||<p>This thesis examines the dynamics of cloud forest bird communities along an altitudinal
gradient on the eastern slopes of the Ecuadorian Andes. Specifically, I examined
the seasonality of breeding, investigated a novel tool for documenting altitudinal
migrations, and compared diets among groups of hummingbirds. In Chapter 2, I compared
the prevalence of breeding condition in mist-netted birds among elevations and months.
Overall, birds breed less seasonally at lower latitudes, but there is substantial
variation in the timing of breeding, which varies with both abiotic conditions like
precipitation (Hau 2004; Tye 1992), photoperiod (Hau et al. 1998) and temperature
(Wikelski et al. 2000), and the biotic variation in plant phenology (Komdeur 1996),
and insect abundance (Poulin et al. 1992). Several of these biotic (e.g. canopy height,
biological diversity) and abiotic (e.g. temperature, pressure) factors vary along
elevational gradients. I compared the percentage of birds captured in breeding condition
along an altitudinal transect in eastern Ecuador, and found that breeding is more
seasonal at higher altitudes. There was a marked increase in breeding birds during
Sept-Nov at higher elevations, but I found no such "breeding season" at lower elevations.
</p><p>I also examined a novel methodology for tracing annual altitudinal migrations
which takes advantage of the natural variation in deuterium abundance from the base
to the peaks of the Andes (Chapter 3). Local migrations by birds in the tropics pose
conservation problems, in part because the movements themselves are difficult to document.
There is a theoretical relationship between Deuterium (or 18O) signature and elevation,
because of fractionation during precipitation events and evapotranspiration. A previous
study had suggested that if a bird had more or less deuterium in its tissues than
theory would suggest, such discrepancy might be used to identify altitudinal migrants.
Unfortunately, when I refined the methodology, I found that the variation within species
and sites was too great to allow such applications. </p><p>In Chapter 4 I shifted
my focus to comparing diet among hummingbirds. Hummingbirds rely on the sugars in
nectar to meet their high metabolic requirements, but most nectars are extremely low
in nitrogen. As a result, the birds must also consume arthropods to meet their protein
requirements. In many hummingbird species, males use nectar resources differently
from females. I hypothesized that the different genders might also differ in their
intake of arthropods, because females have higher nitrogen requirements for breeding.
I used stable nitrogen isotopic analysis of feathers and blood to demonstrate that
females feed at higher trophic levels than males and adults at higher levels than
juveniles, respectively. Feathers from female Coeligena torquata (Collared Inca) showed
significantly higher 15N levels (one-tailed t20=1.73, p<.05) than males. The difference
between genders in Heliodoxa leadbeateri (Violet-fronted brilliant) was smaller (one-tailed
t16=1.63, p=.06). δ15N was significantly lower for juveniles (mean = 6.34, SD =
2.10) than for adults (mean = 7.53, SD = 1.24). It appears that females captured during
the breeding season were also feeding at higher trophic levels than those captured
outside of the breeding season, although the sample sizes were small. Finally, I
also found a slight but unanticipated effect of elevation on δ15N values in feathers.</p>