A partial migrant relies upon a range-wide cue set but uses population-specific weighting for migratory timing

Abstract

Background Many birds species range over vast geographic regions and migrate seasonally between their breeding and overwintering sites. Deciding when to depart for migration is one of the most consequential life-history decisions an individual may make. However, it is still not fully understood which environmental cues are used to time the onset of migration and to what extent their relative importance differs across a range of migratory strategies. We focus on departure decisions of a songbird, the Eurasian blackbird Turdus merula, in which selected Russian and Polish populations are full migrants which travel relatively long-distances, whereas Finnish and German populations exhibit partial migration with shorter migration distances. Methods We used telemetry data from the four populations (610 individuals) to determine which environmental cues individuals from each population use to initiate their autumn migration. Results When departing, individuals in all populations selected nights with high atmospheric pressure and minimal cloud cover. Fully migratory populations departed earlier in autumn, at longer day length, at higher ambient temperatures, and during nights with higher relative atmospheric pressure and more supportive winds than partial migrants; however, they did not depart in higher synchrony. Thus, while all studied populations used the same environmental cues, they used population-specific and locally tuned thresholds to determine the day of departure. Conclusions Our data support the idea that migratory timing is controlled by general, species-wide mechanisms, but fine-tuned thresholds in response to local conditions.

Eurasian Stone-curlews Burhinus oedicnemus breeding in Italy show a remarkable inter and intra-population variability of migratory behaviour

“…In this study we 1) investigated the spring and autumn migratory strategies of Stone-curlews belonging to four Italian populations, two from the North and two from the center of the peninsula; 2) performed a preliminary evaluation of the possible role of exogenous factors in modulating the phenology of autumn migration. We expected a higher proportion of migrants in northern populations than in southern ones, as reported for other Palearctic migrants (Newton & Dale 1996, Newton 2008, Linek et al . 2021) and suggested for the Stone-curlew (Delany et al .…”

“…2) performed a preliminary evaluation of the possible role of exogenous factors in modulating the phenology of autumn migration. We expected a higher proportion of migrants in northern populations than in southern ones, as reported for other Palearctic migrants (Newton & Dale 1996, Newton 2008, Linek et al 2021 and suggested for the Stone-curlew (Delany et al 2009, Biondi & Pietrelli 2015. Northern breeding birds should leave their wintering/breeding areas earlier than birds breeding more in the South as observed in other Palearctic species (Newton & Dale 1996, Newton 2008, Ambrosini et al 2016, Briedis et al 2020.…”

“…birds from northern populations are expected to depart earlier in autumn and later in spring with respect to birds from southern populations (Newton 2008, Conklin et al . 2010, Linek et al . 2021).…”

“…Investigating the migratory strategies of different populations, the geographical range of their movements and the degree of migratory connectivity is an important contribution for understanding the factors affecting populations trends (Webster et al 2002, Taylor & Norris 2010, Finch et al 2017. Furthermore, understanding how migration phenology is influenced by proximate meteorological factors can help to understand how a species would react to the ongoing climate change (Gill et al 2014, Haest et al 2018, Burnside et al 2021, Linek et al 2021.…”

“…Breeding latitude should affect the timing of departure dates, i.e. birds from northern populations are expected to depart earlier in autumn and later in spring with respect to birds from southern populations (Newton 2008, Conklin et al 2010, Linek et al 2021. The timing of migration was expected to be quite variable,as short-distance migrants usually show a quite flexible migratory schedule (Pulido & Widmer 2005, Newton 2008, Pulido & Berthold 2010, Monti et al 2018b; this variability should be particularly high in autumn, when there is no compelling need to reach the wintering sites (Shamoun Baranes et al 2006, Nilsson et al 2013.…”