Náttúrufræðingurinn - 1957, Blaðsíða 62

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NÁTTÚRUFRÆÐINGURINN
the young o£ this species, probably caused by shortage of suitable £ood or incle-
nient weather conditions.
The black-headed gull, which has colonized Iceland in recent years, has by
now started to nest in some colonies o£ the arctic tern on islands along tlie coast
and in lakes. This has led to a sharp competition for nesting-sites between these
two species and as the black-headed guli nests considerably earlier than the
tern the former has an obvious advantage over the latter. As far as I know, how-
ever, the black-headed gull lias nowhere ousted the arctic tern altogether, and
the conflict has usually resulted in these two species dividing the nesting-ground
between tliem. Arctic terns frequently nest on islands supporting eider-duck
colonies. This is generally considered beneficial as the arctic tern is believed to
defend the eider-duck colonies from predators. The arctic tern is wellknown for
its aggressive behaviour on the breeding-ground. It will attack any trespasser
tiercely and will strike freely at the heads of human intruders.
Although a great rnany colonies of the arctic tern in Iceland are old-est;i-
blished and are known to have existed for centuries the desertion of old sites
and the establishing oí new colonies is of common occurrence. The reasons for
such erratic tendencies are, at least in some cases, indiscriminate egg collecting,
periodical shortage of food supply, and catastrophic changes in the nesting-
site itself.
The arctic tern is a summer-resident in Iceland and possibly also a passage-
migrant. The arrival in spring takes place in May, chiefly in the first half of
this month. In S.E. Iceland the vanguard may arrive already at tlie end of
April and in some years as early in N.E. Iceland. In the western parts of the
country first arrivals average about a week later than in the East and in the
North-Western Peninsula (Vestfirdir) first arrivals may be delayed until May
20. Flowever, by the middle of May the main body o£ the birds has arrived and
nesting grounds in most districts have been occupied. The birds begin to leave
again in the second half of July although the exodus chiefly takes place in
August. By the end of that nronth the nesting-grounds have been abandoned and
the majority of the birds have left inshore waters. However, during the first
lialf of September terns are still present in some numbers along the coast and odd
birds may even by met with until early October.
Little is known alrout the migrations of the Icelandic tern population. Only
two ringed terns have been recovered abroad, one on the Belgian coast in spring
and the other in Nigeria, West Africa, in September. The latter had been ringed
as young and was 21 years old when recovered. On the other hand 49 ringed
terns have been recovered or recaptured in the area where they had been ringed
one or more years after ringing. Of 30 terns ringed as adults and recovered in
subsequent years where they had been ringed, no less than 20 were recovered in
the first and second year subsequent to ringing, while the remaining 10 recov-
eries are scattered over a period extending from the 3rd to 17th year subsequent
to ringing. The recoveries o£ 19 terns in areas where tliey had been ringed as
young are distributed over a period extending from the 3rd to 25th year subse-
quent to ringing. It is of interest to note that none of the terns ringed as young
were recovered in the first and second year subsequent to ringing. This indicates