Data from location logging tags have revolutionised our understanding of migration ecology, but methods of tagging that do not compromise survival need to be identified. We compared resighting rates for 156 geolocator-tagged and 316 colour ringed-only whinchats on their African wintering grounds after migration to and from eastern Europe in two separate years. We experimentally varied both light stalk length (0, 5 and 10 mm) and harness material (elastic or non-elastic nylon braid tied on, leg-loop 'Rappole' harnesses) in the second year using a reasonably balanced design (all tags in the first year used an elastic harness and 10 mm light stalk). Tags weighed 0.63 g (0.01 SE), representing 4.1% of average body mass. There was no overall significant reduction in between-year resighting rate (our proxy for survival) comparing tagged and untagged birds in either year. When comparing within tagged birds, however, using a tied harness significantly reduced resighting rate by 53% on average compared to using an elastic harness (in all models), but stalk length effects were not statistically significant in any model considered. There was no strong evidence that the fit (relative tightness) or added tag mass affected survival, although tied tags were fitted more tightly later in the study, and birds fitted with tied tags later may have had lower survival. Overall, on a precautionary principle, deploying tags with non-elastic tied harnesses should be avoided because the necessary fit, so as not to reduce survival, is time-consuming to achieve and does not necessarily improve with experience. Geolocator tags of the recommended percentage of body mass fitted with elastic leg-loop harnesses and with short light stalks can be used without survival effects in small long-distance migrant birds.
The spatial scale of non‐breeding areas used by long‐distance migrant animals can vary from specific, relatively small non‐breeding areas for each independent breeding population (high connectivity) to a distribution over a large non‐breeding area with mixing of breeding populations (low connectivity). Measuring variation in the degree of connectivity and how it arises is crucial to predict how migratory animals can respond to global habitat and climate change because low connectivity is likely to be an adaptation to environmental uncertainty. Here, we assess whether use of non‐breeding areas in a long‐distance migrant may be stochastic by measuring the degree of connectivity, and whether it is annually variable. Twenty‐nine wintering Whinchats tagged with geolocators over 2 years within 40 km2 in central Nigeria were found to be breeding over 2.55 million km2 (26% of the land area of Europe), without an asymptote being approached in the relationship between area and sample size. Ranges differed in size between years by 1.51 million km2 and only 15% of the total breeding range across both years overlapped (8% overlap between years when only first‐year birds were considered), well above the range size difference and below the proportion of overlap that would be predicted from two equivalent groups breeding at random locations within the observed range. Mean distance between breeding locations (i.e. migratory spread) differed significantly between years (604 ± 18 km in 2013 and 869 ± 33 km in 2014). The results showed very low and variable connectivity that was reasonably robust to the errors and assumptions inherent in the use of geolocators, but with the caveat of having only ranges of 2 years to compare, and the sensitivity of range to the breeding locations of a small number of individuals. However, if representative, the results suggest the scope for between‐year variation (cohort effects) to determine migrant distribution on a large scale. Furthermore, for species with similarly low connectivity, we would predict breeding population trends to reflect average conditions across large non‐breeding areas: thus, as large areas of Africa become subject to habitat loss, migrant populations throughout Europe will decline.
The flexibility for migrant land birds to be able to travel long distances rapidly without stopovers, and thus to cross wide inhospitable areas such as deserts and oceans, is likely to be a major determinant of their survival during migration. We measured variation in flight distance, speed and duration of major stopovers (more than 2 days), using geolocator tracks of 35 Whinchats Saxicola rubetra that migrated successfully from central Nigeria to Eastern Europe in spring, and examined how these measures changed, or depended on age, when crossing the barriers of the Sahara or the Mediterranean Sea. In all, 31% of Whinchats crossed at least the Sahara and the Mediterranean before a major stopover and 17% travelled over 4751 km on average without any major stopovers. Flight distance and speed during, and duration of major stopovers after, crossing the Mediterranean Sea were indistinguishable from migration over Continental Europe. Speed during a migration leg was lowest crossing Continental Europe and fastest, with longer duration major stopovers afterwards, when crossing the Sahara, but there was much individual variation, and start date of migration was also a good predictor of stopover duration. As the distance travelled during a leg increased, so major stopover duration afterwards increased (1 day for every 1000 km), but the speed of travel during the leg had no effect. There were no differences in any migration characteristics with age, other than an earlier start date for adult birds. The results suggest that adaptive shortening or even dropping of daily stopovers may occur often, allowing rapid, long-distance migration at the cost of major stopovers afterwards, but such behaviour is not restricted to or always found when crossing barriers, even for birds on their first spring migration. The results may highlight the importance of stopover sites rather than barrier width as the likely key component to successful migration. Individual variation in spring migration may indicate that small passerine migrants like Whinchats may be resilient to future changes in the extent of barriers they encounter, although this may not be true of first autumn migrations or if stopover sites are lost.
