On September 14, 1896, on a smallhill outside of Madison, Wiscon-

sin, Orin Libby counted 3600 callsfrom night-migrating birds in fivehours of listening (Libby 1899). Thiscount is the first published record ofan attempt to quantify the night flightcall phenomenon of North America’savifauna. Native people in Wisconsinlikely heard nocturnal bird migrationsduring the previous 10,000 years.

In the 50 years after Libby’s report,only two other counts of nocturnalflight calls were reported, those byPaul Howes (1914) counting Swain-son’s Thrushes [then called Olive-backed Thrush] in Connecticut andthe extraordinary 15-year study byStanley Ball (1952) counting migrat-ing thrushes in eastern Quebec. Ball’swork was the first to use nocturnalflight call monitoring to produce dataon the migration timing of species in aregion.

Beginning in the 1950s, regular noc-turnal flight call counts were reportedfrom regions throughout easternNorth America in the Audubon FieldNotes. Most of the reports were tallies

of call totals or of estimated numbersof Catharus thrushes passing over dur-ing a portion of an evening. One out-standing effort is the long-term workby Ron Weir who has been countingthrush calls in fall migration nearKingston, Ontario for more than 25years (see American Birds, Lake On-tario region, fall reports).

In the 1950s technological develop-ments enabled the first audio record-ing of nocturnal bird migration. BillGunn recorded some short periods ofcalling from the top of the ImperialOil building in downtown Toronto inlate September 1957. At about thesame time, the team of RichardGraber and Bill Cochran were begin-ning their pioneering nocturnal flightcall study in central Illinois (Fig.1).Some recordings of both of these ef-forts are archived at the Macauley Li-brary of Natural Sounds at CornellUniversity.

Graber and Cochran were associ-ated with the Illinois Natural HistorySurvey. Cochran was an electrical engi-neer and designed the recordingequipment. He was able to configure a

15

Monitoring Avian Night Flight Calls—The New Century Ahead

William R. EvansOld Bird, Inc.

605 W. State StreetIthaca, NY 14850607. 272. 1786

wrevans@clarityconnect.com

16 Monitoring Avian Night Flight Calls—The New Century Ahead

reel-to-reel tape recorder to record fortwo hours and to automatically recordten minutes out of every hour. Byrecording ten minutes out of everyhour all night long, the device wasable to sample night flight callingthrough the course of an evening.This allowed the researchers to sleepwhile the machine automaticallyrecorded sounds in the night sky. Theywould later listen to the recordingsand log any bird calls. The method al-lowed them to study the quantity, andto some degree, the species composi-tion of calling during the course of anight, throughout a migration season,and in different weather conditions.Like Ball, they documented the sea-sonal timing of migration for a num-ber of species and their work laid theconceptual foundation for machine-

based recording of nocturnal flightcalls (Graber and Cochran 1959 &1960).

In his classic 1968 paper, Graber dis-cussed the potential of acoustic moni-toring but pointed out several majorimpediments. One was the labor nec-essary for the analysis of the calls andthe other was deciphering the identityof the many unknown callers. Theconcept of night flight call monitoringclearly existed in the 1960s, but themethod would need some technologi-cal developments before it was readyto advance further.

On a late May night in 1985, I wascamped on a bluff along the St. CroixRiver on the Minnesota/Wisconsinborder. It was a beautiful late springevening with a light wind from thesouth. Shortly after I settled into my

Figure 1. Dick Graber is pictured with night flight call monitoring equipment designed by BillCochran at the Illinois Natural History Survey in the late 1950s.

sleeping bag my attention was drawnto the faint sounds of birds in nightmigration. A decade earlier my fatherhad pointed out this night flight call-ing to me when I was a teenager grow-ing up in southern Minnesota, but Ihad never heard it again until thisnight. With nothing else on my mind,I tuned in and was enthralled in won-der for hours by the beauty of voicestrickling down from the passing riverof birds.

