Geolocators reveal new insights into Ring Ouzel Turdus torquatus migration routes and non-breeding areas

Abstract

Capsule The first details of the migration pattern of a male Ring Ouzel Turdus torquatus, fitted with a geolocator on its Scottish breeding grounds, showed that it wintered in the Algerian Atlas Mountains, substantially east of the suspected main wintering area.

In recent years, light-recording geolocators have increasingly been used to determine migration routes and non-breeding areas for a variety of bird species (Arlt et al. 2013, Bridge et al. 2013, McKinnon et al. 2013). These archival tags allow data on latitude and longitude to be estimated through the recording of light intensity levels, thus estimating the position of birds on the globe to an accuracy of approximately 200 km (Bowlin et al. 2010, McKinnon et al. 2013). The relatively small size, low weight and low cost of geolocators make them especially suitable for studies on smaller migrant passerines that are too small to carry larger and heavier tracking devices (Arlt et al. 2013, Gómez et al. 2014). Detailed knowledge of the migratory strategies of species of conservation concern is of particular importance for conservation biologists wishing to determine key life history stages for such migrants.

Ring Ouzel Turdus torquatus (hereafter ouzel) populations in Fennoscandia and central and southern Europe are considered to be stable, although comprehensive monitoring data are lacking from many areas (BirdLife International 2004). However, the species is of high conservation concern in the UK due to serious long-term declines in breeding population size and range (Eaton et al. 2011, Balmer et al. 2013). These declines are widespread throughout Britain, but the underlying causes and their likely geographical location (i.e. on the breeding grounds, on migration routes and/or in the winter quarters) are poorly understood (Sim et al. 2010). However, a recent study suggests that population growth rate of ouzels is largely determined by first-year and adult survival (Sim et al. 2011), which are most likely influenced by factors acting on the migration routes and wintering areas.

Ringing recovery data suggest that British breeding ouzels migrate through France and Spain during autumn and spring on their way to and from their apparent main winter ranges in the Atlas Mountains, north-west Africa (Wernham et al. 2002; Fig. 1). A large proportion of ringing recoveries refer to dead birds, with 77% of birds with a known cause of death being shot, mainly in France (Wernham et al. 2002). Indeed, the distribution of recoveries is likely to be heavily biased towards those areas where trapping and shooting are most intense and/or centres of human population (Durman 1976). Thus, our current understanding of precise ouzel migration routes, and stopover and wintering areas, is limited and more information on non-breeding areas is therefore required. Such knowledge is likely to be of importance to the conservation of the species, for example through identification and protection of important stopover and wintering areas. Here, we describe for the first time details of the migration timing and routes, stopover sites and wintering area of a single ouzel fitted with a geolocator on the Scottish breeding grounds in June 2013.

Figure 1. Median autumn stopover and wintering areas (ellipses) identified from geolocations from an ouzel tracked from Scotland, and recovery locations of British-ringed ouzels. Stopover and winter location ellipses represent the standard deviation of locations around the median point. British-breeding ouzels recovered in autumn (September–November: stars), winter (December–February: open circles) or spring (March–April: upward triangles) are shown alongside non British-breeding ouzels recovered during September–November (filled circles).

We studied a breeding population of ouzels in Glen Clunie (56°56ʹN 3°25ʹW), Aberdeenshire, north-east Scotland, during 1998–2014 (see Sim et al. 2011 for details). First-year and adult birds were distinguished following Svensson (1992). Apparent annual survival of first-year birds (4%) was much lower than that of breeding adult males (47%) and females (37%; Sim et al. 2011). Thus, in order to maximize the probability of birds fitted with geolocators returning in the following year, we tagged only adult males. Breeding adult ouzels were trapped using mist nets and a decoy stuffed kestrel Falco tinnunculus placed close to nests, which usually initiated a strong defence response (Sim & Rebecca 2003). All trapped adults were fitted with uniquely numbered metal rings, plus individual combinations of darvic plastic colour-rings.

In 2013, we trapped and individually colour-ringed 7 breeding males, in addition to re-sighting a further 13 returning males that had been colour-ringed as adults or nestlings in previous years. During 24 May to 7 July 2013, 10 of these 20 individuals were trapped and fitted with 1.5 g MK14-S geolocators (Biotrack, England). Geolocator weight, including harness, was approximately 1.8 g, or 1.7–1.9% of breeding male body mass of 93–109 g (Sim & Rebecca 2003), and thus well within the suggested upper weight limit of 5% of body mass for such devices (Cochran 1980). Geolocators were attached using leg-loop harnesses made from silastic tubing and a core of elasticated thread to allow the loops to expand when the birds accumulated migratory stores of fat and protein (Rappole & Tipton 1991, Smith et al. 2014). The fitted geolocator was positioned on the back of the bird, with the light sensor located on a 6 mm stalk protruding above the feathers at an angle of 30° to the horizontal, to maximize light sensitivity. Tagged birds, and their nesting attempts, were observed in the days following deployment to check that they continued to feed their nestlings and that their behaviour was not adversely affected. During mid-April to mid-July 2014 all returning adults were checked for colour-ring combinations, and males fitted with geolocators in 2013 were targeted for re-capture.

