan to arrive a half hour before sunset to allow time for set-up. Banding and broadcast equipment are stored in the banding garage.

Put the battery and tape player on the plywood sheet (battery discharge will be less than if on the ground). Attach the leads from the tape player to the battery. Cover the tape player with the plastic bag to protect it from the elements (including condensation on dry nights). Speakers are placed at the centre of each square on the overturned milk crates. The speaker in the western square is pointed to the north, while that in the eastern square is pointed south (see map for compass directions).

Test the tape player, adjusting volume so that the tape, played at the square, is just audible to a listener situated at a point 100 metres away (see Figure 1 for location of listening post). Make sure the settings are at the following positions to reduce distortion of sound [give details]:

4. Banding protocol [example only--modify to describe system at your own site]

Open nets one half hour after official sunset. Times of sunset for the season can be found in the front of the banding notebook. Run for approximately four hours (four 45-min. tape cycles--see below).

Start the tape once nets have been set up, and record the time. Nets should be checked three-quarters of an hour later. The tape should be stopped at both squares before birds are extracted, partly to prevent owls approaching when people are around and partly to reduce feistiness in the birds, which seems to be higher if the tape is still playing. Restart the tape once birds have been extracted, and check nets again 45 min. later. Every time the tape is started or stopped, record the time on the record sheets. After the equivalent of four 45-minute broadcasts has been completed (i.e. 3 hours total broadcast time), the standard monitoring period comes to an end.

Birds will be transported to the banding garage for immediate processing, either in a bag (small owls) or a can. Release owls by placing on the ground outside the banding garage.

Record all information called for on the special banding sheet for owls. Record all information on retraps (once per night--release bird after checking band number if it has already been handled once that night). If a bird is retrapped in a standard 4 hr period that was banded outside standard hours on a previous night, enter its number in the appropriate place on the record sheet for the current night's standard period. (These are counted as "new" birds in trend analysis.)

A sample data sheet for recording coverage data is shown in Figure 4. It must be completed for every owl banding session, even if no owls are caught or if the session is terminated early. [Banding protocol will vary among stations, but all written protocols should include a sample data sheet.]

The bander in charge is responsible for all decisions to close down operations. Netting should stop if:

A bird that is injured and unable to fly should be held overnight. If it has a simple wing sprain, the bird may be released the next night; but if longer rehabilitation is required, it should be transported to [give name and address of an approved facility].

6. Closing down [example only--modify to describe system at your own site]

Furl nets and put on at least 5 ties to hold in place. Return broadcast equipment to banding garage. Attach battery to charger. [If banding site has no electricity, give instructions for frequency and location of recharging.]

The next morning, report to the project coordinator any difficulties with broadcast equipment, nets, headlamps or any other equipment, and any shortage of banding supplies.

APPENDIX I. Starting and ending the season [example only--modify to describe system at your own site]

Start-up: bring maps, nets, 15 foot poles, guy ropes, pulleys, step ladder, flagging tape, warning signs, hammer, nails, saw, brush clippers, 2 milk crates and a 2X3 sheet of 1/2" plywood.

Locate net pole locations according to Figures 1 and 2, and renew the flagging tape on the landmarks if necessary. Clear away debris or vegetation from paths and net lanes. Guy the poles to nearby trees from the top (illustrated in Fig. 3), so that net loops can slide the length of the pole. Pulleys are attached as in Figure 3, so nets can be raised and lowered easily. The poles are placed to leave gaps of about 1 meter width at each corner of the net squares, to allow ready access to the interior. Put nets on poles no earlier than 5 days before the first banding session.

Set up the warning signs for visitors (see Fig. 2). Place the plywood sheet and milk crates in the locations shown on Fig. 2.

Close down: Ensure that landmarks are clearly marked. Remove nets, poles, guy ropes, warning signs, plywood sheet and milk crates. Store all equipment in banding garage except for nets, tapes and the tape player, which should be delivered to the project coordinator for checking and testing prior to storage.

APPENDIX II. Specification of equipment [example only--modify to describe equipment and sources used by your own station]

Mist nets are black nylon, 4-panel, 50 denier, 7'x60' with 1 1/2inch mesh, tethered on the top shelf string.

The tape player used to broadcast the Saw-whet call is a Sparkomatic cassette stereo deck SR37 with three-band graphic equalizer and was purchased at Canadian Tire. It is designed for use in vehicles and runs off a 12-volt supply. It produces 9 watts of power at 10% (RMS) total harmonic distortion (THD) and 7.5 watts at 1% THD. The speakers are Sparkomatic SK690 and came with the deck. They are bolted to an aspenite baffle, approximately 25 x 50 cm, mounted on a frame constructed of 2"x4" lumber.

The lure tape contains a Northern Saw-whet Owl call recorded from "Bird Sounds of Canada Vol. 1" by Monty Brigham (in cooperation with the Canadian Museum of Nature), Holborne Distributing Co. Ltd., P.O. Box 3095, Mount Albert, Ontario, L0G 1M0. The call used is the familiar whistle repeated approximately 95 times a minute. It was recorded from compact disc to a Radio Shack 1-minute continuous play cassette, then transferred on to both sides of a 90-minute Maxell II cassette. The project coordinator records a new tape for each season.