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Researchers decode the migratory pattern of the smallest seabird in the Mediterranean

06/06/2022

  • Geolocation data reveal that the Atlantic Ocean is the main wintering area for storm petrels that nest in the western Mediterranean

 

Photograph: European storm petrel, storm petrel, tern or bronze mannikin (Author: Víctor París)

 

 

Esporles, 7th June 2022. It had always been thought that the Mediterranean population of the storm petrel—the smallest seabird in the Mediterranean—spent the entire year in this sea and only a small part of the population migrated to the Atlantic during the wintering period. Now, a study has revealed that most of the storm petrels that nest in the western Mediterranean move to the Atlantic Ocean as their main wintering area. The results—which have been published in the 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 Group on Seabird Ecology of the Faculty of Biology and the Biodiversity Research Institute (IRBio) of the University of Barcelona. Amongst the participants in the study are Ana Sanz-Aguilar and Andreu Rotger, from 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 which reaches sexual maturity at a relatively late age. It is highly sensitive to threats affecting adult survival and only sets foot on land to breed. There are two subspecies: one that nests in the Mediterranean Sea (H. p. melitensis), and a more abundant one which nests in several European islands of the Atlantic (H. p. pelagicus).

 

 

‘Data from ring recoveries and open-sea sightings on board suggested that the Atlantic subspecies could migrate towards the southern waters of the African continent. As for the Mediterranean subspecies, lecturer Raül Ramos, from the Department of Evolutionary Biology, Ecology and Environmental Sciences, notes that ‘biogeochemical and geolocation data indicate that most of the wintering took place in this sea’.

 

 

In this study, the team used light-level geolocators to determine the location of the Mediterranean population across a full migratory cycle. Using these devices, which were placed on the birds’ tarsus— and not on their backs with a harness, as was usual until now—, researchers have been able to record solar irradiance over nearly a year. This methodology also makes it possible to record complementary information, such as data concerning saltwater conductivity, sea temperature, and so forth.

 

 

‘The results allow us to better understand the activity patterns of seabirds throughout the entire wintering season—i.e. when they fly and when they rest in the water—, information that was previously unknown’, says researcher Teresa Militão, first author of the study.

 

 

‘During the entire wintering period, this species spends more time resting in the water during the day than at night. This tells us that the species forages mainly at night, and probably feeds on zooplankton and small fish that migrate to the sea surface at that time’, the researcher points out.

 

 

Heading for the North Atlantic waters

 

The main feeding areas of this species are still unknown. During the breeding period, it finds food in areas located at distances of up to 500 kilometres from the colony, according to other studies led by IMEDEA researchers Andreu Rotger and Ana Sanz Aguilar. During the rest of the year, ‘everything seems to indicate that, in the case of the population studied, the main feeding areas are in the Atlantic—in a marine area extending from the Canary Islands to the south of Iceland’, say 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 take better advantage of local resources or find the most suitable habitat and climate at each stage of their life cycle. In the population this study analysed, breeding adults return to the breeding colony between February and April to rejoin their partner and prepare for breeding. Between August and December, individuals leave the colony and begin postnuptial migration in a highly variable way.

 

 

‘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, some extrinsic factors, such as oceanic conditions, could favour the migration of the population studied towards the North Atlantic, which would then take advantage of the productive oceanic waters during the wintering period’, says Raül Ramos.

 

 

The natal and breeding population also determine the migration patterns of each individual. There are also other intrinsic factors that cannot be ignored, such as genetic predisposition, age, physical condition, gender, reproductive success or moulting status, amongst others. That is why the authors note that ‘we need more studies and data to understand the influence of all these factors on the migratory movements of the species’.

 

 

Knowing the wintering areas in order to protect the species

 

The paper published in the journal IBIS improves our knowledge of the ecology of these birds and their distribution during their life cycle. ‘Knowing the wintering areas of this species will help to identify the environmental variables that condition its distribution’, say the authors.

 

 

Studies such as this one are crucial to understand the overlap between the areas explored by marine fauna and anthropogenic threats in 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 prey on eggs, chicks and adults, and by the destruction or modification of nesting habitats. In the open ocean, 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, which should be protected’, the researchers conclude.

 

 

Source: IMEDEA (CSIC-UIB)

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