Common guillemot (Uria aalge)

„Spec­ta­cled“ Com­mon guil­l­emot (Ice­land).

Descrip­ti­on: The Com­mon guil­l­emot has a white bel­ly and is other­wi­se black; it is 41 cm tall and weighs 900-1,100 g. The most­ly-sub-arc­tic Com­mon guil­l­emot is very simi­lar to its high-arc­tic rela­ti­ve, the Brünich’s guil­l­emot. The dif­fe­rent beaks enable expe­ri­en­ced eyes to tell the­se two apart: the beak of the Com­mon guil­l­emot is lon­ger and more poin­ted and it does not have the white stri­pe along the upper man­di­ble, which is cha­rac­te­ristic for the Brünich’s guil­l­emot. Also, the white bel­ly can be used for iden­ti­fi­ca­ti­on: The Brünich’s guil­l­emot has a white stri­pe with a rather poin­ted end, run­ning from the bel­ly towards the throat; this is more roun­ded in the case of the Com­mon. A small pro­por­ti­on of the Com­mon guil­l­emots have white ‘spec­ta­cles’.

Distribution/Migrations: The com­mon guil­l­emot does not breed in Spits­ber­gen, apart from a few small occur­ren­ces in Bell­sund and at the nor­t­hern tip of Prins Karls For­land. The nor­t­hern­most lar­ge colo­nies are on Bjørnøya whe­re Com­mon and Brünich’s guil­l­emots breed in mixed colo­nies with hundreds of thou­sands of bree­ding pairs of each spe­ci­es. The Com­mon guil­l­emot thus tends to inha­bit more sub-arc­tic are­as, but is also found ever­y­whe­re at hig­her lati­tu­des and is one of the most num­e­rous sea­birds of the North. Pro­ba­b­ly spen­ding the win­ter in open waters lar­ge­ly in the regi­on of the bree­ding colo­nies, they are not very migra­ti­ve. Com­mon guil­l­emots bree­ding in the nor­t­hern­most parts of the spe­ci­es’ ran­ge move a few hundred kilo­me­t­res south towards the Nor­we­gi­an coast.

Bio­lo­gy: Com­mon guil­l­emots are sea­birds, which means that they spend all of their life at sea wit­hout coming to land, except in the bree­ding sea­son. Their menu is more spe­cia­li­sed than that of the Brünich’s guil­l­emot and com­pri­ses most­ly fish which they catch at depths bet­ween 20 and 30 met­res, excep­tio­nal­ly even bey­ond 100 met­res.

The stee­per and den­ser the colo­ny site, the bet­ter the bree­ding suc­cess of the Com­mon guil­l­emot. One reason for this is pro­tec­tion against pre­da­tors such as Arc­tic foxes and Glau­cous gulls. The bree­ding cycle is very simi­lar to the Brünich’s guil­l­emot. A sin­gle, pear-shaped egg is laid in late May or ear­ly June. Both par­ents sit for 32 days and take part in fora­ging for the chick for ano­ther three weeks until, still unable to fly, the chick has to jump from the cliff into the water or, if neces­sa­ry, down onto the tun­dra. Many young birds die during this hazar­dous ope­ra­ti­on or fall vic­tim to Arc­tic foxes. To redu­ce the risk, the bree­ding cycle is lar­ge­ly syn­chro­ni­zed within a colo­ny: The more chicks wal­king over the tun­dra at the same time, the smal­ler the risk for the indi­vi­du­al bird to end up as fox food.

Mis­cel­la­neous: In 1987, the Com­mon guil­l­emots’ colo­nies on Bjørnøya col­lap­sed by 85 %. Out of appro­xi­m­ate­ly 245,000 bree­ding pairs from the year befo­re, only 36,000 retur­ned to breed. The Brünich’s guil­l­emot, which breeds at the same colo­nies in com­pa­ra­ble num­bers, was not affec­ted. The reason was over­fi­shing and thus a deple­ti­on of the food resour­ces of the Com­mon guil­l­emot, whe­re­as the Brü­nichs were bet­ter off becau­se of their more varied diet. Sin­ce then, the bree­ding popu­la­ti­on of Com­mon guil­l­emots on Bjørnøya is reco­ve­ring, but has not yet rea­ched its ori­gi­nal size.

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