Plumage microbiota covaries with the major histocompatibility complex in blue petrels.

To increase fitness, a wide range of vertebrates preferentially mate with partners that are dissimilar at the major histocompatibility complex (MHC) or that have high MHC diversity. Although MHC often can be assessed through olfactory cues, the mechanism by which MHC genes influence odour remains largely unclear. MHC class IIB molecules, which enable recognition and elimination of extracellular bacteria, have been suggested to influence odour indirectly by shaping odour-producing microbiota, i.e. bacterial communities. However, there is little evidence of the predicted covariation between an animal's MHC genotype and its bacterial communities in scent-producing body surfaces. Here, using high-throughput sequencing, we tested the covariation between MHC class IIB genotypes and feather microbiota in the blue petrel (Halobaena caerulea), a seabird with highly developed olfaction that has been suggested to rely on oduor cues during an MHC-based mate choice. First, we show that individuals with similar MHC class IIB profiles also have similar bacterial assemblages in their feathers. Then, we show that individuals with high MHC diversity have less diverse feather microbiota and also a reduced abundance of a bacterium of the genus Arsenophonus, a genus in which some species are symbionts of avian ectoparasites. Our results, showing that feather microbiota covary with MHC, are consistent with the hypothesis that individual MHC genotype may shape the semiochemical-producing microbiota in birds.

Flexible migratory choices of Cory's shearwaters are not driven by shifts in prevailing air currents.

Wind conditions strongly affect migratory costs and shape flyways and detours for many birds, especially soaring birds. However, whether winds also influence individual variability in migratory choices is an unexplored question. Cory's shearwaters (Calonectris borealis) exhibit migratory flexibility, changing non-breeding destination across the Atlantic Ocean within and between years. Here, we investigated how wind dynamics affect the spatiotemporal migratory behaviour and whether they influence individual choices of non-breeding destination. We analysed 168 GLS tracks of migratory Cory's shearwaters over five years in relation to concurrent wind data. We found no evidence for an association of the use of specific paths or destinations with particular wind conditions. Our results suggest that shearwaters deliberately choose their non-breeding destination, even when the choice entails longer distances and higher energetic costs for displacement due to unfavourable wind conditions en route. Favourable winds trigger migration only when directed towards specific areas but not to others. Despite their dependence on wind for dynamic soaring, Cory's shearwaters show a high individuality in migratory behaviour that cannot be explained by individual birds encountering different meteorological conditions at departure or during migratory movements.

Comment on "Marine plastic debris emits a keystone infochemical for olfactory foraging seabirds" by Savoca et al.

In their recent paper, Savoca and collaborators (2016) showed that plastic debris in the ocean may acquire a dimethyl sulfide (DMS) signature from biofouling developing on their surface. According to them, DMS emission may represent an olfactory trap for foraging seabirds, which explains patterns of plastic ingestion among procellariiform seabirds. This hypothesis is appealing, but some of the data that Savoca et al. used to support their claim are questionable, resulting in a misclassification of species, as well as other decisions regarding the variables to include in their models. Furthermore, with their focus on a single lifestyle trait (nesting habit) of dubious relevance for explaining plastic ingestion, Savoca et al. neglect the opportunity to explore other factors that might provide better ecological insight. Finally, we are deeply concerned by the conservation policy recommendation proposed by Savoca et al.-to increase antifouling properties of consumer plastics-which constitutes a substantial environmental risk and delivers the wrong message to decision-makers. The reduction of plastic consumption, waste prevention, and proactive reuse through a circular economy should be at the heart of policy recommendations for future mitigation efforts.

Olfactory foraging in temperate waters: sensitivity to dimethylsulphide of shearwaters in the Atlantic Ocean and Mediterranean Sea.

Many procellariiforms use olfactory cues to locate food patches over the seemingly featureless ocean surface. In particular, some of them are able to detect and are attracted by dimethylsulphide (DMS), a volatile compound naturally occurring over worldwide oceans in correspondence with productive feeding areas. However, current knowledge is restricted to sub-Antarctic species and to only one study realized under natural conditions at sea. Here, for the first time, we investigated the response to DMS in parallel in two different environments in temperate waters, the Atlantic Ocean and the Mediterranean Sea, employing Cory's (Calonectris borealis) and Scopoli's (Calonectris diomedea) shearwaters as models. To test whether these birds can detect and respond to DMS, we presented them with this substance in a Y-maze. Then, to determine whether they use this molecule in natural conditions, we tested the response to DMS at sea. The number of birds that chose DMS in the Y-maze and that were recruited at DMS-scented slicks at sea suggests that these shearwaters are attracted to DMS in both non-foraging and natural contexts. Our findings show that the use of DMS as a foraging cue may be a strategy adopted by procellariiforms across oceans but that regional differences may exist, giving a worldwide perspective to previous hypotheses concerning the use of DMS as a chemical cue.

