BIRDS OF THE ST. CROIX RIVER VALLEY 



147 



R. 12 W.; Robbins 1974a). Another population is known to occur in north- 

 western Burnett County near Grantsburg. 



Habitat: Bent (1953) described the breeding habitat of the Connecticut 

 warbler as consisting of tamarack bogs or wet coniferous forest. However, 

 Robbins (1974a) reported this warbler in fairly large densities in monotypic 

 stands of jack pine. Further description of this habitat indicates the trees 

 were 4-9 m high and had well-developed lower branches. Trees < 4.6 m tall 

 or those with a scarcity of lower branches were not used by breeding Con- 

 necticut warblers. Robbins' observation of nesting in jack pine is further 

 substantiated by recent (1974-78) records from the Union BBS transect, 

 Burnett County. The habitat along this route also consists of large tracts of 

 jack pine, and Connecticut warblers have been found regularly there. 



Mourning Warbler {Oporornis Philadelphia) 

 Status: Regular migrant and nesting species. 



Migration: Fairly common migrant throughout the Valley. Spring migrants 

 arrive in the Western Upland 10-15 May and reach the Northern Highland 

 15-20 May. Peak spring migration occurs 25 May to 5 June and departure 

 from nonbreeding areas by 10 June. Fall migrants arrive in the Western 

 Upland 20-25 August. Peak fall migration through the Valley occurs 

 5-15 September and departure by 25 September (latest— 29 September 

 1965, Washington County). 



Nesting Season Distribution: Fairly common (locally common) breeding bird 

 in the Northern Highland and the northern half of the Central Plain. Else- 

 where, the mourning warbler is uncommon to rare during the nesting season. 

 A nest with young observed in Washington County on 11 and 13 July 1975 

 (Eckert 1975) represents the southernmost breeding record in the Valley. 

 Breeding Bird Survey data (Table 6) also suggest that the mourning warbler 

 breeding population increases with latitude through the Valley. 



Habitat: Characteristic breeding habitat of the mourning warbler includes 

 Northern Hardwood Forest and Deciduous Clear Cuts. Primary nesting 

 habitat of this warbler appears to be areas of dense understory in mature 

 stands of deciduous forest, such as those resulting from openings in the 

 overstory that allow ample sunUght to penetrate. The edge between 

 medium-aged aspen forest and open fields or highway rights-of-way are also 

 important habitats. I have not recorded this warbler in coniferous habitats 

 during the breeding season. 



Common Yellowthroat {Geothlypis trichas) 

 Status: Regular migrant and nesting species. 



Migration: Common migrant throughout the Valley. Spring migrants arrive 

 in the Western Upland 1-5 May and the Northern Highland about 10 May. 

 Peak spring migration occurs 10-20 May. Fall migration begins about 

 15 August. Peak fall migration through the Valley occurs 5-20 September 

 and departure by 5 October (latest— 17 October 1955, Polk County). 



