104 



NORTH AMERICAN FAUNA 73 



Migration: Common to abundant migrant throughout the Valley. The 

 largest numbers occur in open agricultural regions of the Western Upland 

 and Central Plain. Spring migration begins during late January in the south 

 and the first migrants reach Burnett and Pine counties by 15 February. 

 Peak spring migration occurs between 25 February and 25 March. Peak fall 

 migration occurs between 15 October and 10 November and most have de- 

 parted by 15 December. 



Nesting Season Distribution: Horned larks are common nesting birds in the 

 Western Upland and Central Plain, rare and local in the Northern Highland. 



Winter: During the winter, horned larks present a perplexing problem in 

 determining whether individuals are early or late migrants or actual winter 

 residents. North of St. Croix Falls, this species is generally absent from mid- 

 December to late January. South of St. Croix Falls, occasional small flocks 

 of 5 to 20 individuals can be found in open areas during this period. I usually 

 consider displaying individuals to be returning summer residents and con- 

 sider migration to be commencing with the observation of the first large 

 flocks. 



Habitat: Horned larks are characteristic of agricultural areas. Breeding 

 densities appear largest in short vegetation associated with domestic hay- 

 fields and oat stubble. This species has adapted well to man's increased agri- 

 cultural production and breeding pairs regularly use bare cultivated fields 

 and various row-crop fields. 



FAMILY HIRUNDINIDAE: Swallows 



Tree Swallow [Iridoprocne bicolor) 

 Status: Regular migrant and nesting species. 



Migration: Abundant migrant in all regions. Spring migrants arrive in the 

 Western Upland 25 March to 5 April and reach the Northern Highland 

 5-10 April. Peak spring abundance occurs 20-30 April. Fall migration 

 begins in late July with flocking of family groups. Peak fall abundance 

 occurs 15-30 August and departure by 10 October. During peak fall migra- 

 tion, tree swallows are occasionally observed in massive mixed-species 

 flocks near large water bodies. Concentrations of 4,000 to 5,000 individuals 

 are frequently observed along the lower St. Croix River. 



Nesting Season Distribution: Common nesting species in the Western 

 Upland and Central Plain, common and more local in the Northern High- 

 land. Breeding Bird Survey data (Table 3) show that tree swallows are uni- 

 formly distributed throughout the Valley. 



Habitat: Tree swallows nest in loose, semicolonial associations. Breeding 

 pairs are typically associated with water bodies that are normally near small 

 groves of trees (Western Upland and Central Plain) or along northern Forest 

 Bordered Lakes. Extensive use is made of the edge between deciduous forest 

 and natural openings or agricultural fields. Nests are normally located in 

 natural cavities in tree stumps or fence posts and occasionally in holes in 