For the past 20 years I have beenstudying the night flight calling phe-nomenon across eastern North Amer-ica. I was motivated by the idea that all-night recordings of events like the oneI heard in my camping experiencewould be of interest to people in thefuture. I remember being very in-spired by Audubon’s account of seeinghoards of migrating Passenger Pigeonsdarkening the skies over southernOhio back in the early 19th century. Iremember the sadness evoked from re-alizing I would never get to see thatoriginal North American wilderness.But then I remember the change to aparadoxical optimism in realizing thatthe wilderness was only likely to deteri-orate further and that at least I washere now and there were still lots ofbirds migrating. I believed that the all-night, all-season recordings of aviannight flight calls would in some way re-flect the species composition andnumbers of birds on the continentnow, and that someone might appreci-ate this information hundreds of yearsfrom now.

By 1985, a lot had changed techno-logically since Graber and Cochran’srecording efforts in the late 1950s.The consumer electronics revolutionhad begun and for $1000 one couldbuy a portable hi-fi Video Cassette

Recorder (VCR) and make a record-ing of 8-hours of good quality stereosound. For the next ten years I wouldrecord and listen to thousands ofhours of audio tape recordings of thenight sky.

A sign of the next technological ad-vance was reported in 1987 by VolkerDierschke, studying the nocturnalflight call phenomenon on HelgolandIsland in western Germany (Dierschke1989). Peter Kaetsch, an engineerworking on the project, developed avoice-activated nocturnal flight callrecording system. The tape recorderwas activated when it received a signalof specified loudness in the 3–8 kHzrange. This conserved audio tape andalso saved analysis time in allowingone to bypass the long sections ofnight when no calling occurred.

I have no regrets about the thou-sands of hours I spent listening toaudio tapes, as this experience con-vinced me more than ever that there isutility in monitoring avian night flightcalls. But, the idea of automating thecall detection and extraction processhad been in my sight since my listen-ing experience in 1985. This dreambegan to come together in 1994 whenI teamed with the Cornell Lab of Or-nithology’s newly formed BioacousticsResearch Program (BRP) to work onthe development of automatic noctur-nal flight call detectors. Under the di-rection of Dr. Chris Clark, BRP was pi-oneering the development ofcomputer-based (digital) acousticanalysis software. BRP programmerHarold Mills wrote the first functionalnocturnal flight call detector in 1994(Mills 1995). This software for Macin-tosh computers allowed automatic de-tection of the short, high-pitchedflight calls of warblers and sparrows.

The Passenger Pigeon, Vol. 67, No. 1, 2005 17

This development allowed the auto-matic extraction of their flight callsfrom audio tapes or in real-time di-rectly from an active microphonepointed at the sky.

BRP’s spectrographic analysis soft-ware called Canary, written by SteveMitchell, was then used to study andhelp discriminate similar calls. Time-frequency spectrograms allow one tosee how the frequency of flight callsvaried with time, and Canary enabledone to quickly make a spectrogram ofa call and make measurements of itsfine acoustic features. It was the toolthat opened the door to breaking thecode of nocturnal flight call identifica-tion.

Back in 1990 I had released anaudio cassette tape identificationguide to the flight calls of night-mi-grating thrushes. Pirated copies of thetape made the rounds of the birdingunderground and as I set my sights onthe daunting task of completing thewarblers and sparrows, I was joined inthat effort by Michael O’Brien, an ar-dent night flight call enthusiast fromRockford, MD. Ten years later wewould release a CD-Rom guide to theflight calls of landbirds in easternNorth America (Evans and O’Brien,2002).

In 1998, my myopic focus brought onthe creation of an organization dedi-cated solely to night flight call monitor-ing, and I founded the nonprofit or-ganization called Old Bird. Old Birdcontracted former BRP programmerSteve Mitchell to develop advancednight flight call analysis software. Newdetectors were written that enabled op-eration on the more common PC com-puters. This included the first detectorfor thrush calls. In addition, softwarewas developed for automatic recogni-

tion of one of the most distinctive smallpasserine night flight calls in easternNorth America, the Dickcissel. UsingDickcissel call detection software, thefirst transect of automated night flightcall recording stations was establishedin spring 2000 at a network of highschools in south Texas (see www.oldbird.org/dicks.html).

A big advance at this time wasMitchell’s development of the softwarecalled GlassOFire that greatly facilitatedthe sorting of extracted bird calls intouser-determined species categories.

