Guillemot Diet and Size: Feeding Habits, Predators, and Measurements

Guillemots spend most of their lives out at sea and only come ashore to breed. They tend to breed in large colonies on sea cliffs where they lay a single egg on a narrow ledge and rely on the unique pyramidal shape of the egg to prevent it falling off the cliff.

The Guillemot is a relatively peaceful bird for most of the year but that all changes during the breeding season when they become territorially aggressive. In this article we’ll take a closer look at what Guillemots eat, their breeding habits, size and weight and conservation status. Keep reading to find out more.

Table of Contents

The Dietary Habits Of The Guillemot

Guillemots are primarily diving seabirds, and their diet consists largely of small fish and marine invertebrates. They are excellent divers, using their wings to “fly” underwater, reaching depths of over 100 metres (328 feet) to catch prey.

What Does A Guillemot Eat?

Their primary source of food are fish, mainly sandeels, capelin, herring, sprats, and cod. They also eat crustaceans, molluscs, and planktonic species when fish are less available.

What To Feed A Baby Guillemot?

Guillemot parents feed their chicks small fish and will catch and carry a single fish crosswise in their bill and take it back to the chick. They typically feed the chick small fish like sand eels, sprats and young herring.

These small fish are easy to swallow whole and provide a high energy content which is necessary for the chicks fast growth and development.

Breeding Habits Of Guillemots

Guillemots typically begin their breeding season in late spring, with courtship starting as early as April, and egg-laying occurring in May or June. They breed in large, dense colonies on steep sea cliffs, often with thousands of birds packed closely together on narrow ledges.

Males sometimes perform short flights above the colony to advertise themselves to potential mates. Once a male and female form a bond, they will often preen each other as well as use a series of calls and engage in synchronised behaviours.

They will bow to each other and bob their heads at the same time. Guillemots don’t build nests but simply lay a single egg on the rocky cliff ledge. Each breeding pair will lay an egg which is different in colour and with unique markings like dark spots, speckles or blotches.

Both parents take turns incubating the egg and feeding the chick once it hatches.

Do Guillemots Mate For Life?

Guillemots are generally monogamous, and many pairs reunite at the same breeding site year after year. Long-term pair bonds are typical.

However, the pair might split and find a different partner to form a long-term bond if their breeding is unsuccessful or one partner dies.

The Size & Weight Of Guillemots

Guillemots are similar in size to Razorbills except Razorbills tend to have a larger wingspan.

How Much Does A Guillemot Weigh?

There are some differences in the weight of Guillemots with males generally weighing more than females. The average weight of an adult Guillemot is anywhere between 850 to 1,130 grams.

Females tend to be lighter weighing an average of around 500 to 860 grams. Whereas males can weigh as much as 1,130 grams.

How Big Is A Guillemot?

They can vary in length between 38 to 45 cm and boast a wingspan of around 64 to 73 cm. Females tend to be slightly smaller than males but from a distance it can be hard to distinguish between the two as they have similar colours and markings.

Are Guillemots Under Threat?

Guillemots are not considered to be under threat globally. They are a species of least concern according to the IUCN. However locally there is some concern as to their status. In the UK they are on the UK Birds of Conservation Concern Amber List.

Guillemots face several threats both from natural predators and human activities. Although they are not currently considered endangered, their populations are vulnerable to environmental changes and human impacts.

Climate change leading to ocean warming could impact on their main source of food as could overfishing. Sadly one of the main threats to Guillemots is pollution and oil spills in particular.

As these birds spend a huge amount of time on the water’s surface, they can get coated in oil which can cause serious harm and even death.

Disturbance of their breeding habitats can also pose problems, tourists can cause Guillemots to abandon their nests and eggs and coastal developments can impact on their habitats.

What Eats A Guillemot?

Guillemot chicks and eggs face the threat of predation from large gulls like Herring Gulls and Great Black-Backed Gulls. Ravens and Crows are also a threat to eggs and chicks.

Whereas large birds of prey like Peregrine Falcons take adult Guillemots from the air as they are flying. They also face threats from large marine mammals like seals when foraging at sea for food.

Where Can I See Guillemots In The UK?

Guillemots can be seen in several coastal areas around the UK, particularly along the northern and western coasts where they nest on steep sea cliffs. During the breeding season, they gather in large colonies, making them more visible.

The most reported sightings are found in the following locations;

Scotland

The Isle of May (Firth of Forth)
St. Kilda (Outer Hebrides)
Bass Rock (East Lothian)
Hermaness (Shetland Islands)

England

Bempton Cliffs (Yorkshire)
Farne Islands (Northumberland)

Wales

Skomer Island (Pembrokeshire)
Ramsey Island (Pembrokeshire)
South Stack (Anglesey)

Northern Ireland

Rathlin Island (County Antrim)

The best time to visit these locations is during the breeding season from April to July, when guillemots are nesting on cliffs and are more visible. After the chicks fledge, the adults and young birds move out to sea.

Frequently Asked Questions

Where do guillemots typically nest?

Guillemots nest on narrow, rocky sea cliffs, often in large, dense colonies. They do not build traditional nests but lay their eggs directly on the bare rock ledges, relying on the cliff’s natural features for protection.

What is unique about guillemot eggs?

The unique feature of Guillemot eggs is that they are pear-shaped, which helps prevent them from rolling off the narrow cliff ledges where they are laid. The eggs come in a variety of colours and patterns, which provide camouflage against predators.

What do guillemots eat?

Guillemots primarily feed on small fish like sandeels, herring, and sprats, which they catch by diving underwater. They use their wings to “fly” underwater, reaching depths of over 100 metres to hunt for their prey.