Revista Chilena de Historia Natural 80 (3): 267-273, 2007
RESEARCH ARTICLE
Evaporative water loss and dehydration during the night in hummingbirds
Nectar-feeding birds oscillate between avoiding overhydration when they are feeding and
preventing dehydration during fasts. Here, we examined how resting rates of total evaporative water loss (TEWL) and metabolic water
production (MWP) influence water balance in the green-backed firecrown (Sephanoides sephanoides), a
Chilean hummingbird. We hypothesized that a circadian rhythm in TEWL would assuage the dehydration risk that hummingbirds face
during the night. However, we did not find support for this idea. In resting hummingbirds, rates of TEWL during the day (54 ± 6 µL
h<sup>-1</sup>, n = 8) and night (65 ± 12 µL h<sup>-1</sup>, n = 5) were similar. Rates of MWP were also similar
between the day (22 ± 3 µL h<sup>-1</sup>, n = 8) and night (23 ± 2 µL h<sup>-1</sup>, n = 5). MWP rates were
significantly lower than TEWL rates during both the day and night. Our findings both support the notion that hummingbirds dehydrate
during extended fasts and illustrate that evaporative water loss is an important osmoregulatory consideration in hummingbirds.
However, because the technique we used to estimate rates of TEWL and MWP at night was indirect, our findings should be interpreted
cautiously until direct measurements are available.
Key words:
circadian rhythm,
evaporative water loss, hummingbird, osmoregulation, water balance
