Meeting Abstract
Nestlings have narrow thermal tolerances (35-40˚C) and require nests with good insulation. Smaller nests might require relatively more insulation because of their high surface-to-volume ratio. Since hummingbirds build among the smallest nests, they are ideal for study of structural adaptations that enhance heat retention. We collected broad-billed (BBLH; Cynanthtus latirostris) and black-chinned (BCHU; Archilochus alexandrii) nests near Patagonia, AZ after the nestlings fledged. Detailed morphometric measurements were made on all nests. To test nest insulation a sphere heated to hummingbird body temperature (41˚C) was placed in the nest cup to simulate incubation. Heating and cooling equilibrium times and temperatures of nest walls were recorded at air temperatures between 5-40˚C with thermocouples and infrared thermography. Nest dimensions did not vary between species. Nests were oval in shape due to stretching from incubation and nestling growth. The diameter along the longest axis of the nest cup was 22.1+/-7.1mm (inner) and 40.9+/-8.1mm (outer). Wall thickness ranged from 7-11mm. Temperature gradients between nest surface and ambient temperatures ranged from 9.8 ˚C (Ta=5˚C ) to <1.0˚C (Ta=40˚C) in BBLH and 8.3 to <1.0˚C in BCHU. Cooling time ranged from 7.6-17.9 min in BBLH and 4.1-11.5 min in BCHU. Calculated surface specific conductance for BBLH nests (22.6 W ˚C-1m-2) and BCHU nests (21.7 W ˚C-1m-2) were similar. Total nest conductance for both BBLH nests (60.2 mW ˚C-1) and BCHU nests (58.8 mW ˚C-1) is 2-4 times less than that of bird species ranging in mass from 50-100g (90-200 mW ˚C-1). Longer cooling time in BBLH nests might suggest better insulation than BCHU nests. Lower conductance in hummingbird nests support greater relative insulation than in larger nests.
