Pre-migratory and migratory movements of Montagu's Harrier Circus pygargus, a long-distance migrant, are poorly documented by ring recoveries and observations. In the present study, we tagged 10 adult Montagu's Harriers in NE Spain with satellite transmitters and tracked their movements until their arrival on the wintering grounds. To identify the end of the breeding season and the onset of the migration based on our satellite-telemetry data we developed and applied a statistical approach, defining the time period between these two events as the pre-migratory stage. We then compared habitat preferences during the breeding season and the pre-migratory stage. The duration of the pre-migratory stage averaged 42 days, with harriers leaving the breeding areas between 4 June and 27 July, and the onset of the migration taking place between 25 July and 4 September. Staging areas used during pre-migration were located at higher altitudes than the breeding grounds, and were characterized by the presence of shrublands and cultivated areas, mainly cereals. Montagu's Harriers' use of these sites is likely to be related to food availability: at higher altitudes, peak abundance of Orthoptera, their main prey source, occurs later in the season than in the lowland breeding area. Pre-migratory movements may also play an important role in the search for suitable breeding sites for future use.
The Montagu's Harrier Circus pygargus is a long-distance migrant, breeding in Europe and Asia, and wintering in Africa, the Indian subcontinent and Sri Lanka (Clarke 1996). The species is not globally threatened (BirdLife International 2004) but is considered vulnerable in Spain and France, the strongholds of the western European population (Salamolard et al. 1999, García & Arroyo 2003). In West Africa, the wintering population has declined in recent decades (Thiollay 2006). The main nesting habitat for the species in West Europe is cereal crop, although some populations still breed in natural habitats (Cramp & Simmons 1980, Clarke 1996, Limiñana et al. 2006a). Main threats for the species in Europe are loss of natural habitats, loss of clutches and broods due to harvesting activities in agricultural land, human persecution, and the use of pesticides, which reduce prey populations (Arroyo et al. 2004).
The Montagu's Harrier is a generalist predator that feeds on a range of taxa, including small mammals, birds, lizards and large insects (Underhill-Day 1993, Corbacho et al. 1995, Arroyo 1997). In our study area, the species breeds in shrublands (Limiñana et al. 2006a, 2006b), and mostly feeds on grasshoppers and other medium-sized insects (e.g. 77% of 197 pellets collected between 2001 and 2006 contained grasshoppers or other insects; Limiñana, pers. obs.). Both in natural habitats and croplands the peak of grasshopper abundance occurs later at higher altitudes (Bonnet et al. 1997). In croplands, this peak occurs after harvesting (Tella et al. 1998, Franco & Sutherland 2004, Ursúa et al. 2005), which in our study area takes place in June in the lowlands and at the end of the breeding season (July-August) at higher altitudes.
Many bird species are known to store energy reserves before initiating migration (Blem 1990, Jenni & Jenni-Eiermann 1998, Newton 1998, Berthold 2001). This involves movements from breeding grounds to areas where food availability is higher (e.g. Drewien et al. 1999, Trierweiler et al. 2007). To test whether Montagu's Harriers disperse to areas at higher altitudes after breeding (where food is more abundant), we used satellite telemetry to compare habitat use during the breeding season and the pre-migration stage.
Despite satellite telemetry being widely used in the study of bird migration (see Guan & Higuchi 2000 for review), onset and close of migration are generally chosen subjectively. There is a need to develop and use standardized methods that enable the objective identification of these events, making studies comparable and replicable. In this study, we describe and apply a method to establish onset of migration using movement patterns of tracked birds.
In 2006, we tagged Montagu's Harriers from a breeding population of inland Castellón (NE Spain), where the c. 130 breeding pairs nest in sclerophyllous Mediterranean shrublands composed mainly of Kermes Oaks Quercus coccifera, Gorse Ulex parviflorus and Rosemary Rosmarinus officinalis (Limiñana et al. 2006a, 2006b, Soutullo et al. 2006a). Ten adult birds (six males, four females) were captured between May and June 2006 using dho-gaza nets and a stuffed Eagle Owl Bubo bubo as a decoy. Birds were sexed, weighed and ringed. A Microwave Telemetry's 9.5 g solar-powered PTT-100 platform transmitter terminal (PTT) was fixed to their backs using a Teflon harness (Kenward 2001, Soutullo et al. 2006b). Harriers were captured in the late incubation stage or when they had very small nestlings. Birds were released within 30 minutes of capture. For the first three months of operation the PTTs were programmed on a 6 hours ON / 16 hours OFF duty cycle, and reset to a 10 hours ON / 56 hours OFF duty cycle for the following months. Locations were obtained using the Argos system, and birds were tracked until all birds had arrived on their wintering grounds (see Limiñana et al. 2007 for details on how the end of the migration was established). To avoid excessive autocorrelation, positions obtained less than an hour after the previous one were excluded from the analyses. All data were retrieved and managed with the Satellite Tracking and Analysis Tool (STAT, Coyne & Godley 2005). Tagged harriers are identified by the ID number of the PTT throughout the paper.
For the present study, we only considered the period encompassing the breeding season and the pre-migratory stage. The ‘pre-migratory stage’ is defined as the period between the last record on the breeding grounds and the first record of migration. To establish these dates we employed a modification of the method used by Soutullo et al. (2006c; see also Cadahía et al. 2007). The method is based on the idea that, regardless of the differences among individuals in their movement patterns, for each individual the transition between the more restricted movements undertaken during the breeding season and the wider-ranging movements undertaken during the pre-migratory stage shows as a peak in the variability of the distance covered between consecutive locations. The same applies to the transition between pre-migratory and migratory stages. Hence, for each location we calculated the distance (in km) to the previous one, and then the accumulated coefficient of variation (CV) of the distances between consecutive locations. Starting with the distances between the first three records, we calculated the CV for an increasing number of records, adding a new record each time. To identify the transitions between the breeding season and the pre-migratory stage, and between pre-migratory stage and migration, we then calculated the difference between each pair of consecutive CVs (ΔCV). We assumed that the l