Migration facilitates the use of resources that vary across space and time and is an important event in the annual cycle of many animals. Mobile animals can use migration to avoid unfavorable conditions (e.g., high predation risk, severe weather, low food availability, parasites) or exploit favorable conditions (e.g., abundant or high-quality food resources, mating aggregations, thermal refugia) (Shaw, 2016). In many cases, migrations occur predictably in space and time with animals moving each year between the same locations at approximately the same time; we refer to these as obligate migrations (also termed to-and-fro or calendar migrations; see Box 1 for more on terminology). However, migrations can also occur with much greater flexibility with respect to their timing and destination (see Box 1). For example, individuals may be flexible in whether they undertake migration from one year to the next (e.g., facultative partial migration; Terrill and Able, 1988). Migrations may occur aseasonally, in response to either adverse or advantageous conditions (e.g., irruptive migration and nomadic migration; Dingle, 2014; Teitelbaum and Mueller, 2019). Alternatively, migrations may be irregular with respect to their destination, with individuals moving between varying locations (e.g., nomadic migration; Teitelbaum and Mueller, 2019). Collectively, we refer to these more flexible movement patterns as facultative migrations (following Newton, 2012; Ramenofsky et al., 2012; Watts et al., 2018).

The endocrine system plays a central role in coordinating changes in morphology, physiology, and behavior as animals transition between different stages in their annual cycle, including migration (Jacobs and Wingfield, 2000). At present, much of our understanding of the role of the endocrine system in regulating the transition to a migratory stage comes from studies of obligate migration – and predominately avian obligate migration – with much less known about facultative migrations. Yet facultative migrations occur in a wide array of taxa (Holland et al., 2006; Newton, 2006a; Plotkin, 2010; Teitelbaum and Mueller, 2019), including among species or individuals that also exhibit obligate migrations or are otherwise generally non-migratory (Hahn et al., 2004; Jakes et al., 2018; Teitelbaum et al., 2023; Terrill and Ohmart, 1984). Moreover, growing data on animal movements, facilitated in part by recent advances in tracking technologies, suggest that facultative migrations may be more common than previously recognized (Streby et al., 2015; Teitelbaum et al., 2023). These observations suggest that the study of facultative migrations, including across a diverse array of species, is central to a broad understanding of the endocrine mechanisms involved in mediating migratory transitions.

The diverse array of migratory movements presents a remarkable challenge to understanding the mechanisms that regulate facultative migration. Facultative migrations include nomadic wandering to locate rich but ephemeral food sources (Eby et al., 2014; Fryxell et al., 2004; Pedler et al., 2014; Roshier et al., 2008) as well as more localized to-and-fro movements to temporarily escape inhospitable conditions (e.g., snow storms: Hahn et al., 2004; O'Neill and Parker, 1978; or tropical storms: Udyawer et al., 2013). They are unified by a high degree of variability in spatial or temporal patterning in response to relatively unpredictable biotic or abiotic conditions. Yet, these movements are likely quite heterogenous with respect to the cues that trigger them and the physiological mechanisms that coordinate the response. But an overarching framework to describe and conceptualize this variation has been lacking. Indeed, this challenge is particularly apparent when one considers that varied forms of facultative migration may be routinely exhibited even within a single population or across the lifetime of a single individual (Bennetts and Kitchens, 2000; Newton, 2006a; Newton, 2006b). We propose that a consideration of the conditions at departure provides a unifying framework to describe variation in facultative migrations. This framework can be used to generate hypotheses about cues used to make departure decisions and the physiological mechanisms involved. Further, it can serve as a foundation for understanding variation within and across species in the use of particular cues and mechanisms. Here we give an overview of our framework and then review potential endocrine mechanisms involved in facultative migration in the context of this framework.