Temporal pattern of wetland use by Eurasian and American Wigeon in the Northern Sacramento Valley, California.
Christmas Bird Count data for the Pacific Flyway show an upward trend in EUWI numbers beginning in the 1960s and continuing today (National Audubon Society 2009), although reasons for this trend are unclear (Edgell 1984). Waterfowl Harvest and Parts-Survey data for the Pacific Flyway and Alaska show a similar upward trend for the period of 2005-2008 (USFWS 2014a), with the highest EUWI harvest recorded in 2006 (1712 birds) and 2007 (2232 birds).
Although the ecology of EUWI has been studied extensively in the Old World (Lebret 1950; Donker 1959; Owen and Williams 1976; Rijnsdorp 1986; Jonsson and Gardarsson 2001), there have been no such studies in North America. As California wetlands provide winter habitat for between 4 and 5 million waterfowl, including 70 to 80% of Pacific Flyway wigeon (for example, 76% in 2008; USFWS 2014b), our study goal was to gain a better understanding of the migration chronology and temporal pattern of wetland use by EUWI in the Sacramento Valley, California. Additionally, as there appears to be a very close association between EUWI and AMWI on our study units, we collected comparative data for AMWI as an initial attempt to better understand the sympatric interactions of these 2 species throughout the fall and winter months.
Migratory waterfowl, including EUWI, have been implicated as potential long-distance vectors for avian influenza viruses such as highly pathogenic (HPAI) Subtype H5N1 (Winker and others 2007; Keawcharoen and others 2008; Kim and others 2009; Pearce and others 2009). However, whether HPAI H5N1 is actually maintained in wild duck populations is currently unresolved (Krauss and others, 2007; Kim and others 2009; Takekawa and others 2010). In laboratory studies, ducks that illustrate immunity to HPAI H5N1 are known to shed the virus from their respiratory and digestive tracts (Kim and others 2009). Keawcharoen and others (2008) found the rate of HPAI H5N1 excretion by experimentally infected EUWI to be low relative to several Palearctic pochards (Aythyini) and Mallards (Anas platyrhynchos); of the 6 duck species used in their study, EUWI were considered among the least likely to serve as a long- distance vector for this virus. Nevertheless, their results showed that although EUWI manifest no clinical signs of the disease, they are susceptible to this virus. Also, Keawcharoen and others (2008) isolated HPAI H5N1 virus from 4 of 8 EUWI pharyngeal samples, and they detected productive viral infections in 7 of 8 samples using RT-PCR assays. Considering these results as well as the apparent increase in the transcontinental migration of EUWI into the Pacific Flyway, we believe that attempts to gain a better understanding of the ecology and natural history of this Palearctic wigeon in the Sacramento Valley are warranted.
We conducted our study at the Llano Seco Unit of the North Central Valley Wildlife Management Area (Butte County, California) because of regular use of these wetlands by large numbers of wigeon, exceptional bird visibility, and site accessibility. Surveys were conducted over a 2-y period between fall 2006 and spring 2008 on 2 managed wetlands: Unit T12, a 36.4-ha seasonal wetland (39[degrees]36'20"N, 121[degrees]54'38"W); and Unit T13.3, a 10.1-ha permanent (flooded year round) wetland (39[degrees]36'13"N, 121[degrees]54'57"W). During both field seasons, Unit T12 was flooded from 1 November through 31 March. This seasonal wetland was managed as a moist-soil impoundment dominated by prickle grasses (Crypsis spp.), resulting in a >95% open-water habitat. This wetland also contains numerous loafing islands and several small clumps of Common Tule (Schoenoplectus acutus). Habitat closure was much greater in T13.3, with extensive stands of cattail (Typha spp.) and Common Tule covering over 60% of the unit.
During our initial field season, wigeon surveys were conducted from 12 October 2006 through 2 April 2007. In order to better capture the onset of fall migration during our 2nd field season, we conducted surveys from 29 August 2007 through 2 April 2008.
We conducted 5-6 surveys biweekly (every 2 wk), including 1-2 surveys during the following time periods: Sunrise-2-h post-sunrise; 4-6-h post-sunrise; and 2-h pre-sunset-sunset. All surveys were conducted from viewing platforms adjacent to each unit; binoculars and a 20-60X spotting scope were used during all surveys.
As wigeon migration into the Central Valley begins well before the 1 November flood-up date for the seasonal wetland (Bellrose 1980), all data from late August through 10 November were collected from Unit T13.3. Due to differences in the management strategies of the 2 units, as well as significant temporal differences in our 2 data sets, we quantified migration chronology and wetland use separately by unit.
