BIRDS OF THE ST. CROIX RIVER VALLEY 



161 



1-10 June. During most years, this bird is most numerous during the first 

 10 days of June. Singing ceases about 1 August and birds become difficult 

 to find after that date. There is no discernible peak in fall migration, and 

 most have departed by 25 August (latest— 16 September 1974, Polk 

 County). 



Nesting Season Distribution: Fairly common although highly irregular 

 nesting species in the Western Upland and Central Plain. There are no 

 known nest records for Douglas or Pine counties, although Green and Jans- 

 sen (1975) stated that the breeding range in Minnesota "usually extended 

 only to southern Pine County." Jackson (1943) did not record this species in 

 northwestern Wisconsin during 1919. 



During 1975, the dickcissel was virtually absent from St. Croix and Wash- 

 ington counties during the nesting season. In 1976, I recorded a density of 

 20.4 pairs per 40 ha on Managed Grassland tracts in St. Croix and Polk 

 counties. Breeding Bird Survey data (Table 8) provide another indication of 

 the rapid decrease in abundance moving northward from the Western 

 Upland to Northern Highland. 



Habitat: Characteristic breeding bird of retired agricultural fields that have 

 become overgrown with a rank growth of vegetation. Also an important 

 breeding bird of alfalfa fields and of Managed Grasslands that are main- 

 tained by various State and Federal wildHfe agencies for duck nesting cover. 



Evening Grosbeak (Hesperiphona vespertina) 



Status: Regular migrant and winter resident, casual summer visitor, and 

 possible nesting species. 



Migration: Common yet erratic fall and spring migrant in the Western 

 Upland and Central Plain. Usually a common to abundant migrant in the 

 Northern Highland. Migration periods appear to depend upon the abun- 

 dance of a food source in the northern breeding areas. The first fall migrants 

 usually arrive in the Northern Highland 1-15 October and reach the 

 Western Upland about 15 November, except during invasion years, when 

 they have been recorded in mid-October (15 October 1959, Polk County). 

 Peak fall migration varies with the year, however, usually occurring 15 No- 

 vember to 15 December. Peak spring migration usually occurs during 

 March. During non-invasion years, peak spring migration occurs 25 March 

 to 5 May. The largest late flock on record (600) was observed at Webster 

 (Burnett County) on 25 April 1950. Departure also apparently varies with 

 the year. There are several Polk County departure records that range from 

 4 May (1954) to 17 May (1952). Latest dates from nonsummering areas 

 include 28 May 1962, Polk County and 30 May 1972, Burnett County. 



Nesting Season Distribution: Casual summer resident in the Northern High- 

 land. There are no confirmed nest records for the Valley, although a few 

 pairs may remain to nest after invasion years. Evening grosbeaks were re- 

 corded during the 1972 summer in Pine County (Green and Baumhoffer 

 1972). S. D. Robbins recorded evening grosbeaks along the route of the 

 Minong BBS during mid-June from 1965 to 1977. During June 1974 I ob- 



