A Comparative Study of Foraging Behavior and Disturbance Regimes in Urban Versus Agricultural Habitats used by Cackling Geese Wintering in the Willamette Valley, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/f4752m138

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  • The Cackling goose (Branta hutchinsii minima) population has increased from a low of 20,000 in 1984 to the current population of 220,000-300,000 (Stehn 2012, Sanders 2013). As the Cackling goose population began to recover in the late 1990s, the majority of the population relocated from wintering in California to the Willamette Valley, Oregon (Pacific Flyway Council 1999, Mini 2012). Cackling geese in Oregon now commonly use exurban, suburban, and urban areas (Mini 2012). The reasons for Cackling goose use of urban areas are still unclear as they did not commonly use this habitat type on their traditional wintering areas in California or initially upon showing up in Oregon. Given what we know about habitat selection in geese and the Willamette Valley system, we tested three different hypotheses that seem to have the greatest utility for explaining the recent use of urban habitats in Oregon’s Willamette Valley: 1) Foraging opportunity in urban habitats is higher due to lower perceived, or actual, predation risks, 2) Foraging efficiency in urban habitats is higher in urban landscapes due to a difference in forage characteristics between landscape types, and 3) Quality of forage in urban habitats is higher due to a difference in nutritional content and regrowth rate. My field work centered on collecting data to test predictions deduced from my hypotheses. From November 2013-April 2014 and November 2014-April 2015, I conducted a total of 278 hour-long disturbance surveys and 238 behavioral time- activity budget scans in 109 different sites in the Willamette Valley, Oregon within urban areas in Portland, Eugene, and Salem and agricultural areas in the Willamette Valley National Wildlife Refuge Complex and nearby private fields. Consistent with the Safe Habitat Hypothesis, predator-related disturbances in urban landscapes were significantly lower than in agricultural landscapes: only one avian predator-related disturbance was observed in urban landscapes throughout the entire study. Geese spent more time feeding (69.0 ± 1.9% vs. 55.0 ± 2.1%), less time flying (3.0 ± 1.1% vs. 6.6 ± 1.3%) and less time vigilant (2.1 ± 0.2% vs. 5.6 ± 0.5%) in urban landscapes than in agricultural landscapes. The frequency of the eight disturbance types differed between landscape types (n = 988, χ² = 308, df = 8, P < 0.001). Vigilance (70% vs. 56%, χ² = 22.9, df = 1, P < 0.001) occurred more often in agricultural landscapes and walking away responses (19% vs. 3%, χ² = 64.49, df = 1, P < 0.001) occurred more often in urban landscapes. I measured forage biomass, daily regrowth rate, and nutritional content from 58 plots in December of 2014, and 60 plots in December of 2015 at a total of 6 fields in urban landscapes and 6 fields in agricultural landscapes. Mean grass biomass did not vary significantly between urban (32.0 ± 7.5 g m⁻²) and agricultural landscapes (22.0 ± 4.2 g m⁻², n = 12, P = 0.4), and average daily regrowth rate did not vary significantly between urban (0.08 ± 0.01 cm/d) and agricultural landscapes (0.12 ± 0.02 cm/d, n = 12, P = 0.09). Average grass height in agricultural landscapes was 7.1 ± 0.7 cm and average height in urban landscapes was 3.8 ± 0.3 cm. Percent crude protein did not vary significantly between urban (18.4 ± 0.97%) and agricultural landscapes (17.0 ± 1.3%, n = 12, P = 0.64). Percent ADF did not vary significantly between urban (35.3 ± 1.8%) and agricultural landscapes (36.7 ± 2.4%, n = 12, P = 0.84). My results are consistent with the Safe-habitat Hypothesis which states Cacklers may be now using urban landscapes partially in response to an increasing Bald Eagle population. Ultimately, if nutritional quality of forage is the same between landscapes types, but Cacklers are disturbed more often by predators in agricultural landscapes, Cacklers in urban landscapes may have a higher net energy gain than Cacklers foraging on refuges; therefore, use of urban areas by Cacklers might increase. Coordinated management plans with private landowners, public school districts, and Parks and Recreation departments in the Willamette Valley that maximize preferred foraging conditions on refuges and minimize preferred foraging conditions in urban areas may attract more geese to protected areas in agricultural landscapes.
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