0 the least tern (sternula antillarum , formerly sterna antillarum) is a species of tern that breeds in north america and locally in northern south america .
1  it is closely related to , and was formerly often considered conspecific with , the little tern of the old world .
2  other close relatives include the yellow-billed tern and peruvian tern , both from south america .
3 it is a small tern , 2224 cm (8.79.4 in) long , with a wingspan of 50 cm (20 in), and weighing 3952 g (1.41.8 oz) .
4  the upper parts are a fairly uniform pale gray , and the underparts white .
5  the head is white , with a black cap and line through the eye to the base of the bill , and a small white forehead patch above the bill; in winter , the white forehead is more extensive , with a smaller and less sharply defined black cap .
6  the bill is yellow with a small black tip in summer , all blackish in winter .
7  the legs are yellowish .
8  the wings are mostly pale gray , but with conspicuous black markings on their outermost primaries .
9  it flies over water with fast , jerky wingbeats and a distinctive hunchback appearance , with the bill pointing slightly downward .
10 it is migratory , wintering in central america , the caribbean and northern south america .
11  many spend their whole first year in their wintering area .
12  it has occurred as a vagrant to europe , with one record in great britain .
13 it differs from the little tern mainly in that its rump and tail are gray , not white , and it has a different , more squeaking call; from the yellow-billed tern in being paler gray above and having a black tip to the bill; and from the peruvian tern in being paler gray above and white (not pale gray) below and having a shorter black tip to the bill .
14 the least tern arrives at its breeding grounds in late april .
15  the breeding colonies are not dense and may appear along either marine or estuarine shores , or on sandbar islands in large rivers , in areas free from humans or predators .
16  courtship typically takes place removed from the nesting colony site , usually on an exposed tidal flat or beach .
17  only after courtship has confirmed mate selection does nesting begin by mid-may and is usually complete by mid-june .
18  nests are situated on barren to sparsely vegetated places near water , normally on sandy or gravelly substrates .
19  in the southeastern united states , many breeding sites are on white gravel rooftops .
20  in the san francisco bay region , breeding typically takes place on abandoned salt flats .
21  where the surface is hard , this species may use an artificial indentation (such as a deep dried footprint) to form the nest basin .
22 the nest density may be as low as several per acre , but in san diego county , densities of 200 nests per acre have been observed .
23  most commonly the clutch size is two or three , but it is not rare to consist of either one or four eggs .
24  both female and male incubate the eggs for a period of about three weeks , and both parents tend the semiprecocial young .
25  young birds can fly at age four weeks .
26  after formation of the new families , groupings of birds may appear at lacustrine settings in proximity to the coast .
27  late-season nesting may be renests or the result of late arrivals .
28  in any case , the bulk of the population has left the breeding grounds by the end of august .
29 the least tern hunts primarily in shallow estuaries and lagoons , where smaller fishes are abundant .
30  it hovers until spotting prey , and then plunges into the water without full submersion to extract meal .
31  the most common prey recently for both chicks and adults are silversides smelt (atherinops spp .
32 ) and anchovy (anchoa spp .
33 ) in southern california , as well as shiner perch , and small crustaceans elsewhere .
34  adults in southern california eat kelpfish (most likely giant kelpfish , heterostichus rostratus) .
35  insects are known to be eaten during el nio events .
36  in southern california , least terns feed in bays and lagoons , near shore , and more than 24 km (15 mi) from shore in the open ocean .
37  elsewhere , they feed in proximity to lagoons or bay mouths .
38 adults do not require cover , so that they commonly roost and nest on the open ground .
39  after young chicks are three days old , they are brooded less frequently by parents and require wind blocks and shade , and protection from predators .
40  in some colonies in southern california , spanish roof tiles are placed in colonies so chicks can hide there .
41  notable disruption of colonies can occur from predation by burrowing owls , gull-billed terns and american kestrels .
42  depredation by domestic cats has been observed in at least one colony .
43  predation on inland breeding terns by coyotes , bobcats , feral dogs and cats , great blue herons , mississippi kites , and owls has also been documented .
