Linking life histories: winter distribution of Isle of May Shags

Looking for Shags

We are always looking for reports of colour-ringed shags from any coastal location across the east of Scotland and England.

Birds have a large colour ring on one leg with a three-lettercode, and a metal ring on theother. Codes are read downwards, and we need the location, date, code and background colour of the ring for every sighting. Please keep a look out for rings on any birds seen, and send all sightings to

The European Shag

The European shag is a large black seabird, similar to a cormorant. They breed and roost on cliffs and rocky coastal outcrops, feeding on small benthic fish such as sand eels. Shags are impressive divers, reaching down to 60m in depth for up to a minute at a time. However, they have wettable plumage that requires drying between dives, thought to be the cause of the classic “wings out” pose.

The Isle of May

The Isle of May Long-Term Study (IMLOTS) is a key component of the CEH monitoring network for seabirds. The European shag is one of six intensively studied species breeding on the island, with over 90% of all breeding adults and offspring shags ringed each year with field-readable “darvic rings”. Colour ringing allows the study to identify individuals within and between years, recording breeding success, mate pairing and survival.

Winter distribution

The shag breeding season can extend from March to October, but little is known about the ecology and distribution of birds when they leave the colony.

Colour-ringed birds have been reported as far north as Orkney and as far south as Yorkshire over the winter, indicating a high dispersal rate. However, some shags do not leave the breeding colony at all, showing high variation in the choice of winter location.


The highest mortality rates occur during the winter, especially in young birds that are less efficient at foraging. An improved knowledge of winter ecology is particularly important therefore: not only for understanding the population dynamics of a species, but also in order to be able to plan effective conservation measures. Poor conditions over the winter can also have a knock-on effect on the timing of breeding and subsequent success, and even future survival.

European shag

A ringed shag








The project  

The current project is looking to map the wintering distribution of shags that breed on the Isle of May by collating reports of colour-ringed birds over the winter months. The aim is to be able to answer a range of questions including whether mates and offspring winter together, whether the birds return to the same wintering location year after year, and what effect the choice of location has on their subsequent survival and breeding success.