THE COMMON TERN. 71 L 



tail : outer 140-160, central 65-76, fork 64-86. Juvenile $ and $ 

 wing 243-265, tail: outer 103-123, central 68-77, fork 32-51. 

 Primaries : 1st narrow, pointed, and about two-thirds primary- 

 coverts, 2nd longest, 3rd 10-20 mm. shorter, 4th 30-40 shorter, 5th 

 54-60 shorter. Rest of structure as Sandwich Tern but less than 

 half tibia bare, webs between toes only slightly concave. 



Soft parts. — Bill (ad. winter) most of upper mandible black, 

 vermilion base, lower vermilion, crimson in angle, tip black, (ad. 

 summer) crimson, tip black, (juv.) as winter but black portion 

 browner and base more yellowish ; legs and feet (ad.) vermilion, 

 (juv.) yellow-orange ; iris dark brown. 



Characters and allied eorms. — S. h. tibetana (Tibet, Turkestan) 

 has darker upper- and under-parts. Distinguished from Arctic 

 Tern by longer tarsus and usually longer bill and broader blackish 

 line on inner web of outer primary, from Roseate at all ages by dark 

 outer web of outer tail-feather, longer wing, as well as by other 

 differences described under those species. 



Field -characters. — Adult slightly paler, with shorter outer tail- 

 feathers, and longer tarsus than Arctic, but these differences hardly 

 perceptible unless birds standing side by side. Bill in breeding 

 season orange-red tipped with black, not crimson as in Arctic. 

 Immature birds indistinguishable by colour. Best distinction 

 afforded by cries ; call -notes " kitt, kitt " and "keerie"; alarm- 

 notes long drawn " pee -rah " and " pee-er " ; young just able to 

 fly utter repeated " chit, chit, chit." 



Breeding-habits. — Breeds in colonies, sometimes of vast extent, 

 on shingle -banks, sand-dunes, salt-marshes, rocky islets, etc. 

 Nest. — Scratched out hollow in sand, grass, rock, or shingle, some- 

 times quite unlined but occasionally carelessly lined with marram- 

 grass, bents, feathers, etc. Eggs. — Generally 3, but often 2, while 

 4 are occasionally found. Higher numbers due to two or more hens. 

 Varying from stone-colour to grey or brown, blotched and spotted 

 with dark brown and ashy-grey. Erythristic type with red markings 

 on warm ground is rare and blue eggs with or without markings 

 also met with but more often. Average of 100 British eggs, 40.9 X 

 30.3. Max.: 45x29.1 and 42.2x32.5. Min. : 35.7x28.3 and 

 40.5x27.7 mm. Breeding-season. — Most eggs laid end May and 

 early June ; exceptionally by 10 or 12 May. Incubation. — By both 

 sexes in turn. Period 20-22 days, average about 21 days. Single 

 brooded. 



Food. — Mainly small fish, among which have been recorded : young 

 herrings (Clupea), whiting, coal -fish, codling, sand-eels (Ammodytes), 

 plaice (Pleuronectes), lump-suckers, 15-spined stickleback (Gaster- 

 osteus), also insects, including coleoptera, odonata, diptera (Tipu- 

 lidce and small flies), Crustacea (shrimps), etc. 



Distribution. — British Isles. — Summer -resident (end of second 

 week April (early dates Feb. 1 and 4, 1912, Kintyre ; Feb. 24, 1912, 



