Tiny data-loggers help track Europe’s smallest seabird

European storm petrels are tiny seabirds that are long-lived and have a low reproductive rate. There are two subspecies; H. p. melitensis breeds on islands in the Mediterranean Sea and H. p. pelagicus breeds on several European islands in the Atlantic. Because they are very small, mostly darkly colored and fly fairly fast, they are difficult to observe at sea, even to establish the most basic data such as the subspecies involved. This means that not much is known about the lives of storm petrels when they are not at their breeding colonies on offshore islands.

A recent study published in the International Journal of Avian Science (IBIS) sheds new light on the migratory patterns of the storm petrel populations that breed in the western Mediterranean. This has been facilitated by the use of tiny geolocators, fixed to the legs of ten adult birds as they went about their lives between breeding seasons. The results contradict what was previously known about the species and highlights the fact that geolocator data can be crucial to improving knowledge about the seasonal distribution, migration and at-sea activity patterns of small seabird species.

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

Until recently, the information available suggested that most individuals of the Mediterranean subspecies remain within the Mediterranean Sea basin during the non-breeding period. 

“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. In the Mediterranean subspecies, biogeochemical data and geolocation data indicated that most of the wintering took place in this sea,” explained Raül Ramos, a lecturer in the Department of Evolutionary Biology, Ecology and Environmental Sciences.

In order to track the movements of storm petrels that breed on islands in the western Mediterranean, the researchers fastened a light-level geolocator onto one of the lower legs (tibia) of each of ten adult storm petrels caught at night at their nests. The devices, which weigh no more than 0.8 grams, collect data on solar intensity every five minutes. The locators also collect information on seawater conductivity and temperature. 

Nine of the ten birds fitted with geolocators were recaptured a year later when they returned to the same breeding colonies, and eight of the devices were in working order. This allowed the researchers to identify where the birds had been throughout a complete migratory cycle. 

The results showed that, contrary to expectations, all the tracked individuals spent at least part of the non-breeding period in the Atlantic Ocean, indicating that this ocean is far more important as a non-breeding area than was previously thought, at least for the European storm petrels breeding in the Western Mediterranean. 

“The results allow us to better understand the activity patterns of seabirds throughout the wintering season – [in other words] when they fly and when they rest in the water – information that was previously unknown,” said study first author Teresa Militão.

Although the main feeding areas of this species during the period between breeding seasons are still unknown, the data from the study 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, explained the researchers. “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 move around between distant areas during their non-breeding season to take better advantage of local food resources or to find the most suitable habitat or climate. 

“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 favor the migration of the population studied towards the North Atlantic, which would then take advantage of the productive oceanic waters during the wintering period,” said Raül Ramos.

The researchers also assessed monthly variations in primary productivity in the Mediterranean and in the Atlantic areas visited by the tracked storm petrels to understand whether this was a factor that influenced movement patterns. They concluded that birds potentially were moving from the Mediterranean feeding grounds (during breeding season) to the Atlantic feeding grounds in response to low levels of productivity in the Mediterranean during the winter. Low levels of primary productivity would affect the abundance of the ichthyoplankton that is the preferred prey of storm petrels.

Information from the data-loggers also allowed the scientists to understand more about the at-sea activity patterns of the birds. The data confirmed that the European storm petrel is a nocturnal species during the non-breeding period, as has been found previously in the breeding season. Birds spend most of the daylight time on the water and fly mainly during the night.

“During the entire winter period, the 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,” said Militão.

Furthermore, in the early part of the non-breeding season, most of the tracked birds spent more time on the water during the night than they did in the latter part of the non-breeding period. This increase in time spent on the water, when they should normally have been flying around in search of food, could indicate that the birds were possibly still molting some of their flight feathers. 

The study authors noted that their research improves the state of knowledge about 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,” they said.

Studies such as this one are crucial to understanding the different threats to species at different times of their life cycles.  

“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, and by the destruction or modification of nesting habitats,” said the researchers. “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.”

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By Alison Bosman, Earth.com Staff Writer