Developing a Stopover-CORT hypothesis: Corticosterone predicts body composition and refueling rate in Gray Catbirds during migratory stopover

Does Migration Constrain Glucocorticoid Phenotypes? Testing Corticosterone Levels during Breeding in Migratory Versus Resident Birds

Corticosterone, the main glucocorticoid in birds, is a major mediator of the incredible physiological feat of migration. Corticosterone plays important roles in migration, from preparation to in-flight energy mobilization to refueling, and corticosterone levels often show distinct elevations or depressions during certain stages of the migratory process. Here, we ask whether corticosterone's role in migration shapes its modulation during other life-history stages, as is the case with some other phenotypically flexible traits involved in migration. Specifically, we use a global dataset of corticosterone measures to test whether birds' migratory status (migrant versus resident) predicts corticosterone levels during breeding. Our results indicate that migratory status predicts neither baseline nor stress-induced corticosterone levels in breeding birds; despite corticosterone's role in migration, we find no evidence that migratory corticosterone phenotypes carry over to breeding. We encourage future studies to continue to explore corticosterone in migrants versus residents across the annual cycle. Additionally, future efforts should aim to disentangle the possible effects of environmental conditions and migratory status on corticosterone phenotypes; potentially fruitful avenues include focusing on regions where migrants and residents overlap during breeding. Overall, insights from work in this area could demonstrate whether migration shapes traits during other important life stages, identify tradeoffs or limitations associated with the migratory lifestyle, and ultimately shed light on the evolution of flexible traits and migration.

Linking migration and microbiota at a major stopover site in a long-distance avian migrant

Migration is one of the most physical and energetically demanding periods in an individual bird's life. The composition of the bird's gut or cloacal microbiota can temporarily change during migration, likely due to differences in diets, habitats and other environmental conditions experienced en route. However, how physiological condition, migratory patterns, and other drivers interact to affect microbiota composition of migratory birds is still unclear. We sampled the cloacal bacterial microbiota of a long-distance migrant, the steppe buzzard (Buteo buteo vulpinus), at an important spring stopover bottleneck in Eilat, Israel, after crossing the ca. 1800 km Sahara Desert. We examined whether diversity and composition of the cloacal microbiota varied with body condition, sex, movement patterns (i.e., arrival time and migration distance), and survival. Early arrival to Eilat was associated with better body condition, longer post-Eilat spring migration distance, higher microbial α-diversity, and differences in microbiota composition. Specifically, early arrivals had higher abundance of the phylum Synergistota and five genera, including Jonquetella and Peptococcus, whereas the phylum Proteobacteria and genus Escherichia-Shigella (as well as three other genera) were more abundant in later arrivals. While the differences in α-diversity and Escherichia-Shigella seem to be mainly driven by body condition, other compositional differences associated with arrival date could be indicators of longer migratory journeys (e.g., pre-fueling at wintering grounds or stopover habitats along the way) or migratory performance. No significant differences were found between the microbiota of surviving and non-surviving individuals. Overall, our results indicate that variation in steppe buzzard microbiota is linked to variation in migratory patterns (i.e., capture/arrival date) and body condition, highlighting the importance of sampling the microbiota of GPS-tracked individuals on multiple occasions along their migration routes to gain a more detailed understanding of the links between migration, microbiota, and health in birds.

Understanding the ecological and evolutionary function of stopover in migrating birds

Global movement patterns of migratory birds illustrate their fascinating physical and physiological abilities to cross continents and oceans. During their voyages, most birds land multiple times to make so-called 'stopovers'. Our current knowledge on the functions of stopover is mainly based on the proximate study of departure decisions. However, such studies are insufficient to gauge fully the ecological and evolutionary functions of stopover. If we study how a focal trait, e.g. changes in energy stores, affects the decision to depart from a stopover without considering the trait(s) that actually caused the bird to land, e.g. unfavourable environmental conditions for flight, we misinterpret the function of the stopover. It is thus important to realise and acknowledge that stopovers have many different functions, and that not every migrant has the same (set of) reasons to stop-over. Additionally, we may obtain contradictory results because the significance of different traits to a migrant is context dependent. For instance, late spring migrants may be more prone to risk-taking and depart from a stopover with lower energy stores than early spring migrants. Thus, we neglect that departure decisions are subject to selection to minimise immediate (mortality risk) and/or delayed (low future reproductive output) fitness costs. To alleviate these issues, we first define stopover as an interruption of migratory endurance flight to minimise immediate and/or delayed fitness costs. Second, we review all probable functions of stopover, which include accumulating energy, various forms of physiological recovery and avoiding adverse environmental conditions for flight, and list potential other functions that are less well studied, such as minimising predation, recovery from physical exhaustion and spatiotemporal adjustments to migration. Third, derived from these aspects, we argue for a paradigm shift in stopover ecology research. This includes focusing on why an individual interrupts its migratory flight, which is more likely to identify the individual-specific function(s) of the stopover correctly than departure-decision studies. Moreover, we highlight that the selective forces acting on stopover decisions are context dependent and are expected to differ between, e.g. K-/r-selected species, the sexes and migration strategies. For example, all else being equal, r-selected species (low survival rate, high reproductive rate) should have a stronger urge to continue the migratory endurance flight or resume migration from a stopover because the potential increase in immediate fitness costs suffered from a flight is offset by the expected higher reproductive success in the subsequent breeding season. Finally, we propose to focus less on proximate mechanisms controlling landing and departure decisions, and more on ultimate mechanisms to identify the selective forces shaping stopover decisions. Our ideas are not limited to birds but can be applied to any migratory species. Our revised definition of stopover and the proposed paradigm shift has the potential to stimulate a fruitful discussion towards a better evolutionary ecological understanding of the functions of stopover. Furthermore, identifying the functions of stopover will support targeted measures to conserve and restore the functionality of stopover sites threatened by anthropogenic environmental changes. This is especially important for long-distance migrants, which currently are in alarming decline.

Corticosterone's roles in avian migration: Assessment of three hypotheses

While corticosterone (CORT) is often suggested to be an important hormone regulating processes necessary for avian migration, there has been no systematic assessment of CORT's role in migration. Prior to migration, birds increase fat stores and flight muscle size to prepare for the high energetic costs associated with long-distance flight. After attaining sufficient energetic stores, birds then make the actual decision to depart from their origin site. Once en route birds alternate between periods of flight and stopovers, during which they rest and refuel for their next bouts of endurance flight. Here, we evaluate three non-mutually exclusive hypotheses that have been proposed in the literature for CORT's role in migration. (1) CORT facilitates physiological preparations for migration [e.g. hyperphagia, fattening, and flight muscle hypertrophy]. (2) CORT stimulates departure from origin or stopover sites. (3) CORT supports sustained migratory travel. After examining the literature to test predictions stemming from each of these three hypotheses, we found weak support for a role of CORT in physiological preparation for migration. However, we found moderate support for a role of CORT in stimulating departures, as CORT increases immediately prior to departure and is higher when migratory restlessness is displayed. We also found moderate support for the hypothesis that CORT helps maintain sustained travel, as CORT is generally higher during periods of flight, though few studies have tested this hypothesis. We provide recommendations for future studies that would help to further resolve the role of CORT in migration.