Now, 20 years after I bought my firstVCR, a similarly capable VCR costsonly $50. But, VCRs have become al-most obsolete for night flight callrecording. Advances in the speed, soft-ware, and information storage capac-ity of computers make it much moreefficient to record sound directly to acomputer. Having a digital sound fileallows one to run the automatic calldetection software directly on the file.For a typical modern PC it takes about20 minutes to extract the warbler andsparrow calls on an 8-hour sound file.This way one can capture and archivethe whole night and use different soft-ware to extract different speciesgroups, and apply new software as it isdeveloped in the coming years.

Given the rapid advances in tech-nology in the last 20 years, one canimagine that in the next 20 years therewill be software for automatically iden-tifying many of the species and thatthere will be automatic monitoringnetworks across the continent. I haveno doubt that birders will somedaywake up in the morning and, with acup of shade-grown coffee in hand,tune in to continental images of theprevious night’s migration for manysongbirds across the Americas.

18 Monitoring Avian Night Flight Calls—The New Century Ahead

NEXRAD radar allows us to see themass migrations now, but acoustics isthe only means we have for resolvingthose masses to species.

In the meantime, the study of aviannight flight calls is only in its infancyand holds a lot of promise. Right nowthe phenomenon is wide open for ex-ploration and interesting discoveriesare likely to be made by anyone whoundertakes monitoring. Indeed, thereally exciting discoveries are likely tocome when birders start monitoringfrom their homes and start comparingdata with one another. Building anacoustic monitoring station is also agreat school project, and more than

30 high schools in the eastern U.S.have been involved with night flightcall monitoring.

The www.oldbird.org website has in-structions and the design for a simplebut effective microphone for pickingup night flight calls of migrating birds.It costs about $30 and takes an hourand a half to build. The software forcall extraction and analysis can all bedownloaded from the website. A spec-trogram library of common call typesis available as well as many publica-tions and reports on night flight callmonitoring.

Besides the scientific value of nightflight call monitoring, it is a great way

The Passenger Pigeon, Vol. 67, No. 1, 2005 19

Figure 2. Locations of night flight call monitoring station in the upper Midwest infall 2000 and 2001.

to build your yard list. All sorts of in-teresting birds that we would never seeduring the day pass over in night mi-gration: rails, bitterns, Upland Sand-piper and other shorebirds, waterfowl,and many uncommon songbirds.More than 100 years after Orin Libbytallied thousands of flight calls in anight near Madison, Wisconsin, it isstill possible today to hear thousandsof calls on big migration nights in Wis-consin, though the species propor-tions have likely changed.

In hopes of stimulating others to ex-plore night flight call monitoring inWisconsin, I present some data belowthat were recorded at two upper Mid-west sites in 2000 and 2001 (Fig.2).The Minnesota station was on the roofof my dad’s house in Oronoco, MNand the Wisconsin station was on theroof of the Green Bay Water FiltrationPlant, about 15 km east of Green Bay.The Minnesota station operated in thefall of 2000 and 2001. The Wisconsinstation operated in the fall of 2001.Both stations used the microphone de-sign on the www.oldbird.org websiteand both operated between August 18and October 17. Monitoring occurred

nightly from 8:30 p.m. to 4:30 a.m.EDT (8-hrs./night).

Warbler and sparrow calls were ex-tracted with Old Bird’s Tseep detectorand call totals and temporal occur-rences of four species were analyzed:Canada Warbler (Wilsonia canadensis),Black-and-white Warbler (Mniotiltavaria), American Redstart (Setophagaruticilla), and White-throated Sparrow(Zonotrichia albicollis). Typical flightcall spectrograms of these species areillustrated in Figure 3.

Table 1 shows the total number ofwarbler and sparrow calls detected ateach acoustic station in comparison tothe total number of calls detected foreach of the four species studied. Fig. 4shows the proportion of each species’calls during the nearly two monthmonitoring period.

These preliminary data show someinteresting patterns. Black-and-whiteWarbler and Canada Warbler callingeach composed around the 0.5 per-cent of the total calling. This was truein the fall of 2000 and 2001 in Min-nesota and in the 2001 fall season inWisconsin. White-throated Sparrowcalls composed 1.5–2.0 percent of all

20 Monitoring Avian Night Flight Calls—The New Century Ahead

Table 1. Total calls and calls of four species detected from August 18 through October 17 at acousticstations in Minnesota (2000 & 2001) and Wisconsin (2001). AMRE = American Redstart, WTSP =White-throated Sparrow, BAWW = Black-and-white Warbler, CAWA = Canada Warbler. Monitoring oc-curred nightly from 8:30PM to 4:30AM (8-hrs./night).