Back home at night or out until morning? Nycthemeral variations in homing of anosmic Cory's shearwaters in a diurnal colony.

Olfactory cues have been shown to be important to homing petrels at night, but apparently those procellariiform species that also come back to the colony during the day are not impaired by smell deprivation. However, the nycthemeral distribution of homing, i.e. whether displaced birds released at night return to their burrow by night or during daylight, has never been investigated. To explore this question, we studied the homing behaviour of Cory's shearwater (Calonectris borealis) in the only known population where these birds are active at the colony both during the day and at night. Here, we compared the nocturnal versus diurnal homing schedule of birds treated with zinc sulphate (to induce a reversible but complete anosmia) with that of controls. Our results show that anosmic shearwaters were unable to home in the dark and were constrained to wait for the daylight to find their burrow again. Our results confirm that olfaction is the basic sensory input for homing by night even in a petrel species that is diurnally active at the colony.

Waiting time before release increases the motivation to home in homing pigeons (Columba livia).

When performing homing experiments with individual releases, pigeons have to wait in a transport box for a certain amount of time before being released and hence perceive the departure of companions. Quite often, the last pigeons disappear straightforward from the release site. The question is whether this reflects improved orientation because of prolonged exposure to the release place or whether it reflects increased homing motivation. By releasing pigeons from a familiar site, we investigated the effects of the time spent at the release site on homing performance, recording pigeons' flights with GPS loggers. Our results show that, despite individual peculiarities of flight patterns, the waiting time at release site had a positive effect on homing speed and time, and reduced the time spent circling around the release point. However, the overall path efficiency as derived from GPS tracking was not influenced. These results suggest that a longer waiting time before release improves homing performance and this is related not only to increased navigational abilities but also to increased homing motivation.

EEG responses to visual landmarks in flying pigeons.

BACKGROUND: GPS analysis of flight trajectories of pigeons can reveal that topographic features influence their flight paths. Recording electrical brain activity that reflects attentional processing could indicate objects of interest that do not cause changes in the flight path. Therefore, we investigated whether crossing particular visual landmarks when homing from a familiar release site is associated with changes in EEG. RESULTS: Birds carried both data-loggers for recording GPS position and EEG during flight. First, we classified characteristic EEG frequencies of caged birds and found five main bands: A: 0-3, B: 3-12, C: 12-60, D: 60-130, and E: 130-200 Hz. We analyzed changes in these activity bands when pigeons were released over sea (a featureless environment) and over land. Passing over the coastline and other prominent landmarks produced a pattern of EEG alterations consisting of two phases: activation of EEG in the high-frequency bands (D and/or E), followed by activation of C. Overlaying the EEG activity with GPS tracks allowed us to identify topographical features of interest for the pigeons that were not recognizable by distinct changes of their flight path. CONCLUSIONS: We provide evidence that EEG analysis can identify landmarks and objects of interest during homing. Middle-frequency activity (C) reflects visual perception of prominent landmarks, whereas activation of higher frequencies (D and E) is linked with information processing at a higher level. Activation of E bands is likely to reflect an initial process of orientation and is not necessarily linked with processing of visual information.

The influence of experience in orientation: GPS tracking of homing pigeons released over the sea after directional training.

Flight experience is one of the factors that influences initial orientation of displaced homing pigeons (Columba livia). Prior studies showed a systematic dependence of initial orientation on previously flown direction. Using GPS data loggers, this study sought to examine the effect of previous directional training of 40 homing pigeons when they were released over the sea, in the absence of proximal landmarks, in a direction almost perpendicular to that of previous training flights. Our results demonstrated that previous directional training evoked a systematic and predicted deviation from the beeline over the sea that appeared as a compromise between the direction of training and the direction to the loft. Pigeons were able to efficiently correct their flight direction only once over land, where they flew significantly slower and less directly than over the sea.

Long flights and age affect oxidative status of homing pigeons (Columba livia).

Flying is an energy demanding activity that imposes several physiological challenges on birds, such as increase in energy expenditure. Evidence from sports medicine shows that exhausting exercise may cause oxidative stress. Studies on avian flight have so far considered several blood parameters, such as uric acid, corticosteroids, or circulating free fatty acids, but only one study has analysed markers of oxidative stress in flying birds. In this study, we evaluated, for the first time, how different flight efforts affect the oxidative status using homing pigeons (Columba livia) as a model species. Two groups of pigeons flew for around 60 and 200 km, respectively. Pigeons that flew for 200 km had a 54% increase in oxidative damage as measured by serum reactive oxygen metabolites (ROMs), a 19% drop in total serum antioxidant capacity (OXY) and an 86% increase of oxidative stress (ROMs/OXYx1000). Older pigeons depleted more serum antioxidants regardless of the release distance. Among pigeons that flew the longer distance, heavier ones depleted less serum antioxidants. The results of the study suggest that long flights may cause oxidative stress, and that older individuals may experience higher physiological demands.