MICROCLIMATE OF GROUND NESTS - THE RELATIVE IMPORTANCE OF RADIATIVE COVER AND WIND BREAKS FOR 3 GRASSLAND SPECIES

Three species of ground-nesting birds of the shortgrass prairie demons trate contrasting strategies in nest placement: nests of Lark Buntings (Calamospiza melanocorys) are situated beneath shrubs or bunch grasse s, 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 am bient wind velocity by at least an order of magnitude. Spatial orienta tion of vegetation around nests did not indicate the degree to which n ests were sheltered from wind, however. Nests of Lark Buntings were pl aced on the leeward side of shrubs, but vegetation apparently did not function as wind breaks. Profiles of the relative wind velocity at nes ts (nest-cup versus ambient wind velocities) documented that exposed n ests of longspurs and Homed Larks were more sheltered from wind than n ests of Lark Buntings: The association of bunting nests with overhangi ng vegetation instead may be in response to radiative cover. Nests of Lark Buntings were completely shaded 60% of the day compared to 45% fo r nests of longspurs and larks. Furthermore, bunting nests were shaded 40% of midday hours when solar intensity was at a maximum, nests of l ongspurs and larks were shaded only 5% and 13%, respectively, during t his period. Buntings begin breeding later in the season than either lo ngspurs or Homed Larks and thus are exposed to higher ambient temperat ures and decreased winds. Buntings are unique among the three species in that males incubate. Given that males have black plumage, opportuni ties for radiative cover and convective cooling may be important for n est placement in this species. In contrast, early-nesting longspurs an d larks may accrue thermal benefits from increased exposure of nests t o solar radiation.