Baseline and stress-induced corticosterone levels are higher during spring than autumn migration in European robins

Dietary antioxidants attenuate the endocrine stress response during long-duration flight of a migratory bird

Glucocorticoids (GCs) are metabolic hormones that promote catabolic processes, which release stored energy and support high metabolic demands such as during prolonged flights of migrating birds. Dietary antioxidants (e.g. anthocyanins) support metabolism by quenching excess reactive oxygen species produced during aerobic metabolism and also by activating specific metabolic pathways. For example, similar to GCs' function, anthocyanins promote the release of stored energy, although the extent of complementarity between GCs and dietary antioxidants is not well known. If anthocyanins complement GCs functions, birds consuming anthocyanin-rich food can be expected to limit the secretion of GCs when coping with a metabolically challenging activity, avoiding the exposure to potential hormonal detrimental effects. We tested this hypothesis in European starlings (Sturnus vulgaris) flying in a wind tunnel. We compared levels of corticosterone, the main avian GC, immediately after a sustained flight and at rest for birds that were fed diets with or without an anthocyanin supplement. As predicted, we found (i) higher corticosterone after flight than at rest in both diet groups and (ii) anthocyanin-supplemented birds had less elevated corticosterone after flight than unsupplemented control birds. This provides novel evidence that dietary antioxidants attenuate the activation of the HPA axis (i.e. increased secretion of corticosterone) during long-duration flight.

Stressed by Maternity: Changes of Cortisol Level in Lactating Domestic Cats

Lactation is the most energetically expensive component of maternal care in mammals. Increased reproductive investment can lead to physiological stress for the mothers, based on the exhaustion of energy resources and increase in glucocorticoids level. This study aimed to estimate the changes in cortisol concentrations during lactation in domestic cats and compared the differences among litter sizes. Eleven females gave birth to 27 litters, which were divided in two groups-small (1-3 kittens) and large (4-7 kittens) litters. Blood samples were collected from each female before mating, after parturition, at 4 and 8 weeks of lactation. We showed that the cortisol level in females changed significantly during lactation-the highest concentrations were observed at the peak of lactation at 4 weeks. Cortisol levels varied significantly among females but did not depend on their maternal experience. We also revealed that there were no differences in cortisol levels between females with small and large litters, but at 4 weeks of lactation, the hormone concentrations were higher in females with small litters. It is likely that these females initially invested less in reproduction, giving birth to fewer offspring.

The physiology of movement

Movement, from foraging to migration, is known to be under the influence of the environment. The translation of environmental cues to individual movement decision making is determined by an individual's internal state and anticipated to balance costs and benefits. General body condition, metabolic and hormonal physiology mechanistically underpin this internal state. These physiological determinants are tightly, and often genetically linked with each other and hence central to a mechanistic understanding of movement. We here synthesise the available evidence of the physiological drivers and signatures of movement and review (1) how physiological state as measured in its most coarse way by body condition correlates with movement decisions during foraging, migration and dispersal, (2) how hormonal changes underlie changes in these movement strategies and (3) how these can be linked to molecular pathways. We reveale that a high body condition facilitates the efficiency of routine foraging, dispersal and migration. Dispersal decision making is, however, in some cases stimulated by a decreased individual condition. Many of the biotic and abiotic stressors that induce movement initiate a physiological cascade in vertebrates through the production of stress hormones. Movement is therefore associated with hormone levels in vertebrates but also insects, often in interaction with factors related to body or social condition. The underlying molecular and physiological mechanisms are currently studied in few model species, and show -in congruence with our insights on the role of body condition- a central role of energy metabolism during glycolysis, and the coupling with timing processes during migration. Molecular insights into the physiological basis of movement remain, however, highly refractory. We finalise this review with a critical reflection on the importance of these physiological feedbacks for a better mechanistic understanding of movement and its effects on ecological dynamics at all levels of biological organization.

