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INTRODUCTION

The difficulties associated to the performan-ce of direct counts of large groups of birds (e.g.colonies during the breeding season, or largewinter roosts) are well known (Tellería, 1986).The estimates arising from these counts fre-quently underestimate greatly the real numberof birds present, and errors of up to 20% can bereached when groups of around a thousand in-dividuals are counted (Cantos, 1983; Cantos &Tellería, 1985; Tellería & Cantos, 1989), a fact

that could question the validity of the final re-sults of these counts. The use of conventionalaerial photography and subsequent counts madeon the images obtained has been proposed asthe best method to mitigate these estimationerrors (Tellería, 1986). However, when dealingwith the census of large nocturnal roosts ofbirds the inherent difficulties of estimationerrors cannot be alleviated by this method. Thisis due to the fact that the largest concentrationsof individuals take place after sunset (see, forexample, Russell & Gauthreaux, 1998). In this

Ardeola 46(2), 1999, 187-193

APPLICATION OF SENSORS AND THERMAL CAMERASFOR THE CENSUS OF WINTER ROOSTS OF BIRDS

F. J. CANTOS*, M. JIMÉNEZ**, A. FERNÁNDEZ-RENAU**, J. A. GÓMEZ**, F. DE JUAN**,E. DE MIGUEL** and G. SANGLIER**

SUMMARY.—Application of sensors and thermal cameras for the census of winter roosts of birds . Whencensus are carried out by counting directly large groups of individuals, the errors associated to the estimatescan question the reliability of the results obtained. This problem is aggravated when censusing nocturnal roosts, as no conventional aerial photographs can be taken to measure estimation errors. This paper exploresthe usefulness of thermal infra-red Remote Sensing techniques to census nocturnal roosts of birds. A multi-spectral airborne scanner Daedalus 1268 and a FLIR (Forward Looking Infra-red) MKD-400NDP thermal ca-mera, installed on board of mobile laboratories were used to carry out nocturnal census of winter roosts ofBlack-headed Gulls (Larus ridibundus). The FLIR camera obtained excellent images, but in the case of theDaedalus scanner more field testing is required. The digitalisation and automatic processing of the thermalimages obtained with the FLIR during a nocturnal census in the Santillana Reservoir (Manzanares el Real,Central Spain) allowed us to make automatic counts of the roosting population of gulls present there. Howe-ver, further validation is required before using these methods to census groups of birds which roost during thenight in open country or on the water surface.

Key words: birds census, nocturnal roosts, remote sensing, thermal images, winter.

RESUMEN.—Aplicación de sensores y cámaras térmicas en los censos de dormideros invernales deaves. Cuando los censos de aves se realizan por conteo directo de grandes grupos de individuos, los errores co-metidos en las estimas pueden llegar a cuestionar los resultados obtenidos. Este problema se agrava si se tra-ta además de dormideros nocturnos en los que no se pueden utilizar fotografías aéreas convencionales paracompensar estos errores. En este trabajo se explora la utilidad de dos técnicas de teledetección infrarroja parasolventar estos problemas. Se utilizó un escáner multiespectral aerotransportado Daedalus 1268 y una cámaratérmica tipo FLIR (Forward Looking Infrared) MKD-400NDP, instalada a bordo de un laboratorio móvil, pararealizar censos nocturnos de dormideros de Gaviota Reidora (Larus ridibundus). La cámara FLIR obtuvo imá-genes excelentes, pero el escáner Daedalus 1268 requiere de una mayor experimentación. Mediante la digi-talización y procesamiento automático de las imágenes térmicas obtenidas durante la noche con el FLIR en elEmbalse de Santillana (Manzanares el Real, España central) se pudo realizar un censo de las Gaviotas Rei-doras presentes en el dormidero. Estos resultados, aunque preliminares, son alentadores de cara a la aplicaciónde estos métodos para el censo de aves que se agrupen durante la noche en campo abierto o sobre el agua.

Palabras clave: censos de aves, dormideros nocturnos, imágenes térmicas, invierno, teledetección.

* Programa de Desarrollo de la Estrategia Española de Biodiversidad. Dirección General de Conser-vación de la Naturaleza, Gran Vía de San Francisco 4, 28005 Madrid.

** Laboratorio de teledetección, Instituto Nacional de Técnica Aeroespacial (INTA), 28850 Torrejón deArdoz, Madrid.

context, the only alternative seems to be theuse of Remote Sensing sensors and camerassensitive to thermal infra-red radiation. Theseinstruments allow to obtain images of thegroups of birds at night, so that the census canbe carried out on the images by photo-inter-pretation or by automatic counting using digitalimage processing techniques.

