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Climate change and weather variability are having global impacts on the lives of organisms, particularly on high-trophic level predators such as pelagic seabirds. In the North Atlantic, migratory seabirds are expected to respond to climate variability by adjusting their seasonal events, including the timing of migration and arrival at the breeding site. The timing of these events may be influenced by large-scale atmospheric phenomena like the North Atlantic Oscillation (NAO) and the Atlantic Multi-decadal Oscillation (AMO). The White-tailed Tropicbird (Phaethon lepturus) is a wide-spread tropical migratory seabird breeding at its Atlantic northernmost edge of distribution range in Bermuda Islands (32° 17' 58'' N, 64° 47' 25'' W). Using data from eBird, an online database of bird observations where expert and amateur birdwatchers can report their sightings, we explored trends in Tropicbird first annual observation (proxy for bird arrival time) at the Bermuda breeding ground from 1953 to 2023. Specifically, we examined the relationship between the arrival time of the Tropicbird at its breeding site and the NAO and AMO. We show that the progressive early arrival at the breeding site (20–25 days in advance) of Tropicbirds over the last 70 years positively correlated with the NAO and AMO Indices. This suggests that this tropic seabird breeding in the North Atlantic may be responding to climate-induced changes affecting the Atlantic Ocean. Our findings highlight the fundamental contribution of citizen-science data for ecological long-term studies to understand animals' responses to a changing world.

Parent–offspring recognition (POR) is fundamental in colonial birds when the potential intermingling of chicks is higher due to the large number and proximity of nests. In species with isolated nests, where chick presence in the nest is strong contextual evidence of kinship, there might be circumstances when the parent might doubt the identity of the chick, but not enough to reject it. Olfactory-based recognition of conspecifics and nest sites in birds has gained strong evidence suggesting a potential role of olfaction in POR. Despite that, there are no studies testing it. We used Scopoli's shearwater (Calonectris diomedea) as model colonial single-brooded species with a developed olfactory sense, usually breeding in well-spaced nests with low probability of chicks mixing. We tested the parent's ability to selectively respond to their own chick as opposed to a simpler rule of feeding any chick found in the nest by means of chick-fostering experiments. We designed two cross-fostering experiments using chicks of different ages to test whether the ability of parents to recognize a related chick develops over time, possibly after acquiring an own distinctive odour. Finally, we also manipulated nests’ odour to disentangle the confounding effect of nest site recognition from POR when parents return at night. All experimental chicks were adopted by parents as the weight and bill growth of cross-fostered chicks did not differ significantly from the control group. We recorded a small difference in weight when foster chicks were inside an odour-manipulated nest; although we did not record weight loss in experimental chicks, only a steeper increase in weight was observed in control chicks. In conclusion, adoption in Scopoli's shearwater seems to follow the rule "if the young is in my nest, accept it" proposed by Beecher (1991) for species with spatially separated nests and low chick mobility. Fundação para a Ciência e Tecnologia - FCT info:eu-repo/semantics/acceptedVersion

Flying over the open sea is energetically costly for terrestrial birds. Despite this, over-water journeys of many birds, sometimes hundreds of kilometres long, are uncovered by bio-logging technology. To understand how these birds afford their flights over the open sea, we investigated the role of atmospheric conditions, specifically wind and uplift, in subsidizing over-water flight at a global scale. We first established that ΔT, the temperature difference between sea surface and air, is a meaningful proxy for uplift over water. Using this proxy, we showed that the spatio-temporal patterns of sea-crossing in terrestrial migratory birds are associated with favourable uplift conditions. We then analysed route selection over the open sea for five facultative soaring species, representative of all major migratory flyways. The birds maximized wind support when selecting their sea-crossing routes and selected greater uplift when suitable wind support was available. They also preferred routes with low long-term uncertainty in wind conditions. Our findings suggest that, in addition to wind, uplift may play a key role in the energy seascape for bird migration that in turn determines strategies and associated costs for birds crossing ecological barriers such as the open sea

