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THE WILSON JOURNAL OF ORNITHOLOGY • Vol 124. No. 3. September 2012 
been reported (Haverschmidt 1958:143, Skutch 
1973:81, Schuehmann 1999:514). Presumably, the 
mild climate experienced by resident humming¬ 
birds in the tropics allows females to nest multiple 
times without constraints of weather and migra¬ 
tion that cause some double-brooded females of 
temperate species to renest so quickly that nest 
construction, laying, and start of incubation must 
occur before young from a previous attempt have 
Hedged. 
Hummingbirds are unusual among species with 
altricial young because parental care is performed 
solely by the female. Female hummingbirds may 
seem poorly suited for attending two nests 
simultaneously because they receive no assistance 
from their males or from nest helpers (cooperative 
breeding is unknown in trochilids). Female-only 
care by birds with altricial young is confined to 
species that eat fruit or nectar (Cockburn 2006). 
Perhaps the energy-rich diet of nectar, rapid 
absorption of nutrients (Schuehmann 1999), 
ability to enter torpor during energy shortages 
(Calder 2002), and longer day length at higher 
latitudes allow females of temperate species to 
maintain a positive energy balance while simul¬ 
taneously producing a second clutch of eggs and 
provisioning large nestlings. 
Rapid renesting appears to be a viable repro¬ 
ductive strategy for temperate-breeding hum¬ 
mingbirds in terms of tledgling production, but 
nothing is known about survival of fledglings 
from single-brooded versus multi-brooded fe¬ 
males. Young from first broods must be able to 
care for themselves shortly after fledging for the 
strategy to be evolutionary successful. 
Little has been published on the postfledging 
biology of wild hummingbirds, but the period of 
parental dependency is thought to be short, from 
a few days to a few weeks (Stiles 1973. 
Baltosser and Scott 1996, Robinson et al. 
1996. Russell 1996. Baltosser and Russell 
2000). Young Broad-billed, Anna’s, and Costa's 
hummingbirds raised in outdoor aviaries can 
start feeding themselves 7 days after fledging, 
although they are provisioned for longer periods 
if their mothers do not attempt to renest (Karen 
Krebbs. pers, comm.). Incubation constancy 
normally ranges from 60 to 85% in humming¬ 
birds (Skutch 1962, Baltosser 1996. Calder 
2002). rhus, it is not unusual for hummingbird 
eggs to be left unattended for brief periods, as 
would be necessary when females overlapped 
two nesting attempts. 
ACKNOWLEDGMENTS 
Wc thank John Vanderpoel and Boh Maninka for 
assisting with observations: William Baltosser. Karen 
Krebbs. Karl-Ludwig Schuehmann. Peter Scott, and Sheri 
Williamson for helpful discussions and manuscript reviews: 
and the Kujala and Lamb families for allowing us to 
observe hummingbirds on their respective properties. 
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