I recently collaborated with the excellent Yaara Aharon-Rotman (rapidly becoming my most frequent co-author) on a new paper about migratory birds and their departure decisions when on migration. A major challenge for long-distance migratory birds is that they often rely on multiple sites along their migration route. Migratory birds time their movements to synchronize with resource availability en-route and most importantly on their breeding site.
The cues these long-distance migrants use to start migration, and how to adjust migration timing to match conditions in a distant breeding site, remain poorly understood, especially in passerine bird species.
Our aim was to study the relationship between spring migration timing and environmental factors at the African pre-departure sites and at a stopover site in four long distance migratory bird species. To this end, we first looked for changes in the arrival times of these species to an important stopover site in Israel between 2000 and 2021. Next, we tested if environmental conditions at the departure site in Africa, measured as Enhanced Vegetation Index (EVI, a measure of greenness in the landscape), and local temperatures at the stopover site can explain variation in arrival timing. We also analysed changes in wing length as a proxy for migration distance, to identify potentially different populations within the individuals stopping in Israel or age-biased differences. We hypothesised that high EVI at the departure site and higher spring temperatures at the stopover site will facilitate earlier departure due to fast fuelling and flexibility to fine-tune migration speed as a response to conditions en route.
While Lesser whitethroat and Eastern Bonelli’s warbler advanced arrival to the stopover site, the arrival of Thrush nightingale and Olive-tree warbler did not change throughout the study period and was associated with mean EVI on the pre-departure site in the Horn of Africa during the period before departure. Importantly, we show that for Lesser whitethroat, a species with northern breeding areas, temperatures at the stopover site were correlated to arrival timing, suggesting fine-tuning of stopover arrival to match conditions en-route.
Here, we suggest that some species are able to show flexibility and fine-tuned migration speed in response to local conditions en route. This flexibility may be a result of phenotypic plasticity, but we lack individual data to support this argument. Thus, individuals are likely using constant decision making processes based on local weather and body condition throughout the migration route in order to match optimal breeding conditions.
Limiting conditions at the pre-departure and stopover sites of long distance migratory species may have consequences for reproduction success and survival as a result of mismatched arrival and “thermal delay”, emphasizing the challenges to long-distance migratory species in the race to match global changes to ensure their survival.
Click here to read the full paper.