Thanks to the GPS–GSM tags, we could follow the movements of 10 females (16 partial or complete seasons), and 3 males (6 partial or complete seasons) in detail (Table 1, Supplementary Table S1). During breeding, all individuals were found living in (large) colonies. After breeding, several females moved to other areas in Spain, or performed longer flight stretches before starting migration (Fig. 1a, b). All males (6 seasons) remained in the breeding area throughout the season (Fig. 2a, b), and started migration from there. Details about females’ behaviour and movements are found in the electronic supplementand at https://neuro.uni.kn/harrier.
Table 1 Breeding success, days spent in Spain, pre-migratory flights and migration start for tagged individuals in different years, order by year The Konstanz tags (female animals) reported between 0 and 89 fixes per day (mean 14.0, median 8, n = 10 birds), the Ornitela tags (male animals) reported between 5 and 31 fixes per day (mean 27.8, median 29, n = 3 birds).
Analysis across periods
First, we analysed how space was being used by our tagged animals during different periods. We defined four periods for pre-breeding and breeding: (1) pre-breeding, (2) incubation and early nesting, (3) late nesting, and (4) dependency (see “Methods” for how these periods were defined, in particular for males). For post-breeding, we used either ‘same colony’ when females remained in their colony, or ‘other colony’ when they flew to a different colony. For these periods, we evaluated cumulative daily distances (Fig. 3a), home range estimates (Fig. 3b), and activity radii (Fig. 3c) as well as activity centres (Fig. 4).
We analysed the daily distances flown as the cumulative distance between all fixes in 1 day (Fig. 3a). This statistic is a lower estimate, since additional flight segments might have occurred between two recorded fixes. Furthermore, with fewer fixes, the underestimating error increases: we only used days with a minimum of 15 or 20 fixes for females or males, respectively (see “Methods”). We found that, overall, males covered 63.9 ± 29.3 km every day. Flight distances varied slightly between periods (pre-breeding: 58.1 ± 24.4, incubation and early nesting: 70.5 ± 24.3, late nesting: 52.3 ± 17.4, dependency: 68.2 ± 33.2, ‘same colony’: 49.6 ± 25.3, all in km/day, mean ± SD). A generalised linear model (GLM, quasipoisson error, log link function) indicated that these differences were significant, and that males flew significantly longer distances during incubation and early nesting (p = 0.008). During dependency, males flew longer distances too (p = 0.02).
Females flew 27.2 ± 24.6 km/day on average. Flight distances varied considerably between periods (pre-breeding: 18.1 ± 16.2 n = 36 days, late nesting: 5.0 ± 2.7 n = 53, dependency: 26.6 ± 21.5 n = 143, ‘same colony’: 35.1 ± 25.8 n = 72, ‘other colony’: 46.6 ± 26.7 n = 61, all in km/day, mean ± SD). We excluded incubation and early nesting, because only two birds gave sufficient data for this period (n = 7 days; 4.7 km/day average for these 6 days), since these two birds had excursions to water reservoirs 0.7–1 km away from the nest. All other females stayed at the nest for this period, and movements decreased to a minimum. Females left the nest for food passes from males only for very short flights (visual observations in the field, and Clarke 1996), too short to recharge the batteries. A generalised linear model (GLM, quasipoisson error, log link function, cumulative distance ~ period + bird) indicated distances flown differed significantly across periods analysed (p = 0.001 for dependency, p < 0.001 for all other groups). When selectively testing ‘same colony’ against ‘other colony’ there was no significant difference between these two groups, though the significance was close to 5% (p = 0.056, GLM, quasipoisson error, log link function).