Thermal images have been used in some stu-dies to detect or make a census of certain spe-cies of terrestrial vertebrates, e.g. Polar BearsUrsus maritimus (Brooks, 1970), Elks Alcesalces (Wride & Baker, 1997), Wild Boars Susscrofa (Lenco, 1976) and deers Cervus elaphus(Gill et al., 1997) and Odocoileus virginianus(Wiggers & Beckerman, 1993; Havens &Sharp, 1998). Garner et al. (1995) used thermalsensors to locate individual turkeys (Meleagrisgallopavo) and in the last few years infra-redsensors have also been used to detect the pas-sage of migratory birds during the night as asupport to more conventional radar techniques(Buurma, 1988; Liechti & Steuri, 1995; Liech-ti et al., 1995).

In the present study we explore the possibi-lity of using infra-red Remote Sensing techni-ques to obtain thermal images of nocturnal roosts of birds and to derive automatic censusfrom these images. In order to achieve thisgoal, we specifically address the following ob-jectives:

1. To determine the effectiveness of the sen-sors and thermal cameras made availableby the Spanish Instituto Nacional de Téc-nicas Aeroespaciales (INTA), installed onboard of both earth and aerial platforms,to detect birds resting on water. We testedthe Remote Sensing devices with twobird species: the Mallard Anas platyrhyn-chos, a diurnal-roosting species which isroutinely censused each winter togetherwith other waterfowls; and specially theBlack-headed Gull Larus ridibundus, anocturnal-roosting species whose winterpopulation has been censused annuallysince 1980 in Madrid (Central Spain).This population quadrupled during theperiod 1980-1998 reaching c.a. 80000 in-dividuals, more than 50000 of whichcould group together on a single roostingplace (Cantos & Asensio, 1990; GOEP,1997).

2. To establish the usefulness of the ther-mal images obtained to carry out census,either by direct counts on the imagesfrom manual photo-interpretation or byautomatic counts derived from the digita-lisation and processing of those images.

METHODS

Remote Sensing devices

We used to Remote Sensing devices, madeavailable by the INTA, to obtain images of res-ting birds:

1) The multi-spectral scanner DaedalusAADS 1268 ATM is a passive sensor de-signed to capture and record radiation co-ming from the Earth’s surface from anaerial platform. Radiation records areprocessed to produce digital images ofthe surveyed area. The scanner separatesthe radiation captured in eleven spectralbands (distributed between 0.42 µm and13 µm), ranging from blue, in the visiblespectrum, to thermal infra-red. The scan-ner image can be acquired at varying al-titudes which determine different groundresolutions. Resolution varies from onerecorded surface unit or pixel size of 7 m × 7 m acquired at an altitude of 2753m above the ground to a pixel size of0.86 m × 0.86 m acquired at 344 m ofaltitude.

2) The FLIR (Forward Looking Infrared)-type camera MKD-400NDP (TADIRANcompany) is operative in the 8-12 µmband, and it can be operated either froman aircraft or from a mobile laboratoryon ground. It has an IFOV (instantane-ous field of vision) of 0.07 mrad and three FOV (total field of vision) of 22° ×18o7´, 8° 5´ × 5° 4´ and 2° 4´ × 1° 5´. Be-sides the camera, the system consists ofall the computer and video equipment re-quired to record and process the thermalimages.

Each instrument has been designed for diffe-rent purposes and they obtain different kinds ofdata. The scanner Daedalus 1268 producesmultiespectral images digitalised in a 8-bit for-mat; these images are optimised to carry out

188 CANTOS, F. J., JIMENEZ, M., FERNANDEZ-RENAU, A., GOMEZ, J. A., DE JUAN, F., DE MIGUEL, E & SANGLIER, G.

extensive territory surface surveys from an air-craft. The FLIR camera obtains thermal analo-gical TV images, optimised to detect minimumthermal differences (i.e., to enhance thermalcontrasts).

Image acquisition and processing

In order to check the usefulness of the twoRemote Sensing devices, we first took diurnalimages of groups of birds that could be simul-taneously counted and photographed by con-ventional techniques, and then tried to obtainnocturnal images. The number of test was verylimited because of their high costs. Details onthese tests are as follows.

1) Daylight aerial Campaign with the Dae-dalus 1268 ATM scanner, aerial photo-graphy (RC-10 camera) and direct censusfrom the ground in the gravel bed of «ElPorcal» (Rivas-Vaciamadrid, Madrid).The images were taken on the 16th Octo-ber 1997 at 13:00 h GMT at 344 m ofaltitude over the water surface, on an ab-solutely clear cloud-free day.

2) Nocturnal aerial Campaign with the Dae-dalus 1268 ATM scanner in the «El Por-cal» gravel pit. The flight took place onthe 29th October 1997 at 22:30 h GMTat 344 m of altitude above the water sur-face. The prevailing atmospheric condi-tions were steady rain during the entireafternoon until one hour before the flight.

