Beyond refueling: Investigating the diversity of functions of migratory stopover events

Abstract

Stopovers comprise a significant proportion of the time that many birds spend migrating, and researchers have long relied on these events to define and classify broader migratory strategies. Analyses of stopovers often assume that individuals stop primarily or exclusively in order to replenish energy stores, but other non-fueling behaviors have also been described during stopover events and can influence stopover incidence and duration. Here, we discuss the growing demand for understanding these non-fueling behaviors and for restoring the inherent behavioral complexity to stopover events. We begin by describing how light-weight tracking technologies allow researchers to follow individuals along their entire migratory journeys, capturing stopovers that controvert the traditional stop–refuel–resume paradigm. We then discuss 5 well-identified non-fueling behaviors—recovering, sleeping, waiting, information gathering, and social interactions—and examine how including these behaviors can alter interpretations of individual movement paths. Finally, we outline emerging directions for identifying these behaviors and look to larger implications for population management and site conservation along migratory flyways.

When to depart from a stopover site? Time since arrival matters more than current weather conditions

“…TSA encompasses different functions of the stopover behavior: (1) resting after a migratory flight with the associated physiological processes (McWilliams et al 2004, Skrip et al 2015, Eikenaar et al 2020a, 2020b and ( 2) reaching a sufficient level of fuel load to perform the following migratory flight (Schmaljohann and Eikenaar 2017). TSA also reflects (3) the refueling rate, which depends on environmental conditions and physiological processes (Jenni and Schaub 2003, Schmaljohann and Eikenaar 2017, Eikenaar et al 2020a, Linscott and Senner 2021. Here, TSA has a positive effect on departure probability: birds need to stay a minimum number of days before leaving (virtually no departure is predicted during the very first days; see Figure 2).…”

“…We develop a CR model and analyze a long-term CR dataset from a long-distance migrant songbird at a stopover site. Because improving its internal state through refueling, recovering, and sleeping seems to be the major biological function of stopover (Schmaljohann and Eikenaar 2017, Schmaljohann 2018, Eikenaar et al 2020a, 2020b, Linscott and Senner 2021, we expect that TSA will be the main driver of departure probability in this longdistance migrant and that wind, humidity, cloud cover, and atmospheric pressure will have a secondary but significant effect on departure probability (Packmor et al 2020). Estimated TSA should be closer to the genuine duration of the individual's stay than the time elapsed between the first and the last physical captures of the individual.…”

Stopover use of a large estuarine wetland by dunlins during spring and autumn migrations: Linking local refuelling conditions to migratory strategies

“…Sites along migratory routes where birds spend stationary periods have been classified as either “stopover” or “staging” sites, with the former acting more as a “resting” spot, where birds stay for short periods, and the latter being used for long refuelling periods [ 24 , 25 ]. However, the ability of birds to change migratory behaviour between and within seasons may deliver a variable role to a same stopover site according to individual’s internal state and the external environment [ 26 ]. Therefore, a single site can be differently used by the same population in different seasons, or by different individuals within the same season, according to date and their tightness to the migratory calendar.…”

Avian Use of Agricultural Areas as Migration Stopover Sites: A Review of Crop Management Practices and Ecological Correlates

“…Migratory birds (hereafter "migrants") spend up to one-third of each year migrating (Keast and Morton, 1980). Since most species cannot go from breeding to non-breeding grounds in a single flight, individuals alternate between periods of active migration and temporary suspensions of their migration for feeding, resting, recovering, and waiting for favorable weather conditions (hereafter "stopovers") (Piersma, 1987;Hutto, 1998;Rosenberg et al, 2017;Linscott and Senner, 2021). Migrants rely heavily on stopover sites to quickly replenish fat stores (Péron et al, 2007).…”

“…For birds, temporary primary crops can provide food in the form of seeds, the plants themselves, or vertebrates and invertebrates that live among the crops (Stafford et al, 2010). Furthermore, crops provide shelter from predators and places to rest (Hutto, 1998;Linscott and Senner, 2021). Research focusing on rice paddies has revealed extensive use of this habitat by migrating birds in the United States, Europe, and East Asia, even suggesting some of these areas should formally be designated as "Important Bird and Biodiversity Areas" for bird diversity and conservation (Acosta et al, 2010;Masero et al, 2011;Johnson et al, 2012;Yamaguchi et al, 2012;Grishchenko et al, 2019).…”