EUROPEAN ROLLER (Coracias garrulus)
Unravelling the wintering sites of Portuguese rollers
We used light-level geolocators to track the migratory journey of rollers from its breeding grounds in Portugal to its wintering grounds in southern Africa. During autumn migration, birds followed the western African coast with lengthy stopovers within sub-Saharan countries before crossing the equatorial rainforests towards the wintering areas, mainly in Angola.
Migration routes, stopover sites and wintering grounds of two Portuguese rollers. The breeding site in Castro Verde (Portugal) is represented by a red star. Small white circles correspond to individual locations at stopover and wintering sites, and core areas within these periods are represented by 50 and 75 % kernel density contours. Red circles represent individual locations during active autumn migration. Solid arrows indicate approximate autumn migration routes derived from positioning data.
Migratory connectivity of European rollers
In a collaborative study conducted by our colleague Tom Finch (UEA), we synthesize new geolocator data with existing geolocator, satellite tag and ring recovery data from eight countries across Europe. We expand single-country studies into a pan-European analysis by combining all existing published data with new data from rollers tagged in Portugal, Spain, France, Austria, Montenegro, Latvia and Cyprus.
Why study migratory connectivity?
Multipopulation studies can provide insight into migratory connectivity, that is, the extent of mixing of different breeding populations during the non-breeding season. Migratory connectivity is described along a continuum from complete segregation (‘strong’ connectivity) to complete mixing (‘weak’ connectivity) of different breeding populations during the non-breeding season. The strength of connectivity is expected to underpin the response of populations to habitat loss with strong connectivity increasing their vulnerability, as any local deterioration in non-breeding conditions will be felt by all members of a breeding population.
In western rollers, autumn migration occurred on two fronts: rollers from the south-west of the region took a westerly route along the Atlantic coast of West Africa and then turned east along the savanna belt, whereas those from north-east Spain and southern France took a more direct route across the Sahara. Interestingly, individuals from both sides of this migratory divide eventually converged at similar autumn stopover sites and wintered in the same region of south-west Africa. For eastern rollers, there was no evidence of any detours shortening the southward Sahara crossing. Our study supports previous work identifying the northern savanna zone, particularly the Lake Chad basin, as an important autumn stopover site for rollers from western European populations.
All tracked rollers wintered in the tropical grasslands, savannas and shrublands (Olson et al., 2001) of southern Africa, 4000–8600 km away from their breeding sites. Although the winter ranges of different (and often distant) breeding populations overlapped, the distribution of wintering rollers with respect to their breeding origin was significantly non-random. In addition to strong longitudinal correlations between breeding and winter sites, there was a correlation between pairwise distance matrices of breeding and winter sites. These results suggest that migratory connectivity is neither absent nor strong, but somewhere in the middle of the ‘weak–strong’ continuum defined by Webster et al. (2002).
Migratory connectivity in the European roller revealed by geolocators, satellite tags and a ring recovery (Mollweide equal-area projection). Loxodromic lines (not intended to represent routes taken) connect breeding and winter sites of 33 European rollers from eight countries. Inset top: vertical lines represent latitudinal shift in mean winter position resulting from ± 1.0° to sun elevation angle. Inset bottom: bars show variation (SD) surrounding mean winter locations.
The northern savanna zone also appears to be used on spring migration, when stopovers were generally south of their autumn equivalents (although note large latitudinal uncertainty due to proximity to equinox and equator), as expected given that the northern savanna becomes increasingly dry over winter. All individuals exhibited, to some extent, a loop migration. French and Montenegrin rollers had a small clockwise loop, whereas Latvian rollers had a large anticlockwise loop, taking them to the Horn of Africa and along the Arabian Peninsula.
Impacts of tracking devices on rollers
Tracking devices have contributed enormously to our knowledge of avian migration, although their effects on birds are controversial. In a study leaded by Juan Rodríguez Ruiz, we study the short- and long-term effects of deploying geolocators on rollers and assess the optimal weight of tracking devices to use.
We concluded that in nests in which both parents had geolocators, brood mass was lighter than in nests where only one or neither parent had a geolocator. The year-to-year recapture rate for rollers tagged with geolocators was lower than that for control birds and the recapture rate in different populations was negatively related to the device-to-bird weight ratio, decreasing greatly when the weight ratio exceeded 2.5%.
Relationship between device-to-bird weight ratio and recovery rate of rollers in each locality. Photo: a roller being tagged with a geolocator.
Catry I, Catry T, Granadeiro JP, Franco AMA, Moreira F (2014) Unravelling migration routes and wintering grounds of European rollers using light-level geolocators. Journal of Ornithology 155: 1071-1075.
Finch T, Saunders P, Avilés JM, Bermejo A, Catry I, de la Puente J, Emmenegger T, Mardega I, Mayet P, Parejo D, Račinskis E, Rodríguez-Ruiz J, Sackl P, Schwartz T, Tiefenbach M, Valera F, Hewson C, Franco A, Butler SJ (2015) A pan-European, multipopulation assessment of migratory connectivity in a near-threatened migrant bird. Diversity and Distributions 21: 1051-1062.
Rodríguez-Ruiz J, Parejo D, de la Puente J, Valera F, Calero-Torralbo MA, Bermejo A, Catry I, Avilés JM (2016) Short- and long-term effects of tracking devices on the European Roller Coracias garrulus. Ibis 158: 179-183.