3) Daylight Campaign, from the ground,with the FLIR thermal sensor placed onboard of a mobile laboratory, and directcensus of a lake located within thebounds of the INTA (Torrejón de Ardoz,Madrid). Images of a group of Mallardswere taken on the 18th February 1998 at11:00 h GMT at 300 m on a cloud-freeday.

4) Nocturnal Campaign, from the ground,with the FLIR thermal sensor placed onboard of a mobile laboratory, in the San-tillana Reservoir (Manzanares el Real,Madrid). The images were taken on the18th February 1998 at 22:00 h GMT froma position located at an altitude of 56 mabove the surface of the reservoir and at adistance of 1500 m from a roost of the

Black-headed Gulls. From this positionand using the 18o FOV the camera takes abandwidth of 470 m on the ground. Theatmospheric conditions were favourable(cloud- and wind-free) throughout thewhole day and night

Once the thermal images were obtained withthe FLIR camera, the computer program GLO-BAL LAB Image, version 3.0 was used tomake automatic counts of the individuals pre-sent in the roosting place. First, the video imageof the roosting place recorded by the thermalcamera was divided into independent frameswhich were subsequently digitalised using thesame software. The image, digitalised in a 8-bitformat, was adjusted to 256 shades of grey, thehighest values (white dots) corresponding tothe gull bodies as they are warmer than the wa-ter (black background). The program selectsand defines zones from a specified digital level(the limit between water and gulls) and thenautomatically carries out a count of the selectedzones.

RESULTS

The results obtained with the Daedalus 1268ATM scanner were as follows. In the daylightcampaign, visual photo-interpretation of theimages showed that the near infra-red (band 9:1.55-1.75 µm; Fig. 1) obtained the best correla-tion with both the aerial photographs and thedirect census (62, 63 and 63 gulls, respectively).During the nocturnal flight, however, it was notpossible to pinpoint the gulls on the thermalimage of the water surface. The absence of lightat the time of the nocturnal pass did not allowthe make direct counts or conventional aerialphotographs, so that we could not ascertainwhether this result was due to the absence ofgulls or to a bad performance of the device.

On the other hand, the results obtained usingthe FLIR camera from the ground were highlypositive, both in the daylight campaign (Fig.2) and in the nocturnal one (Fig. 3). In bothcases we were able to detect and count the in-dividuals present from the thermal images ob-tained.

After digitalising and processing the thermalimages of the entire roosting place of Black-headed Gulls recorded during a pass of the

APPPLICATION OF SENSORS AND THERMAL CAMERAS FOR THE CENSUS OF WINTER ROOSTS OF BIRDS 189

FLIR camera, it was possible to estimate thesize of the gull population roosting in the San-tillana Reservoir at the day of the census bothmanually and automatically. The automaticcount gave a quantity of 3094 gulls. The errorin this automatic census was estimated to be

less than 5% in relation to a manual count (dot-ting) of the white dots present in the ten photo-graphs of the printed thermal images (2942gulls). The absence of light prevented the com-parison of these result with estimates based onconventional direct census.


FIG. 1.—Image of the «El Porcal» gravel pit in the medium infra-red range (band 9, 1.55-1.75 µm) taken fromthe air by a multispectral Daedalus 1268 scanner at 344 m of altitude at 13:00 h GMT on the 16th October1998. Extensive black areas are water surfaces (the Jarama river crossing the right part of the image, with the«El Rey» reservoir in its bottom end; and the gravel pit crossed by a sand bar). Grey areas are land covered byvegetation and white areas within them indicates bare ground (paths and ploughed lands). White areasdownstream of the «El Rey» reservoir are riffles. White dots on the water surfaces are birds. The group in theleft-top are Black-headed Gulls, whereas the remaining dots correspond to other waterfowl (mostly Mallards). [Imagen en el infrarrojo medio (banda 9, 1,55-1,75 µm) tomada desde el aire con el escáner multiespectralDaedalus 1268 a 344 m de altura sobre la gravera de El Porcal, a las 13.00 h GMT del día 16 de octubre de1998. Las zonas negras extensas son láminas de agua (el río Jarama cruzando la imagen por su parte dere-cha, con la presa de El Rey en su extremo inferior; y la gravera de El Porcal, cruzada por una barra de are-na. Las zonas grises son tierras firmes cubiertas por vegetación y las zonas claras dentro de ellas áreas desuelo desnudo (caminos y campos labrados). Las zonas claras aguas abajo de la presa de El Rey son rápidosdel río. Los puntos blancos sobre las láminas de agua son aves. El grupo situado en la parte superior iz-quierda son Gaviotas Reidoras, mientras que el resto son otras aves acuáticas (sobre todo Anades Azulones).]

DISCUSSION

Although the results presented here shouldbe taken as very preliminar, they demonstratethe usefulness of the thermal sensors emplo-yed to detect and count birds during the night.In addition, our study is the first one whichuses successfully a FLIR-type sensor to carryout nocturnal census of bird roosts.

