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NORTH AMERICAN FAUNA 73 



about 15 May. Jackson (1943) reported that golden-winged warblers were 

 "common" along the St. Croix River near St. Croix Falls from 21-25 May 

 1919. One pair was observed copulating on 22 May 1919. This warbler appar- 

 ently migrates singly, or in small very loose flocks; thus periods of peak 

 spring abundance are difficult to determine. Spring migrants have departed 

 nonbreeding areas in the Western Upland by 30 May. Fall migrants reach 

 the Western Upland about 10 August. Peak fall movements occur 

 20 August to 5 September. Departure from the Northern Highland occurs 

 by 5 September and elsewhere by 15 September. 



Nesting Season Distribution: Uncommon and local breeding bird in the 

 Central Plain and Northern Highland. One nest record from the Western 

 Upland (12 June 1976; River Falls, St. Croix County). Confirmed breeding 

 records exist for St. Croix and Polk counties. Green and Janssen (1975) cited 

 inferred breeding records for Chisago and Pine counties, although the 

 golden-winged warbler was referred to as "quite numerous" in Pine County 

 during the nesting season. One was reported from Washington County on 

 5 July 1970 (Russell 1970). Breeding Bird Survey data (Table 6) suggest that 

 the center of their breeding range is in the Central Plain. 



Habitat: Golden-winged warblers appear to be characteristic of second- 

 growth deciduous forest in early developmental stages. Data from nearby 

 Barron County (Wisconsin) show that largest densities occur in Deciduous 

 Clear Cuts that are less than 10 years old (9 pairs per 40 ha). This density de- 

 creases to <0.1 pair per 40 ha in mature aspen-maple woods over 40 years 

 old. Breeding golden-winged warblers also use brushy edges of retired agri- 

 cultural fields, openings in spruce woods, and occasionally Black 

 Spruce-Tamarack Bogs. Golden-winged warblers apparently avoid jack 

 pine habitat; this warbler has yet to be recorded along the Union BBS route 

 (Burnett County) which traverses extensive stands of jack pine forest. 



Blue-winged Warbler {Vermivora pinus) 

 Status: Regular migrant and nesting species. 



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

 Most observations consist of birds on or near presumed breeding territories. 

 Spring migrants arrive 15-20 May, becoming most conspicuous 25 May to 

 5 June. One spring record exists for Burnett County, 20 May 1956 (South- 

 ern 1960). Breeding birds disperse early from the breeding areas. The latest 

 fall date is 15 September 1975 (St. Croix County). 



Nesting Season Distribution: Rare and local breeding bird in the Western 

 Upland. Green and Janssen (1975) provided the only evidence of confirmed 

 breeding for the Valley (Washington County). Breeding season birds are 

 largely restricted to areas along major river systems, including the Kin- 

 nickinnic, Willow, and lower St. Croix. The northern limit of blue-winged 

 warbler breeding range in Wisconsin and Minnesota closely approximates 

 the southern Umit of its congener, the golden-winged warbler. As with other 

 regions of limited overlap, hybrids of these species have been observed. 

 Manley Olson observed a hybrid referable to "Brewster's warbler" on 



