Telemetry
From 2007 to 2014, 43 breeding adult Red Kites (29 males, 14 females) were fitted with solar-powered GPS transmitters in an area with a population density of 8.4 pairs per 100 km2 near Weimar (50° 59′N, 11° 19′E, Thuringia, Germany). These units provided more than 329,000 GPS fixes by the autumn of 2014. Gender was determined from morphological characteristics (wing length, body mass, and brood patch; Pfeiffer 2009), which was always confirmed by bird behaviour. The males were of particular interest for the evaluation of space use during the breeding period, because they normally bear the main burden of food provision for the young. Age was known for 13 of the 43 Red Kites when fitted with transmitters because these birds had been ringed as nestlings.
Two types of transmitters were employed in the study. All transmitters weighed less than 26 g and were attached to the bird’s back using a harness made of Teflon ribbon (Meyburg and Fuller 2007).
Details about the trapping of adult birds, fitting of transmitters, and data transmission via the Argos System are presented in Pfeiffer and Meyburg (2009) and Meyburg and Meyburg (2013). From 2007 to 2012 transmitters made by Microwave Telemetry, Inc. (USA) (www.microwavetelemetry.com) were used. The tags were programmed to send a GPS fix every hour, except during the hours of darkness, provided that the battery was adequately charged. In general, the longer the sun shone and the more the bird flew, the more fixes were received. When females were incubating or sheltering the young, the batteries received scarcely any charge; consequently, no, or far fewer, fixes were received for females compared to the males during the same period.
From 2012 onwards, GPS solar radio frequency (RF) tags from e-obs GmbH (Germany) (www.e-obs.de) were also deployed. The fixes from these units were not transmitted by satellite, but had to be read out with a hand receiver in the vicinity of the bird. These units delivered about 10 times as many daily fixes and, with adequate sunshine, at 5-min intervals.
In this study, Red Kites were numbered in the order of transmitter fitting, from 01 to 43. After a hyphen, the sex (M or F) is given, followed by the type of transmitter (S—satellite transmitter, T—RF tag).
Microwave Telemetry, Inc., states a GPS fix positional accuracy of ±18 m longitude and latitude. Our test measurements show that the e-obs tags achieved similar accuracy.
Only the fixes during the daily activity phase of birds were evaluated to calculate the home range and other parameters. As the birds did not always roost in the same location, algorithms based on daily local sunrises and sunsets were used to determine the roost sites. If a bird’s initial morning movement was more than 200 m from the roost, the start of the daily activity phase was assumed. Conversely, the last fix of more than 200 m from the night roost was taken to represent the end of the daily activity phase.
Occupation rate of nesting territories
Occupancy is defined as the number of years in which nesting territories are occupied, and has been studied in the target population since 1985. All nest sites of Red Kite breeding pairs in a 597 km2 study area around Weimar have been localised, the majority of chicks ringed, and the number of fledged young recorded. Based on these data, it was possible to establish how often in the past a nesting territory—defined as a confined area where nests are found and where no more than one pair is known to have bred at one time (Steenhof and Newton 2007)—was used by Red Kites fitted with transmitters. A nesting territory was considered to be occupied if an incubating adult was observed on the nest or if there were other indications of breeding. Only data from 1994 to 2014 were evaluated, to exclude any influences that gradually occur as a result of major changes in land use in East Germany following the political changes in 1990 (George 1995).
Analysis of the nutritional condition of young Red Kite nestlings
To judge the extent to which Red Kite nestlings were nourished (i.e. above or below average), the body masses of 1661 ringed young were recorded from 1989 to 2014. As the body mass development of young birds is markedly dependent on the amount of food consumed, the nutritional condition of an individual bird may be deduced by comparing its body mass against the average mass of birds from the same age class (Pfeiffer 2000). The age of young birds may be determined quite accurately from wing length (Traue and Wuttky 1966). Wing growth is minimally influenced by poor nutrition, disease, and parasites (Mammen and Stubbe 1995). Our own studies, which involved multiple checks of the same nest, have confirmed this observation, showing rare deviations of 2 days at most from the average wing growth curve.