Due to extremely low numbers of EUWI relative to AMWI, we employed different survey techniques for monitoring each species through time. AMWI were sampled using a single spotting scope scan through each wetland unit. Our scope was positioned at a set point on each viewing platform, and set at the same magnification (40x at T12 and 60x at T13.3). Scope magnification differed due to the distance between the 2 viewing platforms and wetlands. Scans covered a pre-determined transect that we considered to be both representative of each unit and beyond the point where wigeon were noticeably affected by our presence. During each scan, the total number of AMWI was recorded. AMWI sex and male age data, to be used in another phase of our research, were also collected at both wetland units, beginning with the 12-25 November sampling period. This biweekly period was chosen in order to avoid potential identification errors associated with eclipse and immature plumages (Wishart 1985). Males were aged, whenever possible, using wing plumage characters (Carney 1992).
EUWI were censused through complete searches of each unit. Unit searches were completed in [less than or equal to] 45 min. During censuses, the total number of EUWI, numbers of each sex, and male age were quantified; male age was determined, whenever possible, using wing plumage characters (Carney 1992). Female EUWI were identified, whenever possible, using plumage characters of the under wing (Bellrose 1980; Carney 1992) and outer body (Madge and Burn 1988; Sibley 2000). Although we fully acknowledge the difficulty in making long-distance identifications of female EUWI under certain light conditions, we are confident in our ability to do so.
We quantified numbers of each wigeon species biweekly, and by management unit, as the average number of wigeon per transect (AMWI) or per unit census (EUWI). As we made no attempt to test for differences among years, we averaged values across years, beginning on 12 October (Unit T13.3) and 12 November (Unit T12), because biweekly sampling-period dates and the number of surveys per sampling period were the same (Baar and others 2008). Due to large differences in the numbers of AMWI and EUWI, and in order to better illustrate their chronological patterns of migration, we chose to illustrate our data cumulatively as the sum of all present and past biweekly means as a percentage of the cumulative total for each species.
We tested for differences in the temporal pattern of wetland use between species and by management unit using Kolmogorov-Smirnov one-sample goodness-of-fit tests, with the null hypothesis being that the biweekly pattern of wetland use for AMWI and EUWI are the same for each management unit. For our analysis, AMWI transect data serve as our single "sample", and EUWI data represent complete unit censuses.
Initial use of T13.3 by AMWI was documented during the 29 August-12 September 2007 biweekly period, whereas our earliest record for EUWI occurred during the 12-25 October 2006 period (Appendix). Mean biweekly numbers of AMWI per transect ranged temporally from 15.8 individuals during the 29 August-12 September 2007 biweekly period to 1.6 individuals per transect during the period of 7-15 March. Mean AMWI numbers at T13.3 peaked at 106.9 individual per transect in late December. Mean number of EUWI per census ranged temporally from 0.15 individuals per census during the 1225 October sampling period to 0.2 individuals per census during the 5-20 January sampling period. Mean EUWI numbers at T13.3 peaked at 1.1 individuals per census in late November. There was a statistically significant difference in the temporal pattern of usage of this unit by EUWI and AMWI (K-S Test Statistic = 0.1140, Critical Value = 0.0753, P < .01). This statistical difference was due due in part to a disproportionate increase in the number of AMWI using this unit during the 12-25 November period (Fig. 1).
Initial use of T12 by AMWI and EUWI occurred immediately following flood-up in early November (Appendix). Biweekly mean values for AMWI ranged temporally from 74.3 individuals during the 12-25 November biweekly sampling period to 10.9 individuals during the period of 27 March-2 April. Mean AMWI numbers at T12 peaked at 129.45 individuals per transect during the 5-20 January sampling period. Mean number of EUWI per census ranged temporally from 2.9 individuals during the 12-25 November biweekly period to 0.2 individuals during the period of 27 March-2 April. Biweekly mean values for EUWI at T12 peaked at 11.75 individuals per census during the period of 5-20 January. The maximum number of EUWI observed on a single wetland unit (n =24) occurred at T12 on 11 January 2008. These 24 EUWI included 20 unpaired ASY (after-second-year) adult males, 1 paired ASY adult male, 2 unpaired SY (second-year) immature males, and 1 paired female of unknown age. There was a statistically significant difference in the temporal pattern of usage of this unit by EUWI and AMWI (K-S Test Statistic = 0.08, Critical Value = 0.056, P < .01). This statistical difference was due in part to a disproportionate increase in the number of EUWI during the 5-20 January sampling period (Fig. 2).
