It has always been thought that the Mediterranean population of the European storm petrel – the smallest seabird in the Mediterranean – spends the year in this sea and that only a small part of the population migrates to the Atlantic during the winter season. Now, a study reveals that most European storm petrels that nest in the western Mediterranean move to the Atlantic Ocean as their main wintering area. The results, published in the journal International Journal of Avian Science (IBIS), contrast with the previously known migratory pattern in the central Mediterranean and outline a new migratory map for this small seabird.

The study is led by researchers Raül Ramos and Teresa Militão, members of the Seabird Ecology Group of the Faculty of Biology and the Biodiversity Research Institute of the University of Barcelona (IRBio-UB). Study participants include Ana Sanz-Aguilar and Andreu Rotger, from the University of the Balearic Islands and the Mediterranean Institute for Advanced Studies (IMEDEA, CSIC-UIB). The smallest seabird in the Mediterranean basin

The European storm petrel (Hydrobates pelagicus) is a long-lived species, with a low reproductive rate and a relatively late age of sexual maturity — very sensitive to threats affecting the survival of adults — which only lands for to reproduce. There are two subspecies: one that nests in the Mediterranean (H. p. melitensis) and another, more abundant, that nests in several European Atlantic islands (H. p. pelagicus).

“Data from ring recoveries and deep-sea sightings on board suggest that the Atlantic subspecies may migrate to southern waters of the African continent. In the Mediterranean subspecies, biogeochemical data and data from geolocation indicated that most of the wintering took place in this sea”, notes the speaker Raül Ramos, of the Department of Evolutionary Biology, Ecology and Environmental Sciences.

In the study, the team used light level geolocators to infer the location of the Mediterranean population over a full migratory cycle. Thanks to these devices, placed on the tarsus of the birds – and not on the back with a harness, as usual until now – the solar intensity has been recorded for almost a year. This methodology allows researchers to collect complementary data, such as salt water conductivity and sea temperature.

“The results allow us to better understand the activity patterns of seabirds throughout the wintering season – that is, when they are flying and when they are resting in the water – information which were previously unknown,” says researcher Teresa Militão, first author of the study.

“Throughout the winter period, the species spends more time resting in the water during the day than at night. This tells us that the species feeds mainly at night, and probably feeds on zooplankton and small fish that migrate to the surface of the sea at this time, “says the researcher.

Heading for the waters of the North Atlantic

The main feeding grounds of this species are still unknown. During the breeding period, it finds food in areas located at distances of up to 500 kilometers from the colony, according to other studies. The rest of the year, “everything indicates that, in the case of the population studied, it feeds mainly in the Atlantic, in a marine area that extends from the Canary Islands to the south of Iceland”, specify the authors. “The results contrast with the case of the Maltese storm-petrel, which maintains its main wintering area in the central Mediterranean.”

Migratory species such as the European storm petrel periodically move between distant areas to better take advantage of local resources or to find the most suitable habitat and climate for each stage of their life cycle. In the population analyzed by this study, breeding adults return to the breeding colony between February and April to join their mate and prepare for breeding. Between August and December, individuals leave the colony and begin a postnuptial migration in a very variable way.

According to the geolocation data collected, the Atlantic Ocean would be the main wintering area of ​​the European storm petrel which nests in the western Mediterranean. In particular, all individuals tagged in 2018 and recaptured in 2019 overwintered in the North Atlantic, a pattern that contrasts with the migratory pattern identified in the central Mediterranean.

“The extrinsic and intrinsic factors that determine the different migration patterns of the European storm petrel are still not well understood. According to the first results of the study, certain extrinsic factors, such as ocean conditions, could favor the migration of the studied population towards the North Atlantic, which would then take advantage of the productive ocean waters during the wintering period,” explains Raül Ramos. . .

The natal and breeding population also determine each individual’s migration patterns. There are also other intrinsic factors that cannot be ignored: genetic predisposition, age, physical condition, sex, reproductive success, state of feather moult or physical condition, among others. This is why “we need more studies and data to understand the influence of all these factors on the migratory movements of the species”, note the authors.

Know the wintering areas to protect the species

The article published in the journal IBIS improves our knowledge of the ecology of these birds and their distribution during their life cycle. “Knowing the wintering grounds of this species will make it possible to identify the environmental variables that condition its distribution”, specify the authors.

Studies such as this are crucial for understanding the intertwining between areas explored by marine wildlife and anthropogenic threats to the marine environment. “On land, during the breeding season, the European storm petrel is threatened by the introduction of invasive predators, such as cats or rats, which feed on eggs, lice and adults, and by the destruction or altering nesting habitats.On the high seas, light pollution from ships or oil stations, climate change, and extreme weather events such as cyclones or tornadoes also affect the survival of these small seabirds, whether they should be protected,” the researchers conclude.

Reference article:

Militao, T.; Sanz-Aguilar, A.; Rotger, A.; Ramos, R.”Non-breeding distribution and activity patterns at sea of ​​the smallest European seabird, the European storm-petrel (Hydrobates pelagicus)“. IBIS, April 2022. Doi: 10.1111/ibi.13068