Modelling spatial area use
Various models were tested to provide the most accurate representation of home range. The most useful model proved to be the kernel density estimation according to Worton (1989). Hovey’s (1999) Home Ranger 1.5 programme was used for the calculation. Only the adaptive kernel method, using the reference value for the smoothing parameter, provided a coherent contour of the 95 % kernel utilization distribution (KUD), and was considered suitable for the analysis of home range sizes. The area delimited by the 95 % KUD was used in this study as the measure for home range size. The disadvantage of choosing this smoothing parameter is that over-smoothing of the areas in the outer zones occurs (Seaman and Powell 1996). The resulting calculated home range area tends to be too large. Therefore, Tables 1 and 2 also present the Minimum-Convex-Polygon for 95 % of the fixes closest to the nest (MCP 95 %) using the Anatrack Ltd. Ranges 8 programme (Kenward et al. 2008). Although the MCP method provides smaller area values, it does not depict the actual shape of the area covered.
Table 1 Home range size of male Red Kites with the breeding success being calculated for the nestling and post-fledging dependent period
Table 2 Home range size of Red Kite females and their breeding success calculated for the period when large young are in the nest plus the post-fledging dependent period
In addition to comparing the home ranges of different individuals and their spatial area use in different years, changes in home range sizes during different periods of a reproductive phase were recorded. During the study, the onset of these periods varied considerably in different years. There were also variations for individual breeding pairs within the course of a year. Therefore, a differentiated calculation of these time periods was made for each nest site every year.
Starting with the date of hatching (H) of the oldest nestling (determined by wing length measurements during ringing; Traue and Wuttky 1966), the different time periods were established as follows:
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Courtship and territory occupancy = 1 March to H − 34 days
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Incubation period of the clutch = H − 33 days to H − 1 day
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Small chicks in the nest = H to H + 25 days
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Large nestlings in the nest = H + 26 days to H + 53 days
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Post-fledging dependent period = H + 54 days to H + 75 days
In cases where it was not possible to climb to the nest, the age of young was estimated based on plumage development. The length of each of the stated time period was determined according to Wasmund (2013) and Nachtigall and Herold (2013).
The highest demand for food occurs during the period when the growing young must be fed. Therefore, the period of time from the hatching of the first chick until the young gain independence was selected as characteristic of the home range requirement of a male during the breeding season.
To compare the number of fledged young with male home ranges, home range size was only calculated for the nestling period, because the number young birds that survive after fledging cannot be determined.
The females only participate in searching for food when the young are older. Therefore, the best time for calculating the female’s home range was when large young were in the nest and during the post-fledging dependent period.
Statistical analyses
All continuously scaled variables were described by the mean, standard deviation, median and the range. In case of a variable exhibiting a skewed distribution (as was the case for home ranges), the median and range (min, max) were used as descriptors.
Regression modelling was performed to investigate the influence of home range size (= independent variable in the models) on other variables (dependent variables). The link function within the model (linear link, logit link) was chosen according the types of variables. In particular, a cumulative logit model was used to determine the effects of home range size on the number of fledglings per successful breeding pair. Because of limited sample size, only pre-specified univariable or bivariable models have been applied without further reduction or selection of variables. Correlation within the data (which may occur if data from several time points exist for individual subjects) was taken into account by applying mixed models (generalized linear mixed model (GLMM) and linear mixed models [LMM]) to investigate robustness of simple models (generalized linear model (GLM), linear model [LM]). Effects with p-values <0.05 were regarded as statistically significant. The particular models used for individual calculations are stated in the results section.
All analyses were performed using the statistical software IBM SPSS Statistics 20 and SAS 9.2 (SAS Institute Inc., Cary, NC, USA).