Fall 2000—MN Fall 2001—MN Fall 2001—WI

AMRE 571 511 204

WTSP 141 234 160

BAWW 44 72 28

CAWA 85 41 49

Total calls 9671 11377 8525

calls. This was also similar betweenboth seasons in Minnesota and the sin-gle season in Wisconsin. AmericanRedstart had the largest percentage ofcalls at both stations, but the percent-age was distinctly higher at the Min-nesota station for these species in bothyears (between 4% and 6% of totalcalls in Minnesota compared with lessthan 2.5% in Wisconsin). This sug-

gests to me that American Redstartsmay be relatively more common inwestern Wisconsin than in easternWisconsin. There are certainly too fewdata here to make any firm conclu-sions, but I pass it along in hopes ofenticing someone to try to confirm ordisprove the pattern.

Here are a few notes to consider be-fore you embark on the road to estab-

The Passenger Pigeon, Vol. 67, No. 1, 2005 21

Figure 3. Distinctive night flight call spectrograms of four species studied in the 2000 and 2001 falldata from Minnesota and Wisconsin.

Canada Warbler flight call

American Redstart flight call White-throated Sparrow flight call

Black-and-white Warbler flight call

lishing a night flight call monitoringstation. There are two important con-siderations in determining a locationfor an acoustic monitoring station.One is extraneous environmentalnoise. If your location is near a popu-lation of Spring Peepers (Hyla crucifer),this will impede your ability to hearand also to automatically detectthrushes. If your location is close toone or more singing insects, this mayimpede your ability to detect warblerand sparrow calls. A distant insect cho-rus is usually present everywhere inthe fall migration and that is usuallynot a problem. What can be a majorproblem, especially if you want to useautomatic call extraction software, is ifyou have individual crickets, etc.singing close to your microphone.Their individual calls can be similar induration and frequency to warbler and

sparrow calls. In such a case, you couldend up with thousands of false detec-tions in an evening and a more time-consuming task in separating bird callsfrom false detections. Oddly enough,proximity to highways and road noiseis not a problem, especially if you arejust monitoring warblers and spar-rows. The calls of these species havefrequencies above the lower rumble oftypical auto traffic.

Artificial lighting is an importantfactor that can cause disoriented indi-vidual birds to call more rapidly whenthere is low cloud cover. Such condi-tions can be detected by the appear-ance of abnormally high call rates. Atell-tale sign is if an individual warbleror sparrow gives a series of calls, eachless than a second apart. In general,residential lighting at night does notcause this phenomenon, but it may be

22 Monitoring Avian Night Flight Calls—The New Century Ahead

Figure 4. Comparative percent of calling from four species studied in the 2000 and 2001 fall datafrom Minnesota and Wisconsin. AMRE = American Redstart, WTSP = White-throated Sparrow,BAWW = Black-and-white Warbler, CAWA = Canada Warbler.

caused by brightly lit conveniencestores, parking lots, and city areas. Infall of 2001, besides the acoustic mon-itoring station on the Green Bay WaterFiltration Plant, I had another acousticstation on the west shore of LakeMichigan, 15 km east-northeast of thewater filtration station. The site of therecording station was Algoma HighSchool. The school was brightly lit atnight and on cloudy nights huge call-ing events were recorded at this sta-tion. On several nights more than5000 warbler and sparrow calls wererecorded, undoubtedly many callsfrom the same individuals. Yet onthese nights the water filtration stationand other sparsely-lighted acoustic sta-tions in the area only logged severalhundred calls.

It is difficult making comparisons ofraw call totals between differentacoustic stations. Differences in equip-ment and environmental variablescomplicate the comparison by typi-cally causing one station to have moreor less background noise. The call de-tectors work by sensing a certainthreshold of sound energy above thebackground noise in a specified fre-quency band. So, if one station hastwice the background noise as an-other, that former station will not de-tect weak calls that the latter stationmight.

