Navigation by extrapolation of geomagnetic cues in a migratory songbird

Displacement experiments have demonstrated that experienced migratory birds translocated thousands of kilometers away from their migratory corridor to unfamiliar areas can orient towards and ultimately reach their intended destinations. This implies that they are capable of “true navigation”, commonly defined as the ability to return to a known goal after displacement to a completely unknown location without relying on familiar surroundings, cues that emanate from the destination, or information collected during the outward journey. In birds, true navigation appears to require previous migratory experience, and it is generally assumed that, to correct for displacements outside the familiar area, birds initially have to gather information within their year-round distribution range, learn predictable spatial gradients of some environmental cues within it and extrapolate from those to cues of unfamiliar magnitude ̶ the gradient hypothesis. However, the nature of the cues used, and evidence for actual extrapolation remains elusive. Geomagnetic cues (inclination, declination and total intensity) provide predictable spatial gradients across large parts of the globe and could serve for navigation. We tested the orientation of long-distance migrants, Eurasian reed warblers (Acrocephalus scirpaceus), exposing them to geomagnetic cues of unfamiliar magnitude only encountered beyond their natural distribution range. The birds demonstrated re-orientation towards their natural migratory corridor as if they were translocated to the corresponding geographic location but only when all naturally occurring magnetic cues were presented, not when declination was changed alone. This result represents direct evidence for migratory birds’ ability to navigate using geomagnetic cues extrapolated beyond the range of magnitude they previously experienced

A magnet attached to the forehead disrupts magnetic compass orientation in a migratory songbird

For studies on magnetic compass orientation and navigation performance in small bird species, controlled experiments with orientation cages inside an electromagnetic coil system are the most prominent methodological paradigm. These are, however, not applicable when studying larger bird species and/or orientation behaviour during free flight. For this, researchers have followed a very different approach. By attaching small magnets to birds, they intended to deprive them of access to meaningful magnetic information. Unfortunately, results from studies using this approach appear rather inconsistent. As these are based on experiments with birds under free flight conditions, which usually do not allow exclusion of other potential orientation cues, an assessment of the overall efficacy of this approach is difficult to conduct. Here, we directly test the efficacy of small magnets for temporarily disrupting magnetic compass orientation in small migratory songbirds using orientation cages under controlled experimental conditions. We found that birds which have access to the Earth’s magnetic field as their sole orientation cue show a general orientation towards their seasonally appropriate migratory direction. When carrying magnets on their forehead under these conditions, the same birds become disoriented. However, under changed conditions that allow birds access to other (i.e. celestial) orientation cues, any disruptive effect of the magnets they carry appears obscured. Our results provide clear evidence for the efficacy of the magnet approach for temporarily disrupting magnetic compass orientation in birds, but also reveal its limitations for application in experiments under free flight conditions

Stopover departure decisions in autumn are not associated with constitutive immune function in Northern Wheatears Oenanthe oenanthe

Stopover periods in between migratory flights determine the speed of migration and as such may affect birds’ fitness. The decision to leave a stopover site is affected by, for example, energy stores and weather conditions. We investigated whether immunological state influences autumn stopover departure decisions in Northern Wheatear (Oenanthe oenanthe), a nocturnal long-distance migrant. We measured three parameters of constitutive immune function: microbial killing capacity, haptoglobin, and total immunoglobulins. None of them explained variation in departure probability, minimum stopover duration, or departure time within the night. Thus, we did not find an effect of the birds’ immunological state on stopover departure decisions

Not just fuel : energy stores are correlated with immune function and oxidative damage in a long-distance migrant.

In many animals, catabolic and anabolic periods are temporally separated. Migratory birds alternate energy expenditure during flight with energy accumulation during stopover. The size of the energy stores at stopover affects the decision to resume migration and thus the temporal organization of migration. We now provide data suggesting that it is not only the size of the energy stores per se that may influence migration scheduling, but also the physiological consequences of flying. In two subspecies of the northern wheatearOenanthe oenanthe, a long-distance migrant, estimated energy stores at a stopover during autumn migration were positively related with bothconstitutive innate and acquired immune function, and negatively related with oxidative damage to lipids. In other words, migrants’ physiological condition was associated with their energetic condition. Although time spent at stopover before sampling may have contributed to this relationship, our results suggest that migrants have to trade-off the depletion of energy stores during flight withincurring physiological costs. This will affect migrants’ decisions when to start and when to terminate a migratory flight. The physiological costs associated with the depletion of energy stores may also help explaining why migrants often arrive at and depart from stopover sites with larger energy stores than expected. We propose that studies on the role of energy stores as drivers of the temporal organization of (avian) migration need to consider physiological condition, such as immunological and oxidative states

Stopover departure decisions in songbirds: do long-distance migrants depart earlier and more independently of weather conditions than medium-distance migrants?

