BIRDS OF THE ST. CROIX RIVER VALLEY 



131 



Migration: Common (locally abundant) migrant throughout the Valley. 

 Spring migrants arrive in the Western Upland 10-15 May, reaching the 

 Northern Highland by 20 May. The peak of migration occurs 20 May to 

 5 June. Fall migration begins 20-25 August. Peak abundance occurs 

 10-15 September and departure by 5 October. 



Nesting Season Distribution: Fairly common nesting species in the Western 

 Upland, common to locally abundant in the Central Plain and Northern 

 Highland. Goddard (1972) reported this vireo to be among the 10 most abun- 

 dant breeding birds in the Kinnickinnic River Valley, Pierce County. Mean 

 density in that area was 25.3 pairs per 40 ha. Analysis of BBS data (Table 6) 

 indicates a gradation in abundance moving northward through the Valley. 

 Documented nesting records exist for Polk, St. Croix, and Washington coun- 

 ties and there is evidence of nesting in the other counties. 



Habitat: Red-eyed vireos are largely restricted to various deciduous forest 

 types during migration and the nesting season. Although not characteristic 

 of any one habitat type, their abundance is predictable when compared 

 among geomorphic regions. 



In the Western Upland, highest breeding densities occur in mature Low- 

 land Deciduous Forest and mature Southern Deciduous Forest. In the Cen- 

 tral Plain, breeding populations are greatest in mature Northern Hardwood 

 Forest dominated by sugar maple and basswood. In the Northern Highland, 

 populations are greatest in aspen forest and mature Northern Hardwood 

 Forest. 



Philadelphia Vireo {Vireo philadelphicus) 

 Status: Regular migrant. 



Migration: Uncommon to rare migrant throughout the VaUey. This is the 

 latest arriving vireo; average first appearance is 15 May in the Western 

 Upland. Migrants arrive in the Northern Highland about 20 May. Observa- 

 tions usually are of single birds or occasionally pairs; therefore, peak migra- 

 tion cannot be determined. Spring departure from the southern regions 

 occurs by 30 May and from the Northern Highland by 5 June. Fall migrants 

 arrive 10-15 September. This species is most frequently encountered 

 15-25 September and departs from 30 September to 5 October (latest— 

 29 October 1970, Washington County). 



Habitat: Philadelphia vireos use a variety of habitat types during migration; 

 however, they are most regularly observed in medium-aged to mature 

 Northern Hardwood Forest. 



Warbling Vireo {Vireo gilvus) 



Status: Regular migrant and nesting species. 



Migration: Common migrant in the Western Upland; fairly common in the 

 Central Plain and Northern Highland. Fairly common fall migrant in the 

 Western Upland, uncommon to rare elsewhere. The mean date of spring arri- 



