Microclimate of Ground Nests: the Relative Importance of Radiative Cover and Wind Breaks for Three Grassland Species

Three species of ground-nesting birds of the shortgrass prairie demonstrate contrasting strategies in nest placement: nests of Lark Buntings (Calamospiza melanocorys) are situated beneath shrubs or bunch grasses, whereas nests of McCown’s Longspurs (Calcarius mccownii) and Homed Larks (Eremophila alpestris) are associated with sparse cover and are relatively exposed. Nests of these species exhibited a reduction in ambient wind velocity by at least an order of magnitude. Spatial orientation of vegetation around nests did not indicate the degree to which nests were sheltered from wind, however. Nests of Lark Buntings were placed on the leeward side of shrubs, but vegetation apparently did not function as wind breaks. Profiles of the relative wind velocity at nests (nest-cup versus ambient wind velocities) documented that exposed nests of longspurs and Homed Larks were more sheltered from wind than nests of Lark Buntings. The association of bunting nests with overhanging vegetation instead may be in response to radiative cover. Nests of Lark Buntings were completely shaded 60% of the day compared to 45% for nests of longspurs and larks. Furthermore, bunting nests were shaded 40% of midday hours when solar intensity was at a maximum; nests of longspurs and larks were shaded only 5% and 13%, respectively, during this period. Buntings begin breeding later in the season than either longspurs or Homed Larks and thus are exposed to higher ambient temperatures and decreased winds. Buntings are unique among the three species in that males incubate. Given that males have black plumage, opportunities for radiative cover and convective cooling may be important for nest placement in this species. In contrast, early-nesting longspurs and larks may accrue thermal benefits from increased exposure of nests to solar radiation.