V 14 No.4 - The Scottish Ornithologists' Club

216 Short Notes
S8 14 (4)
egg on 1 July but the Roseate landed at the
nest carrying a sandeel and was clearly
agitated by our presence. A chick (c. 1 day
old) was found 2 m away but we could not
be absolutely certain that it was from this
nest. As happened in most other parts of
Shetland in 1984 this colony failed, and a
visit on 7 July found only a few Arctic Terns
present, with no sign of the adult Roseate.
This constitutes only the fifth Shetland
record of Roseate Tern; two were recorded
in 1983, one of which associated with
displaying Common Terns Sterna hirundo
on Bressay in late May (p JE, pers. obs.) .
This also represents an extension of breeding
range for Roseate Tern - the previous northerly
limit being set by 3 pairs which bred
in Orkney in 1969 (Cramp et al. 1974).
There has been no previous suggestion
of Roseate and Arctic Terns interbreeding
(Cramp 1985; Nisbet in litt.); these species
rarely breed in the same areas. However, the
breeding ranges of Common and Roseate
Terns overlap considerably and it now appears
that hybridisation between these two
species is regular on a small scale on both
sides of the Atlantic, with mixed pairs often
succeeding in fledging hybrid young (eg.
Robbins 1974; Hays 1975; Nisbet 1976). All
but one of the cases in which the sexes were
known involved a female Roseate Tern.
Although we did not witness pair
behaviour between the Roseate and any
Arctic Terns, the most likely interpretation
is that a female Roseate Tern paired with
a male Arctic Tern, laid an egg, and may
have hatched a chick. Cullen (1956) found
an excess of unmated males in Arctic Tern
colonies on the Fame Islands, many of
which persistently attempted to court the
mated females throughout the season. A few
were accepted by established pairs and
assisted with incubation and chick feeding.
It is thus perhaps not surprising that a lone
female Roseate Tern should accept the advances
of a male Arctic Tern. However, we
were unable to rule out the possibility that
an Arctic Tern became attached to a pair
of breeding Roseate Terns.
Acknowledgements
We thank lan Sandison for the egg photographs,
and Drs. Euan Dunn and lan Nisbet for commenting
on an earlier draft manuscript.
References
Cramp, S., Bourne, W.R.P. & Saunders, D.
1974. The Seabirds of Britain and Ireland.
Collins: London.
Cramp, S. (ed.) 1985. The Birds of the Western
Palearctic, Vol. IV, Oxford Univ . Press:
Oxford.
Cullen, J.M. 1956. A study of the behaviour
of the Arctic Tern Sterna macrura. D. Phil.
thesis, Oxford University.
Hays, H. 1975. Probable Common x Roseate
Tern hybrids. Auk 92: 219-234.
Nisbet,I.C.T. 1976. Early stages in postfledging
dispersal of Common Terns. Bird Banding
47: 163-164.
Robbins, C.S. 1974. Probable interbreeding of
Common and Roseate Terns. Brit. Birds 67:
168-170.
P.I. Ewins, Nee, Archway House, 7 £astcheap, Letchworth, Herts SG6 3DG.
I.D. Okill & P.M. Ellis, Shetland.