Wind‐associated detours promote seasonal migratory connectivity in a flapping flying long‐distance avian migrant

Abstract

It is essential to gain knowledge about the causes and extent of migratory connectivity between stationary periods of migrants to further the understanding of processes affecting populations, and to allow efficient implementation of conservation efforts throughout the annual cycle. Avian migrants likely use optimal routes with respect to mode of locomotion, orientation and migration strategy, influenced by external factors such as wind and topography. In self‐powered flapping flying birds, any increases in fuel loads are associated with added flight costs. Energy‐minimizing migrants are therefore predicted to trade‐off extended detours against reduced travel across ecological barriers with no or limited foraging opportunities. Here, we quantify the extent of detours taken by different populations of European nightjars Caprimulgus europaeus, to test our predictions that they used routes beneficial according to energetic principles and evaluate the effect of route shape on seasonal migratory connectivity. We combined data on birds tracked from breeding sites along a longitudinal gradient from England to Sweden. We analysed the migratory connectivity between breeding and main non‐breeding sites, and en route stopover sites just south of the Sahara desert. We quantified each track's route extension relative to the direct route between breeding and wintering sites, respectively, and contrasted it to the potential detour derived from the barrier reduction along the track while accounting for potential wind effects. Nightjars extended their tracks from the direct route between breeding and main non‐breeding sites as they crossed the Mediterranean Sea–Sahara desert, the major ecological barrier in the Palaearctic–African migration system. These clockwise detours were small for birds from eastern sites but increased from east to west breeding longitude. Routes of the tracked birds were associated with partial reduction in the barrier crossing resulting in a trade‐off between route extension and barrier reduction, as expected in an energy‐minimizing migrant. This study demonstrates how the costs of barrier crossings in prevailing winds can disrupt migratory routes towards slightly different goals, and thereby promote migratory connectivity. This is an important link between individual migration strategies in association with an ecological barrier, and both spatially and demographic population patterns.

A general theory of avian migratory connectivity

“…Knight et al, 2018; Kramer et al, 2018; Ruegg et al, 2014; Trierweiler et al, 2014) and/or exploring the consequences of migratory connectivity, particularly for population dynamics (Dolman & Sutherland, 1994; Taylor & Norris, 2010; Taylor & Stutchbury, 2016). The few studies that investigated the causes of migratory connectivity found that drivers include minimising energetic costs of migration given environmental conditions en route (Norevik et al, 2020), cultural transmission of migratory behaviour (Harrison et al, 2010), land availability (Finch et al, 2017) and natal dispersal and density‐dependent population regulations (Taylor, 2019). However, these studies were largely species specific or only used theoretical simulations.…”

Regional wind patterns likely shape a seasonal migration detour

“…The differing wind patterns described in the two migration regions are consistent between the two years of the study and with other studies that describe air circulation in the region (Erni et al 2005, Athar and Ammar 2016, Patlakas et al 2019). Other loop‐migrations in the Afro‐Palearctic flyways may also be driven by these particular (Finch et al 2015, Briedis et al 2018b) or similar (Norevik et al 2019) wind patterns, and seasonally predictable wind patterns (La Sorte et al 2014) along with the distribution of suitable stopover habitat (Stach et al 2016) may explain observed seasonal differences in migration routes more generally.…”

The interplay of wind and uplift facilitates over-water flight in facultative soaring birds

“…For example, the relatively high uplift during the night compared to daytime (Fig. 2) means less drag and could lead to energetically cheaper flight in nocturnal migrants over the Mediterranean [53,54] and the Caribbean [55,56] Seas (but see [57] for a suggestion that songbirds prefer non-turbulent air for sea-crossing). Moreover, dragonflies [58] and cuckoos (Cuculus spp.…”