Cooperative Fish and Wildlife Research Units Program:
Education, Research and Technical Assistance for Managing Our Natural Resources

Many raptor species (i.e., hawks, eagles) will place green plant material on their nests, not as part of the nest structure, but added to the top perimeter of the nest. The sprigs of greenery are often replenished or replaced daily during incubation and nestling rearing periods. The potential implications of this behavior are not trivial, as the behavior would not have developed if it did not serve an important ecological function. I propose that the addition of greenery to nests has a combined function of 1) increasing humidity and 2) decreasing temperature within the immediate environment of the nest. High temperature and low humidity have been demonstrated to negatively influence nesting success of raptors, and raptors have few mechanisms to cool eggs or nestlings. Further, nestling raptors thermoregulate by gular flutter, which is functionally evaporative cooling; this behavior, however, is conducted at the expense of losing body moisture which can only be replaced via food while in the nestling stage. In a pilot test with replica nests, the effect of adding greenery was rapid and dramatic, influencing the nest humidity for 13 hrs and temperature for 9 hrs, with maximum treatment differences of 64% increased humidity and 7% decreased temperature. In this study, an undergraduate fellow will 1) validate preliminary findings through replication and increase in sample size; 2) test leaf material from multiple tree species to assess species-specific differences in effect on humidity and temperature; 3) conduct observations of Mississippi kites to determine what tree species they select for green material and if, based on our tests, they select those species that provide the greatest influence on humidity and temperature.