Project Owlnet - Owl Migration Monitoring Methodologies - protocols - For Mist Netting or Bird Banding Studies - mist net

All banding or other handling of owls is strictly governed by Federal, State and Provincial laws. Please contact your local wildlife agencies for information regarding permits before beginning any studies. You may also obtain information on federal permits for both Canada and the United States from the Bird Banding Lab of the North American Bird Banding Program.

These methods have been developed by Project Owlnet to assist others in beginning a productive study of saw-whet owl migration. We realize that any study may need to be customized for the particular project at hand. However, adopting the methods of the Project can produce comparable results to be used in regional analysis of owl movements. The measurements requested will be useful as we develop the methods for aging and sexing this little known species. Flexibility is encouraged and the information below is meant to be a framework for creating indivudual studies. We strongly stress that once a protocol is established, after some initial adjustment, that it be strictly followed to produce consistant and meaningful data. These methods can also be used as guidance towards the study of other nocturnally migrating owls.

Saw-whet owl in mist net

Nets: We use 12 meter-61mm mesh mist nets, commonly referred to as "thrush nets". A suitable source is the CTX net available from the Association of Field Ornithologists. With net supply and prices what they are now, another alternative is "Spidertech" in Finland. We have some of their nets and, despite the less than glowing review from Manomet, we are pleased with them (after some initial attention). Larger mesh nets ("duck nets", 121mm, ETX) let too many saw-whet owls through. Smaller mesh "dickey bird" nets will catch saw-whet owls, but the smaller mesh size lets more owls back out than thrush nets.

Net Array: A line of 6 or 7 mist nets is used at most Maryland sites. The net line is basically oriented east-west or perpendicular to the shoreline along coastlines. The Lamb's Knoll site has placed nets perpendicular to the main net line at its midpoint to intercept owls that bounce out of nets and/or fly parallel to the main net line. These nets are productive at this site, but have been found to catch relatively few birds at other Maryland sites. Similar arrangements are also being used near Cape Charles, VA; Hawk Mountain, PA; and at sites in Maine, Rhode Island and West Virginia.

Assateague Island, MD nets

At most Maryland sites each net is 2 nets high, i.e. 12 or 14 nets total. About 60% of captures are in the bottom net and 40% in the top. The one question that we can not answer is how much captures would decrease if the top net were eliminated. Audio lures attract owls to the netting area and may keep them there long enough to effectively negate most of the owls lost by not using a 2-net high set, but we'll probably never be able to quantify the difference. In Maryland, the Cassleman River station and the Lamb's Knoll station have similar capture rates, but they use double and single height nets respectively.

The minimum recommended net array should consist of no less than 4 single height nets with an audio lure placed at the junction of the two middle nets. This would net at least 40-45% of the captures expected from a full double high 6 net array. Increasing the 4 net array to double high nets increases the captures to approximately 65% of a full array.

It has been found that recaptures and smaller owls (possibly males) tend to be caught in the outer reaches of a six net, single height line (1). This may also increase the likelihood of recoveries from other banders. Some sites are using a box or triangle of nets around the lure. Further investigation is needed to determine the effectiveness of these arrays. Good documentation of location of captures in the nets as well as direction of travel is essential to designing a good net array. Changes may be made to maximize captures in the first few years of operation, however it will soon be necessary to maintain a consistant setup to standardize the data collected.

Finzel Swamp, MD nets

Habitat: Any wooded area is a potential place to net. We know very little about habitat preferences of Northern Saw-whet Owls during migration. We suspect that fields are not the most productive places to net.

From what is known of habitat preferences we can suggest potentially more productive habitats. Forests with higher composition of species providing good cover have proven to be good places to net migrant saw-whet owls and locations such as forests with significant Eastern Hemlock occurrence, Appalachian Mountain Red Spruce and coastal Loblolly Pine. Shrub swamps, such as coastal shrub communities and alder/heath swamps in the Appalachian Mountains, have also been productive. Another possibility is hardwood forests with substantial cover from rhododendron and Mountain Laurel. (Lamb's Knoll is heavily oak with a thick laurel understory).

Audio lure: Use of an audio lure is essential to reasonable success and banding of a serious sample of owls. To be effective the sound system used as an audio lure should produce 90-110 dB of sound. Audio lures are usually placed near the middle of the net line with 2 speakers 1-2 m off the ground, one facing in a northerly and the other southerly direction.

This can be accomplished in various ways. The simplest is with a good quality "boom box". This would be the most workable for exploratory efforts. The drawback of boom boxes is that they eat batteries when used all night long, night after night. While a little more complicated, using a 12v deep cycle wet cell (conventional RV or Marine battery) in conjunction with an inexpensive voltage reducer to power a boom box is much more workable. This alternative is more expensive, but it pays for itself in the long run through convenience, reliability and rechargability.

Audio-lure box used in many Maryland sites

A combination of a wet cell, voltage converter, small tape player (hand held, not a boom box), auto stereo amplifier and outdoor speakers is what we use in Maryland. While more expensive, these are long lasting, convenient, and weather proof. At one site we have connected a solar cell to an audio lure to eliminate the need for recharging the wet cell every 7-10 days. A schematic, including Radio Shack part numbers, for wiring this combination can be found on the internet copy of the Second International Symposium on the Biology and Conservation of Owls of the Northern Hemisphere. Scroll to the section on Poster Papers and Other presentations. The relevant source is Increasing Mist Net Captures of Northern Saw-whet Owls with an Audio Lure as presented by Thomas C. Erdman and David F. Brinker, page 533.

