Phenological trends in the pre- and post-breeding migration of long-distance migratory birds.

Kieran B Lawrence,Clive R Barlow,Charles Perez,Stephen G Willis,Keith Bensusan

doi:10.1111/gcb.15916

Abstract

Phenological mismatch is often cited as a putative driver of population declines in long-distance migratory birds. The mechanisms and cues utilized to advance breeding ground arrival will impact the adaptability of species to further warming. Furthermore, timing of post-breeding migration potentially faces diverging selective pressures, with earlier onset of tropical dry seasons favouring migration advancement, while longer growing seasons in temperate areas could facilitate delayed departures. Despite this, few studies exist of migration phenology on the non-breeding grounds or on post-breeding passage. Here, we use first arrival and last departure dates of 20species of trans-Saharan migratory birds from tropical non-breeding grounds (The Gambia), between 1964 and 2019. Additionally, we use first arrival and last departure dates, as well as median arrival and departure dates, at an entry/departure site to/from Europe (Gibraltar), between 1991 and 2018. We assess phenological trends in pre- and post-breeding migration, as well as individual species' durations of stay in breeding and non-breeding areas. Furthermore, we assess the extent to which inter-annual variation in these timings may be explained by meteorological and ecological variables. We find significant advances in pre-breeding migration at both locations, while post-breeding migration is delayed. At Gibraltar, these trends do not differ between first/last and median dates of migration. The combination of these trends suggests substantial changes in the temporal usage of the two continents by migratory birds. Duration of stay (of species, not individuals) within Europe increased by 16days, on average, over the 27-year monitoring period. By contrast, duration of species' stays on the non-breeding range declined by 63days, on average, over the 56-year monitoring period. Taken together these changes suggest substantial, previously unreported alterations to annual routines in Afro-Palaearctic migrants.

Highlights

Long-distance migratory birds have evolved to take advantage of spatially-segregated, ephemeral resource peaks (Newton, 2008)
By studying phenology at these non-breeding localities, we aim to infer whether: (1) pre-breeding migration on the non-breeding grounds and/or at an intermediate migratory site has advanced, in line with those phenological changes observed on the breeding grounds, (2) the phenology of post-breeding migration has undergone any significant change at either site and (3) species are spending longer on European breeding grounds as a result of longer growing seasons and either altering the amount of time on the non-breeding range, or the speed of their migration to the nonbreeding range
As we considered Sahelian normalized difference vegetation index (NDVI) as better representing overall sub-Saharan conditions, we used these data in models in preference to Gambian NDVI

Summary

Introduction

Long-distance migratory birds have evolved to take advantage of spatially-segregated, ephemeral resource peaks (Newton, 2008). Advancement in phenology is one of the most frequently documented biological responses to recent climate change, with events such as leaf bud-burst, insect emergence and bird breeding occurring significantly earlier in the year in temperate regions than they did at the end of the last century (Burgess et al, 2018; Post et al, 2018) These advances exhibit significant spatial variation, due to the relationship between latitude and extent of warming and, the phenology of mid- to high-latitude areas has advanced more rapidly than those of the tropics (Post et al, 2018). The severity of recent phenological shifts may have pushed some species to their physiological limits, with breeding date constrained by the timing of arrival (Both & Visser, 2001)

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