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

Abstract

Phenological mismatch is often cited as a putative driver of population declines in longdistance migratory birds. The mechanisms and cues utilised to advance breeding ground arrival will impact the adaptability of species to further warming. Furthermore, timing of postbreeding migration potentially faces diverging selective pressures, with earlier onset of tropical dry seasons favouring migration advancement, whilst 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 firstarrival and last-departure dates of 20 species of trans-Saharan migratory birds from tropical non-breeding grounds (The Gambia), between 1964 and 2019. Additionally, we use firstarrival 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, whilst 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 16 days, on average, over the 27-year monitoring period. By contrast, duration of species' stays on the non-breeding range declined by 63 days, on average, over the 56-year monitoring period. Taken together these changes suggest substantial, previously unreported alterations to annual routines in Afro-Palearctic migrants.

The significance of partial migration for food web and ecosystem dynamics

“…Seasonal migrations occur annually and are commonly linked to transitions between seasons. For instance, numerous bird species seasonally migrate between breeding ecosystems and wintering ecosystems, which coincides with transitions between seasons (Lawrence et al, 2021). Diel migrations include the return migration of organisms between ecosystems within a 24-h period.…”

“…The environmental drivers of migratory behaviour can be extrinsic to the ecosystems (e.g. climate; Lawrence et al, 2021), but for many taxa, the drivers are often intrinsic ecological components of ecosystems. In particular, field and experimental studies have shown the propensity of individuals to migrate can be inf luenced by resource availability (e.g.…”

“…Examples related to climate change are accumulating, for instance, the advancement of phenological events (e.g. spring flowering) on the timing of seasonal bird migrations (Lawrence et al, 2021;Saino et al, 2009). Habitat change examples include the introduction of landfills and agricultural lands leading to the loss of migratory behaviour from bird and mammal populations, by increasing resource availability in their resident ecosystem year-round (Crone & Schultz, 2021;Gilbert et al, 2016;Xu et al, 2021).…”

“…Among the best‐studied examples of partial migration, and contemporary changes in migratory behaviour, are birds and fish. Many bird species inhabiting tropical to temperate environments, including storks, vultures and many songbirds, have been adjusting their seasonal migration patterns to become partially migratory species within the last few decades (Lawrence et al, 2021; Pulido & Berthold, 2010). Likewise, many fish species are partially migratory (Chapman, Hulthén, et al, 2011).…”

Individual repeatability of avian migration phenology: A systematic review and meta‐analysis

“…Changes in migratory behaviour in response to climate change have been documented in many species (Ambrosini et al, 2019). The most frequent responses are shifts in phenology in parallel with climate warming, for example migrant arrival dates at the breeding grounds in spring are getting earlier in many species (Gordo, 2007; Gunnarsson & Tómasson, 2011; Lawrence et al, 2022). In some species, shifts in migratory routes and wintering destinations (Dias et al, 2011; Sutherland, 1998) or reduced propensity for migration have been recorded, such that part or all of a population has become resident (Chapman et al, 2011; van Vliet et al, 2009).…”