Experimental evidence that nest orientation influences microclimate in a temperate grassland

Birds exhibit an assortment of behavioral strategies to cope with variable
environmental conditions during reproduction, including altering nest
construction behaviors. In species building enclosed domed nests, the
microclimate within nests is influenced not only by its structure and the
surrounding vegetation but also by the orientation of the nest opening.
Many grassland-dependent birds build dome-shaped nests with clear
directionality of openings. We studied two species in northeastern Kansas,
United States that typically orient their nests east to northeast in this
region. However, in a drought year, both Grasshopper Sparrows (Ammodramus
savannarum) and Eastern Meadowlarks (Sturnella manga) shifted orientations
of their nests southward toward prevailing winds. We hypothesized that
this shift reduced the deleterious effects of heat stress on parents and
developing young by diminishing morning solar radiation and increasing
cooling due to the prevailing southerly winds. To test this hypothesis, we
measured temperature, humidity, and wind speed at pairs of unoccupied,
field-collected sparrow and meadowlark nests, experimentally placed to
face south or east (control) in a non-drought year. Nest orientation
affected the daily microclimate patterns, with south-facing nests warming
later in the day relative to east-facing nests. The temperature
differences depended upon humidity, with south-facing nests being
relatively cooler under more humid conditions. This work provides the
first experimental evidence of the benefits of plasticity in nest
construction under challenging thermoregulatory conditions and shows how
ground-nesting birds may reduce thermoregulatory demands during incubation
under climate variation.