details of home-range use and separation of the home-ranges of neighbour- ing birds. ... of technical equipment for many different purpos- es is available ...
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RADIOTRACKING OF GREAT AND BLUE TITS: NEW TOOLS TO ASSESS TERRITORIALITY, HOME-RANGE USE AND RESOURCE DISTRIBUTION BEAT NAEF-DAENZER ABSTRACT With a newly developed small transmitter, radiotracking of birds with a minimum body mass of 7 g is possible. Its high pulse rate (4-9 Hz) allows birds to be located within a few seconds with a precision of ±lAO (SD), i.e. ±2A m at 100 m bearing distance. Data on the use of individual trees by Great and Blue Tits (Parus major and P. caeruleus) revealed details of home-range use and separation of the home-ranges of neighbouring birds. Within 1 min, Great and Blue Tits moved over a median distance of 18 m and 10m, respectively. The range use of the tits was determined to a great extent by the spatial distribution of caterpillars, the main prey during breeding season. Overlaps of neighbouring home-ranges were observed only for areas of low location density. By contrast, the central parts of the home-ranges were used exclusively by the resident pairs. Using the examples given, I discuss four main methodical aspects of radiotracking of small woodland birds: (1) Location probability and basic qualities of the data obtained. (2) Spatial and temporal resolution of tracking data. (3) Tracking of short and medium distance movements. (4) Analysis of territoriality and relationships between neighbours. The new technique greatly improves the observation of small animals under poor visibility conditions. It is, however, limited to a relatively small number of individuals, which has its implications for study design and statistical analysis of the data.
Swiss Ornithological Institute, CH-6204 Sempach, Switzerland.
INTRODUCTION Knowing how animals use space and time and how they react to changing patterns of resources is a key to understanding behavioural, ecological and evolutionary processes. Where direct visual observation is impossible, radiotracking allows remote data collection. The technique was developed shortly after the introduction of semiconductors (Barr 1954) and has given enormous scope to biological studies. Today, a great variety of technical equipment for many different purposes is available commercially. Overviews of the actual technical standard are given by Amlaner and MacDonald (1980) and Kenward (1987). As radio tags should in no way affect the animal's behaviour, the use of the technique in the study of very small animals is still limited by the size and mass of available transmitters (Boag 1972, Amlaner et al. 1978, Massey et at. 1988, Aldridge & Accepted 27 July 1994
Brigham 1988, Brigham 1989, Klaassen et at. 1992). Today, the smallest flying animals to be radio-tagged have a mass of 15-20 g, such as Great Tits (East & Hofer 1985), Bam Swallows Hirundo rustica (Brigham 1989), Chaffinches Fringilla coelebs (Hanski & Haila 1988), or small bats Lasiurus spp. (Hickey & Fenton 1990). During a study on the behavioural ecology of tits (Parus sp.), I developed a new radio tracking system that allows radio tagging of birds with a minimum body mass of 7 g. It consists of a transmitter with a mean mass of 260 mg (without battery), a triangulation method to locate the fast moving birds with high precision, and a software package for the analysis of home-range use with high spatial resolution. A detailed description of the techniques and an evaluation of transmitter loads for small birds is given in Naef-Daenzer 1993a. The high pulse rate of the transmitter (4-9 Hz) allows location within a few seconds. These ARDEA 82: 335-347
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methods and techniques were developed as a part of a large project on the effects of forest decline on woodland bird communities in Switzerland (Mosimann et al. 1987, Naef-Daenzer & Blattner 1989). To estimate home-range structure and the use of individual trees during the breeding season, large numbers of locations had to be collected with high precision, which would have been difficult whithout the new methods (Smith & Sweatman 1974). Herein, I present examples of the use of the new technique and methods of studying the behavioural ecology of tits and other small birds with regard to the following main aspects: (1) Location probability and and basic qualities of the data obtained. (2) Spatial and temporal resolution in the analysis of home-range and resource use.. (3) Tracking of short and medium distance movements. (4) Analysis of territoriality and relationships between neighbours. The general approach of the radiotracking study was to give a detailed view of the use of space and time, resource selection and spatial relationships between neighbours for a relatively small number of birds. I focus on the methodological options and restrictions of radiotracking as a tool to obtain such information and on the implications to be considered in project design and analysis of the data.