Next, we analysed home range areas for males and females and all periods (Fig. 3b). The procedure used considered that recorded fixes were not equidistant in time (biased random bridge, Calenge 2019). The 75% home range indicates the estimated area that a bird has resided in for 75% of the time, that is the area that is most heavily used by that bird during that period. Over the entire season, females covered 75% home ranges of 40 km2 (mean across period). During pre-breeding, females covered smaller home ranges (26 km2). When nesting started, females remained close to the nest with no insolation, with the effect that the tag batteries lost energy and could not send fixes any more (Supplementary Fig. 1). The few remaining fixes were not sufficient to calculate useful home ranges, and therefore, the period incubation and early nesting was excluded for female home range analysis: we estimate an area of close to 0 km2. Home ranges increased gradually during late nesting (0.8 km2) and dependency (29 km2). Males covered home ranges of 74 km2 for the entire season (mean across periods), ranging from a minimum of 45 km2 (late nesting period) to a maximum of 91 km2 (dependency period) for the 75% home range value (Fig. 3b). Home range values were significantly different across periods both for males and for females (GLM, quasipoisson error, log link function). After dependency, some females stayed in the area (‘same colony’), other females flew into another colony (‘other colony’), 90–270 km away (Fig. 1a, b): home range areas increased to mean values of 50 km2 for ‘same colony’, and 120 km2 for ‘other colony’.
As a more conservative way to estimate action ranges, we identified the geometric median of all fixes within each period, and calculated the median distance from this activity centre (see “Methods”). Across all periods, females had a radius of 2.1 km, while males had a radius of 3.5 km. Radii differed significantly across periods (Fig. 3c). In females, the average radii were: 407 m (pre-breeding), 14 m (incubation and early nesting), 76 m (late nesting), 872 m (dependency), 4.4 km (‘same colony’) and 4.9 km (‘other colony’). In males, the average radii were: 556 m (pre-breeding), 1.4 km (incubation and early nesting), 1.3 km (late nesting), 4.9 km (dependency), and 5.5 km (‘same colony’). Differences were significant across periods and across sex (GLM).
The strong differences between territory use during breeding by males and by females indicate that their role in providing shelter and food differs. We wondered whether this is apparent also in their localization. Therefore, we analysed their activity centres over the course of the periods. We found that the activity of females is centred on the nest for the entire breeding season (see example in Fig. 4a), while males move over different places in the colony’s vicinity (see example in Fig. 4b). Accordingly, the displacement of the activity centre in females is very small during breeding (e.g. 17 m when changing from incubation and early nesting to late nesting, and 837 m when changing from late nesting to dependency), while males shifted in space by 7.6 km and 2.4 km for these transitions, respectively (median values, n = 3–5, Fig. 4c). After breeding, activity centres in females shifted significantly to 6.1 km (p = 0.014, GLM). Centre transitions in males did not differ significantly in magnitude over periods (GLM): their movement over time was continuous.
Interactions between individuals and colonies
Common roost
Our loggers reported also positions during night time, allowing us to report about communal roosting (Wiacek 2010), which we define here as nights when fixes were less than 200 m apart. Over the entire year 2020, males had 40 “common roost” nights. In particular, the pair 191643–191644 spent 27% of the observed nights in the same roost (but used different hunting areas, Fig. 2), while the other two pairs had 12% and 2% common roost nights (see Supplementary Table S2). Females were found in separate places at nights during breeding, indicating the separation of their nests, but night fixes were low due to weak batteries in this period. However, during the post-breeding period, different pairs of females (2–3) from different colonies, including non-breeders as well as one female from the remote colony in Mérida used the same location to roost within 100 m, sometimes closer than 10 m. Across bird pairs, values ranged from no common nights, to 7.5% common nights (see Supplementary Table S2). We could not analyse roosts common to male and female birds, since in the overlapping year (2019) tagged females were found further away in the post-breeding period (‘other colony’), whereas males stayed at the breeding sites (‘same colony’).
Flights to other colonies
After dependency, or after nest loss, most females (6 out of 8 breeding females) took off for pre-migratory flights to remote sites (90–270 km away, Fig. 1a, b). They stayed there for periods ranging from single nights to 80 days. Site visits revealed that these areas were occupied by other Harrier colonies and that land use (winter sown cereal, interspersed with pasture and fallow land) was similar to that in ‘La Serena’ (B. Arroyo, M. Calderón, personal visits).