The most sophisticated solution is a digital system with no moving parts. We are currently trying one of these and they look very promising. They produce exceptional sound quality, have no moving parts and draw very little current. The digital solution avoids "eaten" and worn out tapes, broken tape players, worn out tape player heads, etc. and lessens battery problems. Right now the cost is approximately $300.00 from commercial sources, but we are investigating means of significantly reducing this.

In reality, any system that produces sufficient volume will work. A colleague initially used the stereo system from his home by moving the speakers outdoors via a long wire before building an audio lure!

Lure Tape: Lure tapes consist of 3-minute loop cassettes typically used in telephone answering machines with the recording of the solicitation call of a Northern Saw-whet Owl.

We can provide tapes for US$ 7.00, plus postage, each when supplies permit, however the cost means we keep our stock at a minimum and may not have many available. We eventually hope to have a reliable source that can provide this service for us as needed and at a low cost. We currently do not produce CDs as we feel our efforts are better spent looking for an inexpensive digital system.

If you record your own tape, try not to use less than a 3-minute loop. Shorter loops (i.e. 15- or 30-seconds) make many more trips through the cassette per night and self destruct much faster! There are 6- and 12-minute loop tapes available, which have worked well for some sites. We have found that TDK brand, endless loop tapes without a sensing foil, work very well.

An alternative route to follow is to use an auto reversing tape player and regular cassette tapes (i.e. 30-, 60-, 90- minute tapes). This will require a longer initial taping session, but also works quite well from a reliability standpoint.

As a final note, the quality of the recording is of less importance than its presence and volume. The same individual that used the home stereo for exploratory work made his first tape by whistling into a tape player because he did not have a recording of a saw-whet owl. If worked fine!

Phenology: Northern Saw-whet Owls begin migrating as deciduous foliage nears peak color and ends approximately 1 month after leaf fall. Thus, migration begins during mid-September north of 45 degrees north latitude, late September to early October north of 40 degrees, and mid to late October south of 40 degrees. South of 36 degrees north latitude migration probably begins during early November. In Maryland, during most years, we operate from mid October to 1 December.

Moon phase is thought to have some effect on migration. Brighter nights seem to yield fewer owls. This may be due to the increased light of a fuller moon making the nets more visible. It might also be that the owls are avoiding predation from other owls by flying high or not migrating.

Spring migration has been studied much less. Wintering birds have departed Maryland by 1 April.

Is it time to migrate? Assateague Island, MD

Weather: Ideal weather conditions typically follow the passage of a cold front. Clear, dark (little moonlight) nights with light Northerly winds are best, but any light winds are used during the peak of migration. High winds, precipitation and fog seem to limit migration.

The best time of night for captures is site, season, moon phase and weather dependent. For example, when 3 years of effort at Assateague is summarized by 3-hour blocks, 60-70% of captures occur before midnight. But on any given night, when captures will occur is very difficult to predict. At Cape May, New Jersey most saw-whet owls are captured in the 4 hours preceding dawn (a multi- year summary) with 60-70% of captures occurring after midnight. The only good way to determine this for a given site is to operate all night for several seasons. Understandably, many people can not do this and they just do the best that they feel they can.

Net Checks: Many banders make hourly net checks, increasing frequency in relation to poor weather conditions or heavy flights. In Maryland we seldom net over 10 owls a night. During 1993 Cape May had a night when 70 owls were netted. When this constant a movement is occurring constant net checking is a must. In Wisconsin, nights of 35+ owls occur several times a season at all major stations. (During the 1995 and 1999 invasions some stations netted in excess of 100 owls a night!)

Owls are very hardy and much less affected by stress than passerines and even some diurnal raptors. However, their safety can be affected by predation and extreme weather such as wind, rain, snow, or even freezing temperatures that can freeze the toes of these otherwise well insulated owls. It is important to adjust to weather conditions and resolve any predator problems swiftly. A bander faced with an unexpected large flight or poor weather can shut off the lure, effectively stopping any further captures, and close nets as they are cleared until it is safe and responsible to continue netting.

Normalization: So that comparisons can be made between stations it is useful to record three important variables, hours of audio lure time, area of nets, and height of capture, for calculation of catch per effort.

When similar sets of 6-7 double high nets are run it may be adequate to normalize as owls netted per hour of audio lure time. This is because the effect of the audio lure is so overwhelming that small differences in net area may not be significant.

To equalize for different net arrays it is also valuable to normalize as owls netted per 10 square meters of net per 100 hours of audio lure time.

If double high net sets are used, recording height of capture allows comparison of captures in the bottom net with other stations where only single high nets are used.

Measuring Wing Chord

Measurements: To facilitate comparisons, over time, of recaptured owls the following set of data should be collected from each bird: release time, age, sex, weight, tail length, natural wing chord, flattened wing chord and bill from nostril. it is also extremely useful to prepare a molt card for AHY individuals detailing old and new primaries and secondaries on both wings.

Weighing a saw-whet owl. The owls are placed in a can to calm and immobilize them.

Weighing - a 6 ounce juice can is used to imobilize and calm the owl

1 - (Whalen, David M. and B.D. Watts. 1999. The Influence of Audio-Lures on Capture Patterns of Migrant Northern Saw-whet Owls. Journal of Field Ornithology. 70:163-168)