MATERIAL AND METHODS
The study was carried out in an oak-rich deciduous forest near Basel, Switzerland. The main tree species in the 'Birsfelder Hard' are Oak Quercus robur (c. 30% of trees in the canopy), Beech Fagus silvatica (18%), Hornbeam Carpinus betulus (c. 10%) and Ash Fraxinus excelsior (c. 36%). About 300 nestboxes were placed along the forest roads (Mosimann et at. 1987 and Gebhardt-Henrich & Van Noordwijk 1991). During the breeding seasons 1988-1990, the home-range use of Great and Blue Tits was observed by radiotracking. For this purpose, I developed a new transmitter of 260 mg without batteries. With the smallest batteries, complete tags
have a mass of 390 mg and run for 5-6 days. A modified temperature-sensitive design was mounted in artificial eggs of the same size as a Blue Tit egg (Fig. 1). For Great Tits, transmitters with a total mass of 900 mg and a life of 50-60 days were used. Blue Tits carried radio tags of 620 mg, running for 15-25 days. The detection range of all transmitters was 200-400 m in dense vegetation and up to 1000 m in the open. The birds were trapped with mist nets mounted 0.5-lm from the nestboxes. The feathers on the back were clipped over an area of 5xlO mm, leaving about 1-2 mm of the feather shafts. The tag was glued onto them, using 40-50 mg of cyanoacrylate glue (Raim 1978, Kenward 1987). All birds were released within 10-15 minutes of capture. The birds adapted to the tags in 0.5 to 2 hours and the transmitters remained on the birds, backs for 2 to 20 days with a median of 6 days. For technical details see Naef-Daenzer (1993a). The birds were located by triangulation from two fixed stations. Each station consisted of two phase-shifted six-element Yagis on top of a 5.511.0 m pole (antennas and poles built by Wicker & Biirki AG, Rtimlang, Switzerland). The two antennas were positioned at a distance of 60-100 m from each other and the nest box, rendering the bearings from the central parts of the home-range most accurately (White 1985). High synchronization of the two stations was needed to avoid triangulation errors due to movements by the birds (Schmutz & White 1990). Data were collected by two persons taking bearings simultaneously. The two stations communicated with walkie-talkies, without interruption of the signal from the observed bird. Fixes were taken once a minute; if it was not possible to get an exact direction within 5 seconds, bearings were classified as unsuccessful. The mean rate of successful bearings was 49 ± 13%, i.e. 22 to 37 locations per hour. In addition to the standard technique, a small sample of locations with a bearing interval of 30 seconds was collected. Large excursions out of the home-range were recorded on maps using handheld H-Antennas. The triangulation errors were normally dis-
Naef-Daenzer: RADIOTRACKING OF TITS
Fig. 1.
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450 mg tracking transmitter (left), telemetering egg (open and closed, mid) and real Blue Tit egg (right).
tributed with a standard deviation of ±1.4°. At 100 m observation distance, this corresponds to a standard deviation of the locations of ±2.4 m and a 95% range of ±4.7 m, respectively. This precision was achieved only for birds sitting more than 0.5 wavelength (approx. 1 m) above ground. Birds sitting near or on the ground were not located with sufficient precision (Naef-Daenzer 1993a). The food available in the middle and upper layer of the canopy of each tree was estimated by collection of caterpillar faeces and by taking branch samples (Van Balen 1973, Liebhold & Elkington 1988). Frass fall was collected continuously with plastic funnels of 39 cm diameter. Fractions of 2-3 days were sampled, and the duration of each sample measured to the nearest hour. The frass samples proved to be a good estimate for caterpillar biomasses in the trees (Fischbacher et
ai. 1993). There was, however, a considerable effect of caterpillar size on the frass sampling rates. The pellets of small caterpillars were collected less efficiently than those from larger animals. Therefore, biomasses of the caterpillars in tree species carrying large caterpillar species (Beech, Ash) were somewhat overestimated. For the examples given herein, frass samples were not corrected for caterpillar size. The positions of the trees were mapped using a Zeiss Elta theodolite. To estimate location densities, I used a modified bivariate normal kernel estimation as provided by the newly developed software ('GRID') to calculate home-range use. This program is distributed by the Swiss Ornithological Institute and written for MS-DOS computers with hard disk. It includes routines to compute kernel estimations and contour or area plots. Areas of different use density may be calculated and also the overlap-
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ping parts of different home-ranges. Output files are suitable for further processing or graphical presentation (Naef-Daenzer 1993b, Worton 1989). This program allows the location data to be analyzed with enhanced spatial resolution and includes all necessary routines for density calculation and estimation of areas. All values for location densities are given as a frequency relative to 100 locations: A density value of 2 means that 2% of the locations were within a 2.5 m radius of a given grid point. To test whether parts of the research area were used exclusively by single individuals, the following procedure was applied: (l) Identically structured density matrices covering the same area were computed for each bird. (2) In order to compensate for different location samples, all 10cation densities were calculated as relative to 100 locations. (3) The density matrices of all birds were added to compute the total use density at each grid point. (4) The percentage of the contribution of a selected bird to the total use density at a given grid point was calculated.