Times of daily sunrise and sunset from the retrieved geolocator were extracted with a light intensity threshold value of two to define sunrise and sunset events, and a six-hour minimum dark period to remove false light transitions, with locations calculated using BASTrak and Transedit software (Fox 2010). Clock drift was less than one minute, and therefore no adjustment for clock drift was necessary (Fox 2010). Day length was used to calculate latitude and time of midday was used to calculate longitude. A sun angle of −4.3° was selected from calibration of locations during the assumed stationary period (as indicated by light data and estimated latitude) at the breeding site between 2 August and 29 August using LocatorAid in BASTrack. Migration around both equinoxes (22 September 2013 and 20 March 2014), and data from the northward spring migration, prevented using the Hill–Ekstrom method of calibration using stationary periods either side of an equinox. Locations 14 days either side of the equinoxes were excluded. We estimated the accuracy of the calculated breeding location data by comparing the distance between the true position of the breeding site and the inferred median geolocator position for the same period as used for the sun angle calibration. The accuracy of the calculated stopover site location was estimated using the standard deviation of the median 13–29 October location, the stationary period of the stopover as assumed from the abrupt change in latitude in the estimated geolocation data immediately before and after these dates. The variation of geolocation at the wintering location was estimated using the standard deviation of the median location for the period 31 October–27 February. We used the median of locations as this is less sensitive to outliers than the mean. Differences in kilometres for each estimated location from the respective median stopover or winter location were calculated, and the standard deviation of latitude and longitude calculated from the differences.

To visually compare geolocations with ringing recovery data, all overseas recoveries of British-ringed ouzels were obtained from the BTO database. These records were classified to differentiate between British-breeding birds (those ringed in Britain, excluding Shetland and Fair Isle, during April–August, plus one September-ringed ouzel captured in likely breeding habitat), and non-British-breeding birds (those ringed in Britain during September–November at coastal locations and thus most likely to have bred outside Britain and be on passage). We excluded recoveries of four British-ringed ouzels recovered in only probable breeding locations; two in Norway and two in northern Germany.

Of the 10 male ouzels fitted with geolocators in 2013, 3 (30%) were re-sighted in 2014, compared with 6 out of 10 (60%) males fitted with colour-rings only. This apparent difference was not statistically significant (Fisher's exact test P > 0.37), but sample sizes were small. Of the three returning tagged birds, one male was re-trapped at its second breeding attempt and the geolocator retrieved. Unsuccessful attempts were made to re-capture the other two males. One of these had almost certainly lost its geolocator, since it could not be seen on the bird despite good views being obtained with a 13–40× zoom telescope. The third male was shy, elusive and difficult to observe at close quarters, and we could not be certain whether or not it had retained its geolocator.

The calibrated breeding location used positioned the bird 24.5 km south-west from the actual breeding site. Standard deviation of the median wintering location was 168.7 km (latitude) and 42.1 km (longitude). The retrieved data suggest departure from north-east Scotland after 4 October 2013. An abrupt change in estimated latitude suggested that the bird arrived in the Bordeaux region of south-west France on 13 October. It stayed there for 17 days (median stopover location 44°81ʹN 0°04ʹW), before leaving around 29–30 October and flying south to the Atlas Mountains in north-west Algeria close to the border with Morocco (median location 32°05ʹN 0°83ʹW). An abrupt change in estimated latitude suggested it had arrived there by 31 October. It spent the next four months in this region, with the next abrupt change in the estimated latitude indicating that it departed around 4 March 2014. Due to departing the wintering grounds and migrating during the period of the spring equinox nothing can be inferred about the spring migration. The ouzel had arrived back in Glen Clunie by 6 April 2014. Spring migration was therefore no more than 32 days in duration, compared to 28 days for autumn migration which included 17 days at a stopover site.

