Differential changes in the morphology and fuel loads of obligatory and partial migrant passerines over half a century in Britain

Bird ringing, by Rob Read / BTO

Author(s): Pickett, H.R.W., Robinson, R.A. & Nudds, R.L.

Published: September 2024  

Journal: Movement Ecology Volume: 12

Article No.: 60

Digital Identifier No. (DOI): 10.1186/s40462-024-00497-3

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Populations of many species will be affected by climate change, but migratory species are expected to be affected more than most, in part because change can occur on both the breeding and non-breeding grounds, but at different rates.

Climate change can also lead to changes in distribution, at either the northern or southern end of their migratory route. In turn, this might lead to changes in the distance individuals can, or need to, migrate. Flight efficiency is closely tied to wing shape; longer, more pointed wings allow faster flight, albeit at the cost of reduced manoeuvrability. Therefore species that migrate longer distances tend to have longer, more pointed wings, a
difference that holds even between populations of the same species.

This study set out to ask whether we could observe, from measurements taking when birds are being ringed, whether there have been changes in wing length or body mass in the last 60 years as climates have changed? The authors looked at wing length and body weight data collected from individuals of 15 species at three bird observatories on the east and south coasts of England which were likely to catch birds on their migratory journeys. Changes in either wing or weight over time were found in nine of the 15 species although the direction and size of these changes differed between species indicating that the impacts of climate change are unlikely to have a universal predictable effect.

For some species, all long-distance migrants, there was a tendency for increased mass (and hence a greater potential flight range), which could indicate the need to cross larger areas of inhospitable terrain. For short-distance migratory species, on the other hand, we found the opposite pattern, with reductions in predicted flight range, which may indicate that these species are able to adapt better to the changed environmental conditions. These results highlight the importance of conditions on migratory stopover sites, perhaps especially in the Mediterranean and North Africa that immediately precede major geographical barriers, and the need to have habitats of sufficient quality to ensure migrating birds can complete their journey successfully.

Abstract
Migratory distances and stopover locations are changing for many passerines in response to climate change. Morphological changes have been linked to rising global temperatures in both migrants and residents, but the implications of these changes on fuel loads, and associated flight ranges are little studied. Wing length and body mass changes between 1964 and 2020 were calculated for 15 migrant and partially migrant passerines in Britain. Changes in fuel load and lean body mass were also estimated and used to predict flight ranges. Twelve of the species have undergone morphological changes and eight species, estimated fuel load changes. Nine species were estimated to have reduced flight ranges, indicating that the morphological changes have not compensated fully for the reduction in flight range experienced since 1964. Partial migrants showed greater decreases in flight ranges than did full migrants, which may indicate greater behavioural plasticity in the former. Those species which do not adapt morphologically or behaviourally may be unable to complete long migrations, resulting in restriction to sub-optimal breeding/wintering habitats, or a need for a sooner first stop and more stops en route. This highlights the importance of conserving migratory stopover sites, particularly in the Mediterranean and North Africa that immediately precede major geographical barriers, as-well-as breeding and wintering grounds.

Notes

BTO provided all ringing data. 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 NatureScot and the Department of the Environment Northern Ireland), The National Parks and Wildlife Service (Ireland) and the ringers themselves. Thank you also to the Rye Bay Ringing Group, Gibraltar Point Bird Observatory and Portland Bird Observatory for all their vital front-line work ringing and migration monitoring.
Staff Author(s)

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