RESULTS During the study period a total of 6305 locations of Great Tits and 8559 of Blue Tits were collected. For 8 Great Tits and 5 Blue Tits samples of more than 300 locations were collected, which was considered to be sufficient to outline the individual home-ranges. Table 1 gives sample sizes and areas of the home-ranges. As an estimate of total home-range size, the limit of 0.2% location probability within a radius of 2.5 m is used. For 1990, data from 2 Great Tits and 4 Blue Tits nesting in neighbouring boxes were available. Data on caterpillar distribution are based on 2990 frass samples from 601 trees.
Location probability and and basic data qualities 58% of the successful bearings were followed by another location 1 min later. In about 19% of the cases the time lag in the sequence of the locations was 2 min (Fig. 2). The frequencies of the
observed time lags accord well with the frequency expected for a random distribution based on the mean bearing success of 49 ± 13% (Wilcoxon test, P = 0.96, see also Naef-Daenzer 1993a). Therefore, the time sequence of the bearings is considered as random. Figure 3 gives values of the autocorrelation of the consecutive positions for different time intervals between the locations. For statistical reasons, only pairs of locations with no successful bearings inbetween were included in the analysis for time lags of more than 1 min. For bearings with 1 min time interval, the autocorrelation of the positions is high. With an increasing time lag, the coefficient of autocorrelation decreases substantially. The high autocorrelation indicates that the locations are not independent in the statistical sense. This fact may be important for the application of statistical methods. Biologically, it shows that the birds stay for few minutes in the same region of their home-range. However, this result does not indicate that the location density at a given point is biased due to poor resolution of bearings: the spatial resolution of the bearings is sufficient to detect distances smaller than those covered by the birds within the interval of 1 min: Figure 4 shows the frequency distribution of distances moved by the birds within 1 min. In only 2% of the cases did the birds move less than 5 m. In conjunction with the bearing errors of less than ±4.7 m (95% range), possible bias in the estimation of location frequencies at a given point is estimated to be below 2%. Therefore, locations were not affected by the location one minute earlier and were considered as an unbiased sample out of the behavioural sequence (Aebischer et al. 1993). The autocorrelation of the bearings is not sufficiently high to track the searching paths of the birds. Figure 5 gives two examples, showing that the sequence of locations is unable to reproduce the route followed by a bird, even if the bearing interval is reduced to 30 seconds (Fig 5b). Because of the frequent and fast movements, searching trips could be reproduced only if the bearing interval was reduced to 5 to 10 s, which is impossible with the triangulation method described.