In 2016, female 5005 from a big colony (> 20 breeding pairs) lost her nest with already big chicks. Thereafter, she visited another big colony (> 10 breeding pairs) during 8 consecutive days, about 10 km away (Fig. 5), daily distances amounted to 21.4 km on average. During the next 2 weeks her movements were concentrated about a central point 2 km north-west from the original breeding place; average daily distances decreased to 6.0 km—similar to the distances typically covered during late nesting period. During the following 29 days (putative dependency period), her daily distances averaged 21.5 km, corresponding to daily distances that females flew during dependency period. We, therefore, hypothesise that she acted as a foster parent for another nest in a nearby colony.
Post-breeding period and pre-migratory movements
Post-breeding behaviour was diverse: all males (three individuals, six seasons) and three females (one successful breeder, two non-breeders, five seasons) stayed exclusively around the breeding site with only occasional daily flight excursions to sites up to 50 km away. Two females (two seasons) left the breeding site for only a short visit to a destination farther away: Female 5043 from Mérida (Fig. 1b, orange trace) visited the colonies of ‘La Serena’ (110 km East), stayed for one night and returned to her breeding site. Female 5005 (Fig. 1a, blue trace) took off south to Andalusia (270 km) on 9th August 2016 but returned north to her breeding site before starting migration (see also interactive plots at https://neuro.uni.kn/harrier).
Three females (successful and failed breeders, six seasons) flew to areas 90–200 km away from their breeding site (Fig. 1b). In two consecutive years, they took the same route and spent up to 80 days there. Successful breeders took off from their breeding site 21–24 days after fledging of the chicks and stayed 4–20 days at the remote place that was occupied by other colonies of Montagu’s Harrier.
We found only one female (4490-17, tagged in Mérida) that explored different sites during her pre-migratory flights (purple in Fig. 1a). During 12 days, she visited twice the colonies of ‘La Serena’ but went on, flying more than 1500 km until she stopped at high altitude (1100 m MSL) in an elevated plateau northeast of Madrid in the Sierra de Guadalajara where she spent 17 days (daily distances there: 16.5 km, home range 75% 17 km2) before starting migration from there. See also interactive material at https://neuro.uni.kn/harrier. The total distance flown during her post-breeding period amounted to 1860 km.
Start of migration
All three males and three females (4488, 4489, 6003) started migration from the breeding site without any pre-migratory flights to distant sites (Fig. 6). Most females (4 out of 6) that had visited remote destinations during the pre-migratory period returned to their breeding site and started migration from there (Fig. 6). Only two females (4490, 6001) started migration directly from their pre-migratory location without returning to the breeding site.
In 2017, females that were successful initiated migration 11–16 days earlier than failed females and even 23 days earlier than the non-breeder 4488, the difference was significant (Table 1; two-tailed t = 4.096; df = 3; p = 0.026). Number of days from arrival at the breeding site until migration start was 116 and 118 days for successful females compared to 130 and 132 days for failed breeders (Table 1) and 133, 138 and 141 days for males. In 2018, breeding season was delayed for about 3–4 weeks, migration was also late and concentrated at the end of August.
Site fidelity
All three males in our study returned to the same breeding area, using the same hunting sites as the previous year, with strongly overlapping home ranges (Fig. 2). We could not assign the individuals to a particular nest based on their movement patterns; therefore, we do not know whether they were (successful) breeders or not.
Of the five nesting females that we recorded across seasons (four successful, one failed breeder), four selected a nest site close to the previous year’s site (distance up to 3 km). The fifth (tag 6001), after spring migration, moved east to a site 90 km away (Supplementary Table S3). There was no relationship between previous year’s breeding success and current year’s nest location: female 5005 nested just 190 m away from the previous year’s predated nest, while female 6001 moved 90 km away from the previous year’s successful nest.
All tagged females that we could track over two seasons used the same site for their post-breeding/pre-migratory stay. Females 4489 (successful in both years) and 5005 (failed in both years) stayed at their breeding grounds of the previous year during their post-breeding period. Female 6001 had two different breeding sites over consecutive years, but showed site fidelity to her pre-migratory location. Females 4400 and 6002 were faithful both to their breeding and pre-migratory sites in both consecutive years.