82 



NORTH AMERICAN FAUNA 73 



Forster's Tern (Sterna forsteri) 



Status: Regular migrant and casual summer resident. The WDNR listed this 

 species as endangered (Les 1979). 



Migration: Fairly common spring and rare fall migrant in the Western 

 Upland and Central Plain, casual in the Northern Highland. Spring mi- 

 grants arrive 20-25 April and are most commonly observed 5-15 May. De- 

 parture of spring migrants occurs by 25 May. FaU migrants arrive between 

 1-5 September and depart by 1 October. 



Summer: A casual summer resident in St. Croix and Washington counties. 

 The presence of summering birds suggests nesting; however, no nests or 

 young have been recorded. This tern occurs with greatest frequency during 

 the summer on the wetlands in St. Croix and Washington counties. 



Habitat: Migrants are observed in association with large semipermanently 

 and permanently flooded wetlands. Observations of summering birds have 

 been restricted primarily to permanently flooded wetlands that support 

 extensive growths of cattail and hardstem bulrush along the periphery of 

 the basin. 



Caspian Tern {Sterna caspia) 

 Status: Regular migrant. 



Migration: Rare migrant in the Western Upland, casual or absent elsewhere. 

 Spring migrants arrive about 1 May (earliest— 8 April 1961, Burnett 

 County). Peak abundance occurs 10-20 May and departure by 25 May (lat- 

 est-30 May 1924, Chisago County; Roberts 1938: 2 June 1974, Wash- 

 ington County; Eckert 1975). Fall migrants arrive about 1 September and 

 have departed by 25 September. During both migration periods, Caspian 

 terns are most regularly observed on the St. Croix River. Away from the 

 river, they are less common and more irregular in occurrence. 



Habitat: Largely restricted to open expanses of the St. Croix River and large 

 permanently flooded wetlands. 



Black Tern (Chlidonias niger) 



Status: Regular migrant and nesting species. 



Migration: Fairly common migrant in the Western Upland and Central 

 Plain; uncommon in the Northern Highland. Spring migrants arrive 25 April 

 to 1 May, reaching peak abundance about 15 May. FaU migrants reach peak 

 numbers about 10 August and have departed by 1 September (latest— 

 4 October 1965, Washington County). 



Nesting Season Distribution: A fairly common nesting species in the prairie 

 wetland region of St. Croix, Washington, and southern Polk counties, 

 becoming less common in the forested regions. Breeding populations of the 

 black tern have decUned sharply in Wisconsin. During 1966-75, Robbins 

 (1977) noted a 14% annual decrease in the statewide breeding population. A 