However, if one is using reasonablysensitive microphones, one can com-pare the ratio of the number of callsdetected from a species to the totalcalls detected and/or the ratio be-tweeen different species’ calls. In thesecomparative methods, the equipmentand environmental variables tend todrop out. The ratio data between thenumber of calls of different speciescan even be compared to the ratios ob-

tained from banding stations if appro-priate species are selected. For exam-ple, it would be interesting to see howpasserine banding ratios from Wiscon-sin compare with the night flight callratios presented in this paper. AreCanada Warbler and Black-and-whiteWarbler banded in approximately oneto one ratios? Of course, in order todo this, one would need to evaluateother variables like whether the netswere set so as to have a good chance ofcatching both species. It is unlikelythat comparing the proportion of call-ing from one species with all calls de-tected would compare favorably with asimilar banding comparison. This isbecause many species (e.g., grasslandsparrows) are detected acousticallythat would not be represented by band-ing data. Conversely, some species willbe banded that are not known to giveregular flight calls (e.g., vireos, catbird).

The flights will be happening againsoon. At this point, I simply must en-courage you to get out and listen upor, if you are moved to do so, buildyour own monitoring station and startnetworking with others. I concludewith a beautiful description of thenight flight call phenomenon fromLibby (1899).

Nothing but an actual experience of asimilar kind can at all adequately conveythe impression produced by such obser-vations. The air seemed at times fairlyalive with invisible birds as the calls rangout, now sharply and near at hand, andnow faintly and far away. Repeatedly itseemed as if some of the nearer onesmust be visible, so vividly was their pres-ence felt as they passed overhead. All va-rieties of bird calls came sounding outof the darkness that evening. The harshsquawk of a water bird would be fol-lowed by the musical chink of the

The Passenger Pigeon, Vol. 67, No. 1, 2005 23

23

Bobolink. Almost human many of themseemed, too, and it was not difficult toimagine that they expressed a wholerange of emotions from anxiety and fearup to good-fellowship and joy. The fineshrill notes of the smaller Sparrows orWarblers were heard only close at handbut the louder ones came from all alongthe line, east and west. More than oncean entire flock, distinct by the unity oftheir calls, came into range and passedout of hearing, keeping up their regularformation with the precision of a swiftlymoving but orderly body of horsemen.The great space of air above swarmedwith life. Singly or in groups, large andsmall, or more seldom in a great throngthe hurrying myriads pressed south-ward. It was a marvel and a mystery en-acted under the cover of night, and ofwhich only fugitive tidings reached thelisteners below.—Orin Libby 1899

LITERATURE CITED

Ball, S. C. 1952. Fall bird migration in the GaspePenninsula. Peabody Museum of Natural His-tory. Yale University Bulletin 7: 1–211.

Dierschke, V. 1989. Automatisch-akustische er-fassung des nachtlichen vogelzuges bei Hel-

goland im Sommer 1987. Die Vogelwarte 35:115–131.

Evans, W.R. and O’Brien M. 2002. Flight calls ofmigratory birds—eastern North AmericanLandbirds. CD-Rom. Old Bird, Inc. www.oldbird.org

Graber, R. R. and W. W. Cochran. 1959. Anaudio technique for the study of nocturnalmigration of birds. Wilson Bulletin 71:220–236.

Graber, R. R. and W. W. Cochran. 1960. An eval-uation of an aural record of nocturnal migra-tion. Wilson Bulletin 72: 253–273.

Graber, R. R. 1968. Nocturnal migration inIllinois—different points of view. Wilson Bul-letin 80: 36–71.

Howes, P. G. 1914. Fall migration of the Olive-backed Thrush, 1912. The Oologist 31:162–167.

Libby, O. G. 1899. The nocturnal flight of mi-grating birds. Auk 16: 140–145.

Mills, H. 1995. Automatic detection and classifi-cation of nocturnal migrant bird calls. Jour-nal of the Acoustical Society of America 97(5). [Abstract].

Bill Evans lives with his wife and son infourth-growth habitat in central New Yorkstate. He is director of Old Bird, Inc., a501(c)(3) non-profit organization whoseprimary research is in using acoustics as apopulation monitoring index for varioussongbird species and for mitigating birdmortality at tall manmade structures.

24 Monitoring Avian Night Flight Calls—The New Century Ahead

Red-legged Honeycreeper, pair