In total, 39 British-breeding and 46 non British-breeding ouzels were recovered outside Britain (Fig. 1). Ouzels were recovered mainly in France, Spain and Morocco during autumn and spring migrations and the overwinter period. The British-breeding ringing recovery locations in south-west France were apparently congruent with recoveries of non-British-breeding ouzels, and the tracked ouzel stopover area (Fig. 1). Only one wintering ground recovery was located in the western Algerian Atlas Mountains, where the tracked bird wintered. Most recoveries were of shot birds: 77% of British-breeding and 57% of non-British-breeding ringing recoveries.

The single male ouzel that returned to Glen Clunie in the year after being fitted with a geolocator has provided the first details of the specific timing of migration, routes and use of a stopover site, and indicated a wintering area substantially further east than suggested by most ringing recoveries. Previous data indicated that autumn migration from Britain begins in late August and continues into October, with most birds apparently arriving in north-west Africa from late October onwards (Durman 1976, Cramp 1988). This general migration schedule thus fits well with the migration timings shown by the tracked individual in this study.

The only previous knowledge of potential stopover sites and migration routes came from ringing recoveries, which tend to be biased to areas with high hunting pressure and/or centres of human population (Durman 1976). However, the area in south-west France where the tracked ouzel was stationary for 17 days coincides with an apparent concentration of ringing recoveries, which raises the possibility that this area might be a key stopover and refuelling site for both British-breeding and non-British-breeding ouzels on migration (Fig. 1). The tracked individual then wintered in the Algerian Atlas Mountains, close to the border with Morocco, and a substantial way east of the main wintering area in the Moroccan Atlas Mountains, as indicated by ringing recoveries (Fig. 1). This area is seldom visited by ornithologists, but may well contain substantial amounts of juniper Juniperus spp. forest, the berries of which are a major food source for ouzels in winter (Heim de Balsac 1931, Ryall & Briggs 2006).

Due to the timing of the ouzel's spring migration around the spring equinox, geolocation provided no detail of the spring route or whether it made a stopover. Perhaps surprisingly, spring migration was several days longer in duration compared to the autumn migration, which included 17 days at a stopover site. This suggests an unidentified stopover site on spring migration. If the similarity between the location of autumn ringing recoveries and the autumn stopover site is also the case for the spring migration, then we speculate that south-west France seems a likely spring stopover location as this is where the majority of March and April British breeding ring recoveries were located. However, it is entirely possible that other spring stopover sites, for example in eastern Spain, are important for this species.

Although information obtained from tracking devices is invaluable to those interested in bird migration and timings, such data are only of value if devices have no impact upon behaviour and survival in the study species (Bowlin et al. 2010, Arlt et al. 2013, Gómez et al. 2014). All ten tagged ouzels appeared to behave normally and continued to feed their nestlings post-tagging in 2013, and all three tagged males which returned in 2014 made second breeding attempts after successfully fledging young from their first attempts. The return rate of males fitted with geolocators (30%) was half that of those fitted with colour-ring combinations only (60%), which could indicate that tagged male ouzels suffered higher mortality. However, sample sizes were small and the apparent difference was not statistically significant. Further work to minimize the impact of tracking devices on different species, and to report both positive and negative results, is therefore to be encouraged.

The information obtained from this tagged ouzel is a first step in answering some of our questions about migration routes, and stopover and wintering areas, but inevitably raises others. For instance, was this bird's migration timing and route typical, was the stopover area in south-west France a key staging area, and could Algeria be an important alternate wintering area to Morocco? The ability of geolocators to answer such questions is limited, due to their relative geographical inaccuracy compared to other tracking devices, and inaccurate location data around the equinoxes. Future studies into migration ecology are increasingly likely to use miniaturized GPS or satellite tags, which provide much more accurate location data than geolocators, but without some of their limitations (García-Ripollés et al. 2010, Bohrer et al. 2012). The future use of GPS tags, which provide location accuracy to tens of metres, is likely to assist conservationists in more precisely determining key stopover sites and wintering areas for species of conservation concern, and thus to more effectively target conservation efforts in these areas.

ACKNOWLEDGEMENTS

We thank Invercauld Estate for co-operation with access to Glen Clunie, Ecology Matters for funding the purchase of the ten geolocators, and Mark Bolton for advice on data analysis. Ringing recovery data was provided by the BTO.

Funding

The BTO Ringing Scheme is funded by a partnership of the British Trust for Ornithology, the Joint Nature Conservation Committee (on behalf of: Natural England, Natural Resources Wales and Scottish Natural Heritage and the Department of the Environment Northern Ireland), The National Parks and Wildlife Service (Ireland) and the ringers themselves.

ORCID

Malcolm D. Burgess http://orcid.org/0000-0003-1288-1231