Corticosterone negative feedback is weaker during spring vs. autumn migration in a songbird (Junco hyemalis)

Birds face many challenges during seasonal migrations and must make important decisions about whether to accelerate, maintain, or delay travel to their final destinations. Spring migration is likely more challenging than autumn migration as spring journeys are completed more quickly and weather conditions are harsher during this time. These differential challenges may be reflected in the endocrine stress response, as the hypothalamic-pituitary-adrenal (HPA) axis is important for both daily energetic needs and coping with stressors. Indeed, most avian studies have found that both baseline and stress-induced corticosterone (CORT) levels tend to be higher in spring migrants than in autumn migrants. We hypothesized that CORT negative feedback efficacy also differs across the season, and is likely weaker during times of year when birds must be most sensitive to stressors. We therefore predicted that CORT negative feedback efficacy would be weaker during spring vs. autumn migration as spring migrants are more likely to encounter situations where they must decide whether to turn back or delay their travel. We examined male dark-eyed juncos (Junco hyemalis) during their spring and autumn stopovers in Fargo, ND, USA. Our prediction was met as we did find that negative feedback efficacy was weaker during spring vs. autumn, although we notably did not find any seasonal differences in baseline and stress-induced CORT. We also found that spring migrants were heavier, had greater subcutaneous fat stores, and had slightly higher hematocrit compared to autumn migrants. These findings suggest that CORT negative feedback sensitivity may help migrating birds effectively cope with the differential challenges of autumn and spring migration.

Migratory Species Show Distinct Patterns in Corticosterone Levels during Spring and Autumn Migrations

Twice a year billions of birds migrate between breeding and wintering grounds. To facilitate migrations, birds develop migratory disposition, a complex suite of behavioral and physiological adjustments. Glucocorticoid hormone corticosterone is involved in the regulation of migratory behavior and physiology, however no consensus on its exact role in controlling avian migration exists. Using a large dataset on seven songbird species (long- and short-distance migrants) obtained during eleven consecutive migratory seasons on the Courish Spit of the Baltic Sea, we showed the general tendency of similar baseline corticosterone concentrations during both migrations, although stress-induced levels were generally much higher during spring. No difference between long- and short-distance migrants was found in either baseline or stress-induced levels, while there was substantial between-species variation, especially in baseline concentrations. The distinct patterns of corticosterone secretion during seasonal migrations even in ecologically similar species indicate that it is likely to be a species-specific trait. Thus, our study corroborates the inconsistency found in earlier studies and demonstrates how scientific understanding of the role of corticosterone during migration is still evolving. Rather low baseline corticosterone concentrations observed in this study emphasize that birds in both migratory seasons were not in a “stressed” state before capture.

The method matters: The effect of handling time on cortisol level and blood parameters in wild cats

Blood analysis has recently become a popular tool to assess the welfare of the wild cats. However, the estimates of blood parameters may depend on the sampling method. We have tested (1) if the sampling procedure influences blood parameters and (2) what parameters are the most efficient in assessing the physiological status in wild cat species. We assessed the effect of handling time on red blood cells (RBC) and white blood cells (WBC) counts, the ratio of neutrophils to lymphocytes (N/L ratio), and serum cortisol level within 1 hr after the capture of the animal in six far-east wild cats (Prionailurus bengalensis euptilura). Also, we analyzed literature data in 17 cat species to assess the effect of place of study, type of immobilization, and handling time on WBC count and N/L ratio. Serum cortisol level varied significantly with the handling time. RBC and WBC counts were strongly affected by the handling time. N/L ratio was very robust and did not depend on the handling time. However, the analysis of literature data has shown that the prolonged handling time (over 1 hr) and the type of immobilization significantly influence the N/L ratio, whereas the WBC count does not depend on any of considered factors. We conclude that while most blood parameters of cats are affected by routine handling time, the N/L ratio does not vary if the samples are collected within 1 hr after the capture of the animal. All other tested parameters should be treated with caution.