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Naef-Daenzer: RADIOTRACKING OF TITS
Table 1. Size of areas of different location density for Great and Blue Tits. The 0.2% density limit is defined as the border of the home range. GT = Great Tit, BT = Blue Tit, f = female, m = male. For more details see text. Nest no
Year (n locations)
Relative location density (%probability of location within 2.5 m radius) >0.2 >0.5 >1
GT 350m GT91 m GT 12m+f GT345 m+f GT 17 m+f GT372m
1988 (349) 1989 (610) 1989 (686) 1989 (712) 1990 (587) 1990 (1162)
3266 3202 2455 3281 2615 3766
1442 1259 991 1031 840 1068
484 412 410 282 325 226
BT373 m BT 373 m BT 373 f BT 16m BT371 m
1989 (1764) 1990 (490) 1990 (1590) 1990 (1350) 1990 (1798)
2730 3030 2651 3452 2590
1264 1674 720 1331 1342
529 476 295 391 483
observed (n=7583) expected from mean bearing success (49±13%)
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Spatial and temporal changes in the use of home-ranges The method applied allows the tagged birds to be located very precisely and the high location rate gives large samples in a relatively short time. This is very important in recording how the birds
behave at different stages of brood rearing or in response to changing food distributions. Figs. 6 and 7 give an example of the home-range use by a male Blue Tit before and after hatching of the young. During the incubation phase (Fig. 6a) the nest is not visited very often, but after hatching
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ARDEA 82(2), 1994
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Tracking of short and medium distance movements The examples given in figure 5, show that the method was not adequate to track the searching paths of the birds when foraging near the nest. Nevertheless, the distance covered by birds within I min gives interesting information about the movements of Great and Blue Tits during foraging: individuals of both species usually moved between 5 and 25 m per minute, but there is a remarkable difference in the distributions: Blue Tits moved most often in small steps of 5 to 10 m, whereas Great Tits, frequently moved across their whole home-range (chi-square, P < 0.0005;
Naef-Daenzer: RADIOTRACKING OF TITS
Fig. 4). The median distance moved within 1 min was 18 m for the Great Tit and 10 m for the Blue Tit. Several individuals carrying transmitters left their home-ranges for quite large excursions lasting between 30 min and half a day. The transmission range was sufficient to locate these birds without problems and to map their routes using handheld antennas. The longest distance recorded was 600 m from the nestbox. Usually, these excursions served to reach rare resources, such as small pools for bathing. In addition, birds went out of their home-range several times to sit and rest at an undisturbed site. The map in figure 8 gives some examples for excursions recorded in 1990.
Range overlap In the breeding season of 1990, it was possible to collect data sets from 4 Blue and 2 Great Tits occupying neighbouring nestboxes. Only two birds could be recorded simultaneously, but all data were collected in the same nestling period. Although the data give no information on the social relations between the birds, they allow the analysis of a central aspect of territoriality: the area used almost or completely exclusively by one or a group of individuals and defended against conspecifics. The location data can be used to test whether or not the different birds used exclusive parts of the research area, but not to determine what social interactions caused only such separation. The first example in figure 9 shows the home-ranges of two Blue Tit males. Both birds were observed in some parts of the area of 1 ha but the areas of higher location density are clearly separated: at a relative density of 0.2%, the areas overlap by about 15%, whereas at a density level of 0.5%, the areas do not overlap at all. Hence, a large part but not the whole of the home-range is used exclusively. For 7 pairs of direct neighbours, the size of the overlapping parts of the homeranges was computed for different levels of location density (Fig. 10). At low levels of location density, the home-ranges overlap considerably, but the overlapping area decreases rapidly with
341
increasing location density. Places with a location density exceeding 0.6 are used almost exclusively by an individual from one pair. (The maximal relative location density in a home-range is 2 to 8.) The curves for the 7 pairs of home-ranges are similar in shape, although the absolute values vary. This indicates that the feeding ranges of the observed birds are not territories in the strict sense. On the other hand, the fact that the sites of high location density are used exclusively, indicates a high level of interaction between neighbours. Occasional observations of aggressive interactions between tagged birds support the idea that important resources are defended against neighbours. In figure 11 the spatial pattern of four Blue Tit 'territories' is shown. The shaded areas give those parts of each home-range where the location density by the nest owner was more than half of the total use density by all four birds. The figure shows that considerable parts of the surroundings of each nest were used predominantly by the resident pair. The amazing shape of the home-ranges of males 016 and 373 might be caused by the fact that male 373 established his home-range late in the year. The Blue Tits evicted a pair of Great Tits already incubating 8 eggs.
DISCUSSION Analysis of the use of space and time by animals is indispensable for the modelling of behavioural, ecological and evolutionary processes. Visual observation of fast moving Great and Blue Tits in dense vegetation is almost impossible. Using radiotracking and improved methods of homerange analysis, however, it was possible to get detailed information on home-range use and foraging behaviour. Rather than biological results, I discuss herein the methodological difficulties that could be resolved by radiotracking and some problems that remain unsolved. The new methods will improve investigations on range use and ecology of small animals. However, radio tracking itself is not a powerful tech-
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Fig.7a. Location densities of a male Blue Tit on an area of 1 ha compared with the distribution of caterpillars. 40r----------,-----,-~--------
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Fig. 9. Overlapping home-ranges of two neighbouring Blue Tit males. The home-ranges overlapped only where the location density of both individuals was low.
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