Our AMWI data are consistent with published chronological data for California's Central Valley (Bellrose 1980) and local waterfowl survey data collected by the Sacramento National Wildlife Refuge Complex (USFWS 2014b). Although no comparative EUWI data exist for the Pacific Flyway, our EUWI arrival dates in early October are consistent with fall records of migrating EUWI on both the Aleutian and Bering Sea (for example, St. Lawrence) islands (Gibson 1981; Lehman 2005).
Data from the permanent wetland unit (T13.3) show decreased usage by wigeon beginning in early January. Periodic shifts in the use of specific habitats by waterfowl throughout the fall and winter are common, and can be explained, in large part, by changes in habitat and food resource availability, harsh weather conditions, as well as the effects of predators (Fleskes and others 2005); for example, decreased usage of the seasonal wetland (T12) by AMWI and other ducks, during the period of 28 December-4 January, corresponded with moderate to high winds, rain, and fog during 5 of 11 field surveys. In addition, the regular presence of Bald Eagles (Haliaeetus leucocephalus) and Peregrine Falcons (Falco peregrinus) was responsible for the temporary movement of ducks off of our study units. We therefore suggest that differences in the temporal pattern of wetland use by AMWI and EUWI that we found in our study are likely due to one or more of these environmental factors. Differences may also be due, in part, to the effects of highly disparate AMWI and EUWI population sizes, and the possibility that the significance of the Kolmogorov-Smirnov test results were inflated due to temporal autocorrelation.
Key words: American Wigeon, Anas americana, Anas penelope, Eurasian Wigeon, Sacramento Valley
Acknowledgments.--We would like to thank the staff at the Sacramento National Wildlife Refuge Complex for granting us access to our study units at the Rancho Llano Seco Unit of the North Sacramento Valley WMA. We would also like to thank Drs. N Carter and N Schwertman, Department of Mathematics and Statistics, California State University, Chico, California for their assistance with our statistical analysis, as well as S Kirn for her review of this manuscript.
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Department of Biological Sciences, California State University, Chico, CA 95929 USA. (RJB, SRF), email@example.com; 3561 San Juan Road, Sacramento, CA 95833 USA. (JBA); California Department of Fish and Wildlife, PO Box 36, Butte City, CA 95920 USA. (LEC); PO Box 335, Richvale, CA 95974 USA. (MLM); 490 E 3rd Street, Chico, CA 95926 USA. (JTR). Submitted 29 October 2014, accepted 17 May 2015. Corresponding Editor: Joan Hagar.
APPENDIX. Mean biweekly values for American (AMWI) and Eurasian Wigeon (EUWI) at the Llano Seco Unit of the North Central Valley Wildlife Management Area, Butte County, California, 2006-2008. Biweekly Sampling Period 29 Aug - 13 Sept - 28 Sept - 12 Oct - Unit 12 Sept 26 Sept 10 Oct 25 Oct 13.3 EUWI 0 0 0 0.15 AMWI 15.8 30 5.3 12.15 12 EUWI 0 0 0 0 AMWI 0 0 0 0 28 Oct - 12 Nov - 29 Nov - 12 Dec - Unit 10 Nov 25 Nov 9 Dec 22 Dec 13.3 EUWI 0 1.1 0.85 0.65 AMWI 6.65 83.5 102.55 106.9 12 EUWI 0 2.9 6.5 8.85 AMWI 0 74.3 103.8 126.85 28 Dec - 5 Jan - 24 Jan - 7 Feb - Unit 4 Jan 20 Jan 4 Feb 17 Feb 13.3 EUWI 0.5 0.2 0 0 AMWI 47.2 19.7 1.25 1.4 12 EUWI 8.2 11.75 6.1 5.25 AMWI 113.85 129.45 111.2 76.4 21 Feb - 7 Mar- 27 Mar - Unit 4 Mar 15 Mar 2 Apr 13.3 EUWI 0 0 0 AMWI 0.3 1.6 0 12 EUWI 0.8 0.8 0.2 AMWI 45.9 33 10.9
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|Author:||Bogiatto, Raymond J.; Ashe, Jeremey B.; Cockrell, Laura E.; Foster, Stephanie R.; Mattson, Michelle|
|Publication:||Northwestern Naturalist: A Journal of Vertebrate Biology|
|Date:||Dec 22, 2015|
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