Understanding general migration characteristics and how breeding and non-breeding sites are connected is crucial for predicting the response of long-distance migratory bird populations to environmental changes. We use data collected from six geolocators to describe migratory routes and identify breeding and non-breeding locations, migratory behaviour and differences between spring and autumn migration of Common Whitethroats Curruca communis, an Afro-Palearctic migrant, wintering in Nigeria. Most individuals departed on spring migration in April, following a north-easterly direction, arriving at their breeding grounds across central-eastern Europe (~425,000 km2) in May. Departures from breeding grounds took place between July and August in a south-westerly direction. During spring migration individuals travelled longer distances at faster rates making its overall duration shorter than autumn migration. We suggest that, while Whitethroats can cross the Sahara Desert and Mediterranean Sea in a single flight, they are likely to refuel before and after crossing. Results indicate that Whitethroats undertook loop migration and visited two wintering sites: first in the Sahel, then in Nigeria, where they remained until spring migration. Geolocator results and data from the European Union for Bird Migration’s (EURING) ringing database suggest that Whitethroats have a relatively high migratory spread—individuals from a single non-breeding site breed across a wide area of Europe. Our research is the first to track and describe the complete annual cycle of Whitethroats and one of the few studies to do so for any Afro-Palearctic migrant from non-breeding grounds. We identified the Sahel as an important refuelling and first wintering site indicating its conservation, alongside other stopover sites, is crucial for the species. We believe that changes in this region will have severe effects on a subset of individuals of specific European breeding populations, but these effects will greatly depend on the severity of the changes and at what spatial scale they occur.
Capsule: There is a substantial gain in precision and accuracy of geolocator locations when using a light stalk. Aims: Light stalks or tubes increase the accuracy of geolocators when tracking migrant birds because they reduce potential shading of the light sensor by feathers but may increase detrimental tag effects. We aimed to determine how adding light stalks to geolocator tags increased accuracy and precision of locations. Methods: We quantified how precision and accuracy of geolocator locations was affected by comparing variation of sunrise and sunset times from tags with variable length light stalks (6 of 0 mm, 8 of 5 mm and 21 of 10 mm). Tags were fitted to Whinchats Saxicola rubetra in central Nigeria (the known location to compare accuracy), and variance in latitude and longitude of geolocator estimated locations were also compared across light stalk lengths during spring migration stationary locations, and at breeding sites in Eastern Europe, for both Geolight and FlightR methods. Results: Without a light stalk, the standard deviation of sunset and sunrise times increased by 50% and 100% respectively (i.e. less precise): confidence intervals for latitude were larger by about 4.3 degrees at non-breeding low latitudes and 1.8 degrees at stopover latitudes, or confidence intervals for longitude were larger by 2.3 degrees, dependent on analysis method. Estimated sun elevation angles were significantly less accurate and so calculated non-breeding locations were significantly less accurate by about 8 degrees of latitude. Precision in sunrise, sunset times, latitude and longitude, was similar when using a 5mm or 10mm stalk. Conclusions: The results show a substantial gain in precision and accuracy of low latitude geolocator locations when using a light stalk that brings the sensor above covering feathers. There is no advantage from longer light stalk lengths than those necessary to just expose the light sensor above the feathers, at least for small passerines.
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