Assessing Nutrient Reserves and Local Population Dynamics of Wintering Lesser Scaup in East-Central Florida
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Date
2003-09-04
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Abstract
The current decline of the continental population of scaup coincides with a decline in body condition of birds arriving on the breeding grounds. The Spring Condition Hypothesis is one of the leading explanations for the current decline, and states that lesser scaup are arriving in poor body condition on the breeding grounds, which has the potential to impair their reproductive capabilities. However, it is not known if this decline in body condition is related to habitat on the wintering or staging areas along the flyways, or a combination of both. I examined changes in body condition (fresh body mass, corrected body mass, lipids, protein, mineral reserves) between mid and late winter on the most significant wintering grounds for scaup on the Atlantic Flyway; the Indian River Lagoon System during the winters of 2002 - 2003. If acquiring elevated levels of reserves prior to migration were critical to scaup, a significant change could be detected between mid and late winter (energy maximization strategy). Additionally, I examined changes in time spent foraging during mid and late winter to understand how scaup behavior might be related to their body condition. Diurnal foraging was similar between mid and late winter (P = 0.06). Nocturnal foraging increased by 70% (P = 0.03) in females and 263% (P = <0.0001) in males from mid to late winter. When time spent feeding was examined throughout the diel cycle, female scaup increased foraging by only 5%, or from 4.62 to 4.83 hr/day, while males increased foraging by 28%, or from 4.69 to 6.02 h. There was no change between mid and late winter for corrected body mass (female P = 0.09, male P = 0.65), lipids (female P = 0.09, male P = 0.96), mineral reserves (female P = 0.74, male P = 0.11) and protein (2002 female P = 0.74; 2002 male P = 0.12; 2003 male P = 0.12). Protein levels in females dropped seasonally by 4%. It is not known if this change is important physiologically.
My results lend support to recent research findings that contend that nutrient reserve acquisition prior to arriving at the breeding grounds is most likely constrained by changes in habitat conditions (e.g., prey availability, wetland degradation) on the northern staging areas, not conditions on the southern wintering grounds. Although it does not seem that nutrient reserves increased dramatically, it is noteworthy that recorded fat levels in this study were 70-80% of levels recorded on scaup just prior to the onset of reproduction. Therefore, wintering grounds remain as an integral habitat of the annual and season cycle of scaups to meet basic energetic requirements. Consequently, preventing habitat degradation should remain a critical component of scaup management efforts. Use of a wide variety of habitats (estuarine and impounded wetlands) by wintering scaup increases the likelihood of continued habitat availability on the wintering grounds.
Period specific survival estimates are necessary to understand the ecological basis of this decline, but such data are not available for scaup. I report winter survival of scaup in central Florida, where 62% of the Atlantic Flyway population overwinters. Estimates of survival were not confounded by hunting mortality as data were collected posthunting in 2002. The Kaplan-Meier survival estimate for females was 0.95 ± 0.04 (SE), and for males was 0.90 ± 0.09 (SE) for the period of 11 January — 14 March. These estimates were not different (P = 0.64). Pooled survival was 0.93 ± 0.04 (SE). Temporary emigration was exhibited by 24% of the birds, but their absence was short (24-hr intervals) and had little effect on precision of survival estimates as censored birds were added back into the model. These results were consistent with the dietary freshwater hypothesis, which states that scaup most likely flew inland to acquire freshwater after foraging in saline systems. These findings also suggested that posthunting season winter survival was high. Hence, sources of mortality, other than hunting, likely occur during other stages of their annual cycle. Estimating survival during other stages of their annual cycle is necessary to identify the underlying reasons for continental population declines.
Florida is the most important wintering site for lesser scaup in the Atlantic Flyway, yet understanding how habitats are used by wintering scaup and why in Florida is inadequate. Continental population declines suggest that addressing this information void is important to evaluate habitat conditions and how habitat conditions may relate to the health and fitness of scaup. I used radio telemetry to test habitat use within the constraints of the Functional Unit System theory (FUS). The FUS theory suggests that wintering waterfowl will use separate units or regions for comfort and feeding activities. I further tested for differences in movements and home range between males and females to determine if they used different habitats. Lastly, I used data on movements and home range to coarsely evaluate habitat quality and potential effects of human disturbance. I found that sexes traveled similar distances in mid and late winter between diurnal and nocturnal sites (P > 0.05). Scaup increased distance between diurnal and nocturnal sites by 5% in late winter (P = 0.03) from 2.4 km to 2.6 km. Male and female fixed kernel home ranges (P > 0.05) and core use areas (P > 0.05) did not differ. Mean pooled fixed kernel 95% home range and 50% core use areas were 15.1 ± 4.2 km2 and 2.7 ± 1.1 km2, representing 3% and 0.5% of available habitats. Results suggested that scaup did use different habitats for comfort and feeding activities. Sexes appeared to use habitats similarly and short distances traveled between diurnal and nocturnal sites suggested that habitat conditions were similar across the impounded wetlands and shallow portions of the Indian and Banana Rivers, and that disturbance was likely negligible. Scaup appeared to locate suitable habitats early in the winter period (January) and remain there throughout much of the season. Accordingly, I believe that the IRL system provides adequate winter habitat, at least around Merritt Island National Wildlife Refuge.
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Keywords
, Aythya affinis, winter fitness, body condition, foraging ecology, winter ecology
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Degree
MS
Discipline
Fisheries and Wildlife Sciences