The results obtained with the Daedalus 1268scanner were somewhat contradictory, as it de-tected gulls during the daylight flight but notduring the nocturnal one. Although the absenceof resting gulls on the water surface at the timeof the nocturnal flight (22:30 h GMT) cannotbe confirmed by alternative methods, as it is

impossible to obtain direct counts at that timeof the night, such absence appears to be unli-kely as the estimate of the roosting populationat dusk (15:00 h GMT) on that particular daywas 20000 individuals and massive nocturnalmovements of roosting gulls are not frequent(pers. obs.). The absence of gulls in the imagesobtained with the Daedalus 1268 during thatnight could have been due to the high level ofenvironmental humidity caused by the intenserainfalls which took place in the area understudy up to a few hours before the flight. Highhumidity levels in the atmosphere reduces thethermal contrasts at the surface level and mo-dify the signal received by the sensor. Althoughthe thermal bands to which the scanner is sen-


FIG. 2.—Image of a group of Mallards Anas platyrhynchos taken during the ground campaign carried out the18th January 1998 at 11:00 h GMT in a lake near Torrejón de Ardoz (Madrid). The image was obtained witha FLIR (Forward Looking Infrared) type sensor in the thermal infra-red range.[Imagen de un grupo de Anades Azulones Anas platyrhynchos tomada durante la campaña terrestre realizadael día 18 de enero de 1998 a las 11.00 h GMT, en una laguna próxima a Torrejón de Ardoz (Madrid). La ima-gen se obtuvo con un sensor tipo FLIR (Forward Looking Infrared) en el infrarrojo térmico.]


FIG. 3.—Series of images obtained at 22:00 h GMT the 18th February 1998 during the ground census of a lar-ge roost of Black-headed Gulls Larus ridibundus in the Santillana Reservoir (Manzanares el Real, Madrid).Images were obtained with a FLIR-type sensor in the thermal infra-red range.

sitive are located on wavelengths of minimalatmospheric influence, the readings can be af-fected by the absorption of part if the radiationby water vapour (Varanasi, 1988; Ottle & Stoll,1993). We have reproduced this effect on a si-mulation made for the band 12 (8.5-13 m) withthe radiative transfer model LOWTRAN-7(Kneizys et al., 1988). Results showed that, fora typical winter night in Madrid (1.3 °C on ave-rage), an atmosphere saturated with water va-pour (100% relative humidity) has a much lo-wer transmittance than an atmosphere with thenormal humidity (70%; transmittances of 0.4and 0.9, respectively). Further tests of this de-vice at varying atmospheric conditions are thusrequired to establish its usefulness for nocturnalbird counts.

As regards to the FLIR camera, results weremuch more encouraging. However, it shouldbe necessary to determine the time when thedifference between the temperature of the watersurface and that of the external plumage of thebirds is at its maximum to optimise the detec-tion of such thermal contrast by the sensors.For doing this, further tests are needed to cali-brate the effects of the meteorological condi-tions, the density and behaviour of the birds,the water temperature and the time of day onthese thermal contrasts and their detection bythe sensor. In fact, we have observed that thecontrast between the white dots representingthe warm bodies of the gulls and the black,cold background vanishes in the FLIR imagesas night advances. This could be due to a pro-gressive reduction of the bird’s activity and anincreasing smothness of their plumage. For thisreason, the best time to carry out this type ofcensus would be the first hours after sunset.Finally, the use of FLIR cameras from an aerial

platform would significantly improve the re-sults obtained, as this will avoid the overlapamong individuals in the images when suchimages are taken obliquely from the ground.

The good results obtained in these tests sug-gest that it would be interesting to adjust thismethod to other groups of birds that concentra-tes in large roosts in open country or on thewater (e.g. cranes or geese). However, iden-tifying species from thermal images may bedifficult, so that mixed species roosts could behard to census with these techniques. Finally, itwould be also interesting to develop this tech-nique for the detection of birds in areas of spar-se vegetation (e.g. bustards) or nest of largebirds of prey in the crowns of trees (e.g. BlackVultures). This could prove of great interestfor monitoring certain endangered species withminimum disturbance. A general use of thistechnique would in turn improve the interpre-tation of the images obtained and a considera-ble reduction of the economic costs of cam-paigns.

ACKNOWLEDMENTS.—We wish to express our gra-titude to the «Centro Cartográfico y Fotográfico delEjército» (CECAF) for its collaboration in the ae-rial campaigns. To the ARIPRESA company and tothe «Sociedad Española de Ornitología» SEO/BirdLife for granting us the permissions required for theaccess to the «El Porcal» gravel pit. Comments fromtwo anonymous reviewers and from the editor gre-atly improved former drafts of the manuscript.

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[Recibido: 12-2-99][Aceptado: 12-10-99]