Background: Songbirds following distinct migration strategies (e.g. long- vs. short- to medium-distance migrants) often differ in their speed of migration during autumn and, thus, are assumed to face different time constraints. During migration, most songbird species alternate migratory flights with stopover periods. Many of them restrict these migratory flights to the night, i.e., they are nocturnal migrants. At stopover, nocturnal migrants need to select a specific night (night-to-night decision) and time of night (within-night decision) to resume migration. These departure decisions, which largely determine the speed of migration, are jointly affected by a set of intrinsic and extrinsic factors, i.e., departure cues. Here we aim to assess whether the set of intrinsic and extrinsic factors and the magnitude of their respective effects on stopover departure decisions differs between nocturnally migrating songbird species, depending on their migration strategy and associated time constraints. Methods: We radio-tracked migrating Northern Wheatears (Oenanthe oenanthe; long-distance migrant), European robins (Erithacus rubecula) and Common Blackbirds (Turdus merula; both medium-distance migrants) during autumn stopover and analysed their night-to-night and within-night departure timing in relation to intrinsic and extrinsic factors. Results: Species generally differed in their departure timing on both temporal scales, with shortest stopovers and earliest nocturnal departures in the long-distance migrant. Some factors, such as day of year, fuel load, cloud cover and crosswind, had consistent effects on stopover departure decisions in all three species. However, species differed in the effects of tailwind assistance, change in atmospheric pressure and air temperature on their stopover departure decisions. Whereas night-to-night decisions were affected by these extrinsic factors in either both or one of the medium-distance migrants, such effects were not found in the long-distance migrant. Conclusions: Our results suggest that the general timing of departures in songbirds is affected by the species-specific migration strategy and associated time constraints. Further, they imply that the assessment and usage of specific extrinsic factors, i.e., weather conditions, as departure cues is adjusted based on this migration strategy, with the long-distance migrants being least selective at departure. Other intrinsic and extrinsic factors, however, seem to be used as departure cues independent of migration strategy

Do departure and flight route decisions correlate with immune parameters in migratory songbirds?

Many songbirds migrate twice a year to exploit seasonally available resources. These journeys are energetically demanding and energy reserves of these small birds are limited. Accordingly, most of the time migrating is spent during stopovers that serve to rest, replenish resources and recover. While external influences, like prevailing weather conditions and resource availability, are well studied with regard to stopover behaviour and departure decisions, studies on how birds' individual physiological condition and stopover decisions may be linked are scarce. We used a largeâscale radioâtelemetry network covering the German Bight (SE North Sea) to study how birds' immunological constitution at stopover may correlate with departure and flight behaviour in five species of shortâ to mediumâdistance migratory songbirds. We measured markers of the innate (bacterial killing activity, lysozyme concentration, natural antibodies and complement titres) and acquired immune function (immunoglobulin Y) in the birds' plasma. After sampling, we tracked the birds' behaviour in terms of stopover duration as well as flight routes and flight distances within the telemetry network after departure. We found that stopover durations were positively related to natural antibodies and immunoglobulin Y across species and to the bacterial killing activity solely in song thrushes in spring, while no relations became apparent in fall. Flight distances were linked positively to immunoglobulin Y concentrations in both spring and fall, though not in all of the investigated species. Coastal and offshoreâoriented routes were taken independent of the birds' immune status. Our study for the first time shows that migratory behaviour of songbirds in the wild is correlated with their immune status, not only during but also beyond stopover. Furthermore, birds with increased complement titres and Immunoglobulin Y concentrations, either due to recent infection or greater investment in their immune function, took additional time at their stopover sites, probably to reach their breeding grounds in good condition. During the less timeâconstrained fall season, stopovers were generally prolonged, independent from the birds' immune status and any delays taken to improve immune status are most likely not detrimental in terms of fitness

Highly pathogenic avian influenza causes mass mortality in Sandwich Tern Thalasseus sandvicensis breeding colonies across north-western Europe

In 2022, highly pathogenic avian influenza (HPAI) A(H5N1) virus clade 2.3.4.4b became enzootic and caused mass mortality in Sandwich TernThalasseus sandvicensis and other seabird species across north-western Europe. We present data on the characteristics of the spread of the virus between and within breeding colonies and the number of dead adult Sandwich Terns recorded at breeding sites throughout north-western Europe. Within two months of the first reported mortalities, 20,531 adult Sandwich Terns were found dead, which is >17% of the total north-western European breeding population. This is probably an under-representation of total mortality, as many carcasses are likely to have gone unnoticed and unreported. Within affected colonies, almost all chicks died. After the peak of the outbreak, in a colony established by late breeders, 25.7% of tested adults showed immunity to HPAI subtype H5. Removal of carcasses was associated with lower levels of mortality at affected colonies. More research on the sources and modes of transmission, incubation times, effective containment, and immunity is urgently needed to combat this major threat for colonial seabirds