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

Abstract

Birds migrating different distances experience different temporal, energetic, physiological, and physical constraints throughout migration, which is reflected in their migration strategy. Consequently, we predict different behavioral decisions to similar environmental cues between short-and long-distance migrants, which has been documented for autumn migration. Here, we focus on the question whether tradeoff decisions regarding departure, routing, and landing when alternating between migratory endurance flights and stopovers also differ during spring migration. As early arrivals at the breeding grounds should be ultimately favored regardless of migration distance, selection may favor more similar behavioral decisions in spring than in autumn. We radio-tagged short-and long-distance migratory songbirds at stopover sites along the German North Sea coast during spring and automatically tracked their migratory behavior using a large-scale network of receiver stations. Once departed, birds could either cross the sea or detour along the coast. We corrected for spatially biased detection data, using a hierarchical multistate model to assess how birds respond to variation in environmental conditions in their day-to-day departure decisions and route selection. The day-to-day departure probability was higher in long-distance migrants independently of the routing decision. Irrespective of migration distance, all species more likely departed under light winds and rainless conditions, while the influence of air pressure change and relative humidity was species-specific. By accounting for detection probabilities, we estimated that about half of all individuals of each species crossed the sea but did not find differences between short-and longdistance migrants. Offshore flights were more likely when winds blew offshore and began earlier within the night compared with onshore flights. Our results suggest that selection more similarly affects birds of different migration distances in spring than in autumn. These findings put the focus toward how ultimate mechanisms may shape departure and routing decisions differently between migration seasons.

Seasonal migration patterns of Siberian Rubythroat (Calliope calliope) facing the Qinghai-Tibet Plateau

“…Nevertheless, the strategy taking the shorter path in spring associated with higher risks suggests that the bene t must be substantial. The fast journey and earlier arrival can be bene cial to a higher tness during the breeding phase, e.g., to allow the birds to be more competitive among males and favored by females at the breeding site (Kokko, 1999;Rotics et al, 2018;Rüppel et al, 2023;Smith & Moore, 2005). In addition, Siberian Rubythroats are highly faithful to their breeding territory (Heim et al, 2018; Zhao, personal observational data).…”