Studying the causes and consequences of route selection in animal migration is important for understanding the evolution of migratory systems and how they may be affected by environmental factors at various spatial and temporal scales. One key decision during migration is whether to cross “high transport cost” areas, or to circumvent them. Soaring birds may face this choice when encountering waterbodies where convective updrafts are weak or scarce. Crossing these waterbodies requires flying using energetically costly flapping flight, while circumventing them over land permits energetically cheap soaring.We tested how several atmospheric factors (e.g., wind, thermal uplift) and geographic, seasonal and state-related factors (sex and age) affected route selection in migrating white storks (Ciconia ciconia). We used 196 GPS tracks of 70 individuals either crossing or circumventing the north-easternmost section of the Mediterranean Sea, over Iskenderun Bay in southern Turkey.We found that westward and southward winds promoted a cross-bay journey in spring and autumn, respectively, acting as tailwinds. Also, overall weaker winds promoted a sea crossing in spring. Sea crossing was associated with flapping flight and higher values of Overall Dynamic Body Acceleration (ODBA) and resulted in higher ground speed than travel over land.The combined environmental conditions and the effects of route selection on movement-related energy costs and speed were likely responsible for an increase in the time spent flying and distance travelled of migrating storks that decided to cross the bay during spring. Notably, daily travel distances of spring migrants crossing the bay were 60 kilometres longer than those of land-detouring birds, allowing them to reach their destination faster but likely incurring a higher energetic flight cost. No such benefit was found during autumn.Our findings confirm that atmospheric conditions can strongly affect bird route selection. Consequently, migration timing, speed and movement-related energy expenditure differed considerably between the two migratory seasons and the two route choices, highlighting a time-energy trade-off in the migration of white storks.

Billions of migrating birds travel between their breeding and over-wintering areas twice a year, encountering various environmental conditions along their migration routes. Wind is of utmost importance for birds as wind speed and direction may strongly affect the birds’ flight speed and metabolism. Specifically, tailwinds were found to initiate flight and facilitate higher migration intensity and faster migratory movement while lowering the energetic cost of flight. Using radar, field observations and local meteorological measurements, we studied spring migrating raptors in a migration bottleneck in Italy near the Strait of Messina, between Sicily and Calabria. We explored the effects of wind on flight speed and the intensity of migration in soaring migrants. We found that the birds’ ground speed was positively affected by tailwind speed, and thus, tailwind likely allowed the birds to reach their destination faster. In addition, bird airspeed decreased under increasing tailwind speed, presumably lowering the energetic cost of flight. These findings are in line with predictions of optimal migration flight theory, yet, tailwind had an unexpected negative effect on migration intensity. We suggest that tailwind conditions induced a change in route selection by the migrants at a regional scale, causing a local decline in migration intensity. This change involves the undertaking of cross sea flight rather than overland detour. Furthermore, we found a modular response to crosswinds as birds compensated for winds blowing towards the sea and drifted when winds blew towards land. Our findings suggest that migrating raptors respond to en route wind conditions and coastline geography by adjusting several features of their flight in a manner that will increase their travel speed, reduce the energetic cost of flight, and permit a safe journey. Soaring birds exploit tailwinds to move faster and presumably to reduce the energetic cost of flight during migration. Using radar, we tested how bird flight is affected by tailwind and further predicted that the due to the advantages of using tailwinds, as increased number of birds migrating over the study area. We found that radar-tracked migrating birds flew faster with tailwinds over the study area, but despite this advantage, surprisingly low number of birds migrated when tailwinds prevailed. Furthermore, birds seem to drift with winds blowing from the sea to the land, but not in the opposite direction, suggesting that birds try to avoid drifting over the sea where it is riskier to fly. Hence, our study highlights the complexity of migrant flight behavior in relation to the wind and suggests a flexible response to different wind conditions and coastline geography at multiple spatiotemporal scales.

Most seabirds are diurnal foragers, but some species may also feed at night. In Peruvian pelicans (Pelecanus thagus), the evidence for nocturnal foraging is sparse and anecdotal. We used GPS-dataloggers on five incubating Peruvian pelicans from Isla Lobos de Tierra, Peru, to examine their nocturnality, foraging movements and activities patterns at sea. All instrumented pelicans undertook nocturnal trips during a 5-7 day tracking period. Eighty-seven percent of these trips (n = 13) were strictly nocturnal, whereas the remaining occurred during the day and night. Most birds departed from the island after sunset and returned a few hours after sunrise. Birds traveled south of the island for single-day trips at a maximum range of 82.8 km. Overall, 22% of the tracking period was spent at sea, whereas the remaining time was spent on the island. In the intermediate section of the trip (between inbound and outbound commutes), birds spent 77% of the trip time in floating bouts interspersed by short flying bouts, the former being on average three times longer than the latter. Taken together, the high sinuosity of the bird's tracks during floating bouts, the exclusively nocturnal trips of most individuals, and the fact that all birds returned to the island within a few hours after sunrise suggest that pelicans were actively feeding at night. The nocturnal foraging strategy of Peruvian pelicans may reduce food competition with the sympatric and strictly diurnal Guanay cormorants (Phalacrocorax bougainvillii), Peruvian boobies (Sula variegata) and Blue-footed boobies (S. nebouxii), which were present on the island in large numbers. Likewise, plankton bioluminescence might be used by pelicans as indirect cues to locate anchovies during their upward migration at night. The foraging success of pelicans at night may be enhanced by seizing prey close to the sea surface using a sit-and-wait strategy.