Bioenergetics and habitat use of green-winged teal across the annual period
Date
2025
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Green-winged Teal (Anas crecca; hereafter teal) have not been extensively studied in the Atlantic Flyway, presumably because of their stable population status across their range. However, many waterfowl managers in the south Atlantic Flyway manage for teal, as they are a numerous waterfowl species across the Carolinas. Therefore, a bioenergetic carrying capacity model could identify which habitat types provide the most calories for teal, as well as calculate the number of teal that the region could support. I created a bioenergetic carrying capacity daily ration model for teal in the coastal plain of North Carolina and South Carolina. I identified preferred teal foods from 16 published studies as well as identified food items found in 73 gizzards collected from hunter-harvested teal in North Carolina. One hundred and twenty three core samples and 312 sweep-net samples were collected across seven distinct habitat types including high marsh, low marsh/mudflat, freshwater, moist-soil impoundment, millet impoundment, rice impoundment, and corn impoundment to calculate food availability on the landscape for teal. I collected samples over early, middle, and late winter to assess caloric depletion through time. Millet, corn, and rice impoundments had high caloric availability to teal in the form of seeds and crops. High marsh and low marsh/mudflat had high animal caloric availability to teal, while freshwater and moist- soil impoundments had comparatively low caloric availability. Over winter, available energy in rice declined, but increased in low marsh/mudflat and high marsh, making it necessary to calculate carrying capacity separately for each time period. To identify energy expenditure, I conducted 429 instantaneous scan samples and used 17,358 accelerometry bursts from teal marked with GPS transmitters to identify behavioral proportions, combined with real-time weather data to account for thermoregulation. Based on instantaneous scan sampling, teal were feeding/swimming 72.6%, resting 8.32%, and flying 19.08% of the time. Based on accelerometry data averaged from teal at both sites, teal were feeding/swimming 51.93%, resting 46.83%, and flying 1.24%. I calculated daily energy expenditure as 140.75 ± 24.7 kcal/bird/day for instantaneous scan sampling, 143.3 ± 1.8 kcal/bird/day for accelerometry data from individuals marked at Mackay Island NWR, and 140.5 ± 1.8 kcal/bird/day for accelerometry data from individuals marked at Orton. Lastly, I used the National Wetlands Inventory spatial data layer, as well as two spatial data layers for saltmarsh and one spatial data layer for impoundments, to calculate available habitat for teal, and extrapolated energy at a landscape level. Using these predicted energy values combined with energy expenditure estimates, I estimated duck-energy days (DED) in our study area at various scales over the wintering period. Orton could support 2,171 teal during the early period, 1,655 teal during the middle period, and 3,361 teal during the late period. My study area of the Carolina Flatwoods region of coastal North and South Carolina can support 976,705 teal during the early period, 679,106 teal during the middle period, and 1,774,812 teal during the late period, suggesting that teal may not be food limited during the wintering period, although providing better quality habitat with higher calories in the form of impoundments may allow teal to be in better body condition going into spring migration in the form of carry-over effects, and may otherwise improve spring migration survival and breeding success. ☐ There is also a noticeable lack of previous telemetry studies on Green-winged teal across their range due to their small body size. However, recent technological advances have allowed for 10-gram GPS/GSM units to collect high-frequency GPS data on teal across their life cycle. I attached 74 transmitters to teal in North Carolina in 2021-2023 and collected GPS points across the nonbreeding, migration and breeding periods. I compared home range size and habitat selection during the nonbreeding period across our two capture sites and compared day/night habitat use and other metrics. The average 95% autocorrelated kernel-density estimate home range size of wintering teal was 2,638.9 ha for teal marked at Mackay Island NWR (N = 10) and 8,062 ha for teal marked at Orton (N = 23). Teal at Mackay Island NWR showed habitat preference for saltmarsh and moist-soil impoundments, while teal at Orton showed habitat preference for saltmarsh and corn impoundments. I also identified winter philopatry in teal returning to the capture area through banding returns or transmitter GPS data. Return rate for teal within 100,000 km2 of the original marking location is 42.0% for teal marked with transmitters (N = 7) and 47.8% for teal marked with bands only (N = 23). ☐ In addition, I identified migration stopover information for marked teal, including migration duration, migration stopover duration, number of migration stopovers, percent habitat used, and percent public land used, and compared spring and fall migrations. I identified spring and fall migration information for 48 and 12 teal, respectively. Teal had stopover sites in 19 U.S. states and five Canadian provinces. The average migration duration was 74.2 days (range: 40-133 days) and 51.0 days (range: 9- 127 days), respectively. The average number of migration stopovers was 11.2 stopovers in spring and 11.6 stopovers in fall. The duration of the stopover predicted the distance between the next stopover site. Teal that committed to long refuel stopovers (> 48 h) flew further to the next stopover site than teal that stayed at a short rest (< 6 h) or a short refuel (6-48 h) stopover site. Teal in the spring used habitats such as temporarily- flooded agricultural fields, hereafter sheetwater wetlands, as well as freshwater marshes, saltmarshes, larger waterbodies, and some impoundments, and habitat use differed among stopover duration categories. In the fall, teal used all habitat types except for sheetwater wetlands, and habitat use differed among stopover duration categories. Behavioral proportions across stopover categories were significantly different for both spring and fall migration. Thirty two point nine percent of spring migration stopovers and 56.8% of fall migration stopovers within the U.S. had at least one GPS point on public land. I identified frequent short-rest stopovers which lasted <6 hours during the night, often in temporarily-flooded sheetwater wetlands, indicating that this habitat may be important for migrating teal to rest and feed as they move across the landscape.
Description
Keywords
Bioenergetics, Green-winged teal, Teal, Telemetry, Waterfowl, Wetland management