Autumn migrating passerines at a desert edge: Do birds depart for migration after reaching a threshold fuel load or vary it according to the rate of fuel deposition?

Abstract

Fuel deposition rate is predicted to determine departure fuel load during stopover in two models of optimal behavior of migrating birds. Yet, near ecological barriers, such as wide deserts, birds may switch to a different strategy of departing with just enough fuel to enable the long cross-barrier flight, thus reaching a threshold of fuel load regardless of the rate of fuel deposition. To test these predictions we studied autumn migrating Red-backed Shrikes (Lanius collurio) before they departed for a ∼2,000 km journey across the Sahara Desert. The body mass of fourteen individuals was measured on a daily basis throughout their stopover using field-deployed scales while being tracked by the ATLAS biotelemetry system in the Hula Valley, Israel. Statistical analysis found that the natural log of departure fuel load was positively related to both the capture fuel load and the fuel deposition rate. Hence, the results of this analysis suggest that bird condition at departure depended on the rate of fuel deposition, as predicted by models of time-minimization migration and the minimization of the total energy cost of migration. Departure fuel load and stopover duration were negatively related to each other as birds that remained for a long time in stopover departed with relatively low fuel loads. These findings suggest that even near a wide ecological barrier, departure fuel load is sensitive to the rate of fuel deposition, especially at lower values of fuel deposition rate. Birds that were able to accumulate fuel at higher rates showed a nearly constant departure fuel load and as such we could not exclude the possibility that the birds were trying to reach a certain threshold of fuel stores. Randomized 1,000 repeats of the aforementioned correlation suggest that the correlation between fuel deposition rate and the log of departure fuel load is valid and does not represent a spurious result. Following bird migration simulation using the program Flight, we conclude that fuel loads allowed most individual to accomplish the journey across the desert. Our findings suggest high between-individual variation in stopover parameters with likely consequences for bird migration performance and survival.

Competition is a major limiting factor of refueling in migratory passerines during stopover

“…However, even though the proximity of a barrier may elevate the importance of energy efficiency (Weber et al 1998, Schaub et al 2008, Buler and Moore 2011), as the adjustments mentioned above might indicate, we found no difference either in mean FDR or in stopover duration between the autumn and spring migrations, implying comparable importance of fuel reserves either prior to or post the barrier crossing. These results are consistent with previous findings on red‐backed shrikes Lanius collurio approaching the crossing of the Sahara, showing highly variable FDRs and no clear evidence of barrier‐related minimal departure fuel load (Zinßmeister et al 2022).…”

“…The commonly proposed mechanisms for inter‐seasonal discrepancies in such stopover dynamics are distinct motivations driving the migrants when travelling to or from their breeding grounds (Schmaljohann et al 2022). For instance, if the south‐bound long‐distance migrants aim to attain a certain threshold of fuel reserves in anticipation of a barrier crossing (Zinßmeister et al 2022), they may intensify their foraging efforts and hence raise the likelihood of interference competition in autumn as compared to spring (Helm et al 2006, Schmaljohann et al 2022, Smith and McWilliams 2014). In spring, by contrast, the migrants reaching the northern edge of the Sahara Desert might lower the priority of fuel stores buildup, given a focus shift from barrier crossing towards securing a breeding territory and mating (Nilsson et al 2013, Duijns et al 2019, Schmaljohann et al 2022).…”

Endocrine mechanisms controlling the migratory disposition in birds

“…К настоящему моменту известно, что механизмы эндокринной регуляции (Wingfield et al 1990;Romero and Wingfield 1999;Loschagina et al 2018;Sharma et al 2018) При изучении регуляции миграционного состояния необходимо количественно выразить отдельные компоненты этого состояния. В природе с помощью разных методик мы можем оценить: продолжительность остановки, количество потребленного корма, скорость жи-ронакопления (прирост массы энергетических резервов), время ночного старта (относительно захода солнца), дальность перемещений в течение дня и другие аспекты пространственного поведения (Chernetsov 2012;Bulyuk and Tsvey 2013;Müller et al 2016;Zinßmeister et al 2022). В связи со значительным прогрессом в развитии прослеживающих устройств к настоящему моменту можно оценить также длительность полета, временной график полетов в сезоне миграции, выбор оптимальной высоты и даже расход энергии в полете (Wikelski et al 2003;Schmaljo hann et al 2009;Bäckman et al 2017;Sjöberg et al 2021).…”