Changes in fat mass affect the motivation to migrate in northern wheatears

Abstract

To fuel their migratory endurance flights, most birds accumulate large quantities of fat prior to departure. It therefore seems logical that the decision to depart on a migratory flight depends on fuel stores, at least to some extent; very small fuel stores prohibit migratory endurance flight. However, studies linking migrants' estimated fuel stores (or sometimes actual fat stores) to departure likelihood or to migratory restlessness measured in captive birds (an accurate proxy for migrants' motivation to depart from stopover) have provided inconsistent results. Fuel stores are not static though, but highly dynamic. Instead of investigating fuel stores at a given time, we therefore focussed on quantifying the dynamic process of fuelling and how it affects migrants' motivation to depart. We did so by temporarily caging wild northern wheatears Oenanthe oenanthe at a stopover site during autumn migration. Birds were kept under controlled conditions, receiving ad libitum food. Changes in fat mass (measured by quantitative magnetic resonance) and migratory restlessness were closely monitored. We found that the change in fat mass was a good predictor of the change in migratory restlessness. Individuals accumulating little fat did not show much change in their restlessness, while individuals accumulating much fat showed a strong increase in restlessness. Our study therewith provides clear support for the idea that fat stores affect birds' motivation to migrate, and shows that for migrating birds, increasing fat stores are an important cue to depart from stopover. Fat accumulation affected the change in migratory restlessness in the same manner for relatively lean and relatively fat birds. Additionally, food intake or changes therein did not explain the change in migratory restlessness we observed. We therefore hypothesize that the fat tissue itself signals changes in energetic state, likely through production and release of hormones, affecting birds' motivation to migrate.

A magnetic pulse does not affect free-flight navigation behaviour of a medium-distance songbird migrant in spring

“…accumulating energy stores. By that, we ensured that all birds carried sufficient and comparable energy stores important for high motivation to resume migration, as many studies have shown a significant positive correlation between energy stores and departure probability ( Schmaljohann et al, 2022 ; Schmaljohann and Eikenaar, 2017 ; Klinner et al, 2020 ; Eikenaar et al, 2021 ).…”

A magnetic pulse does not affect free-flight navigation behaviour of a medium-distance songbird migrant in spring

“…refuelling on energy stores. By that, we ensured that all birds carried sufficient and comparable energy stores important for high motivation to resume migration, as many studies have shown a significant positive correlation between energy stores and departure probability (Schmaljohann and Eikenaar 2017; Klinner et al 2020; Eikenaar, Karwinkel, and Hessler 2021).…”

The urge to breed early: Similar responses to environmental conditions in short‐ and long‐distance migrants during spring migration

“…Departure decisions have two temporal scales, day‐to‐day and within‐night (Müller et al, 2016 ), and both are directly linked to the routing decision (Schmaljohann & Naef‐Daenzer, 2011 ). Notably, birds adjust their migratory decisions based on intrinsic factors, for example, energy stores (Deppe et al, 2015 ; Eikenaar et al, 2016 , 2021 ; Goymann et al, 2010 ; Schmaljohann et al, 2013 ) and immune function (Brust, Eikenaar, et al, 2022 ; Hegemann et al, 2018 ), and react to extrinsic conditions such as weather (Brust et al, 2019 ; Klinner & Schmaljohann, 2020 ; Schmaljohann et al, 2017 ) and ecological barriers (Deppe et al, 2015 ; Sandberg & Moore, 1996 ; Smolinsky et al, 2013 ). As birds en route constantly react to changing environmental conditions, spatial and temporal migration patterns can be highly flexible among and within individuals (Åkesson & Helm, 2020 ; Stanley et al, 2012 ).…”