The endocrine mechanism regulating stopover refueling and departure in avian migrants
Final Report Abstract
Most of the billions of migrating birds cannot reach their final destination (wintering or breeding grounds) in a single flight and have to make so-called stopovers in between flights. Successful completion of migration and timely arrival at the breeding or wintering grounds requires rapid refueling at stopover sites and departing from stopover at the appropriate time. Whereas the factors influencing the rate of refueling and departure from stopover are well known, our understanding, and even identification, of the physiological mechanism(s) involved in these crucial processes is very limited. To increase the limited knowledge of the endocrine mechanisms involved in refueling and departure from stopover, key behaviors of avian migration, we performed a series studies on free-flying and both long-term and short-term captive migrants. A prerequisite of the studies on short-term captive wild migrants was that these birds show their ‘natural’ behaviour in the cages, i.e., that the birds are not stressed by the caging. We showed that our focal species (northern wheatears, Oenanthe oenanthe) indeed is not stressed by caging; the birds immediately start to eat the food provided and corticosterone levels of caged birds are not different from those of conspecifics sampled under free-flying conditions. Our series of subsequent endocrine studies indicate that corticosterone does not stimulate stopover refueling, but that it does play a prominent role in the regulation of departure decision making. It appears that corticosterone has a mediating role between departure cues, such as wind conditions and birds’ fuel stores, and actual departure. Corticosterone levels increase when departure cues are stronger, and departure likelihood is greater and departure within the night earlier, when corticosterone levels are higher. Corticosterone levels seem to increase most prominently in the hours immediately prior to actual departure, suggesting that departure decisions are made relatively late in the day. Ghrelin does not appear to play a role in the upregulation of corticosterone, nor are departure likelihood and timing within the night associated with circulating levels of this hormone. Unexpectedly, pharmacological down-regulation of circulating corticosterone level by administration of dexamethasone, a previously used method, resulted in unwanted decrease of food intake and loss of body mass in northern wheatears. This disqualified this method for (part of) our planned manipulative experiments. Therefore, we put more emphasis on our non-experimental work and in addition started another line of physiological studies investigating whether immune function impacts stopover behaviour and vice versa. This work indicates that migratory endurance flight negatively impacts the immunological state of migrating birds, but that they can rapidly recover their immunological state during stopover. This suggests that stopovers may not only function to refuel, but also allow migrants to recover physiologically, a notion that has the potential to change our view of the temporal organization of avian migration.
Publications
- 2017. Endocrine regulation of fueling by hyperphagia in migratory birds. Journal of Comparative Physiology Part A 203: 439-445
Eikenaar C
(See online at https://doi.org/10.1007/s00359-017-1152-1) - 2018. Cues, corticosterone and departure decisions in a partial migrant. General and Comparative Endocrinology, 26, 59-66
Eikenaar C, Ballstaedt E, Hessler S, Klinner T, Müller, Schmaljohann H
(See online at https://doi.org/10.1016/j.ygcen.2018.01.023) - 2018. Endocrine regulation of migratory departure from stopover: Evidence from a longitudinal migratory restlessness study on northern wheatears. Hormones and Behavior 99, 9-13
Eikenaar C, Müller F, Rüppel G, Stöwe M
(See online at https://doi.org/10.1016/j.yhbeh.2018.01.008) - 2018. Ghrelin, corticosterone and the resumption of migration from stopover, an automated telemetry study. Physiology and Behavior 194, 450-455
Eikenaar C, Hessler S, Ballstaedt E, Schmaljohann H, Kaiya H
(See online at https://doi.org/10.1016/j.physbeh.2018.06.036) - 2019. An exception to the rule: captivity does not stress wild migrating northern wheatears. General and Comparative Endocrinology 275, 25-29
Eikenaar C, Hessler S, Fischer S, Bairlein F
(See online at https://doi.org/10.1016/j.ygcen.2019.02.010) - 2020. Diel variation in corticosterone and departure decision making in migrating birds. Hormones and Behavior 122
Eikenaar C, Schäfer J, Hessler S, Packmor F, Schmaljohann H
(See online at https://doi.org/10.1016/j.yhbeh.2020.104746)
