Comparative spring-staging ecology of sympatric arctic-nesting geese in south-central Nebraska.
Migrating birds must continually search for safe and suitable staging habitats containing abundant resources while traveling between wintering and breeding grounds. Replenishing energy expended during long distance flight and nutrient storage necessary for breeding represent extensive energetic challenges migratory birds must overcome (Ankney and MacInnes, 1978). Migration also can be a time when species that use distinct winter or breeding areas congregate at stopover areas within a common migration corridor. Species exploiting similar and limited resources during migration may experience intensified interspecific interactions because of increased energetic requirements and high densities of potential competitors (Moore and Yong, 1991).
Understanding niche overlap during various portions of the annual cycle of avian species, including migration, provides insight into their distributions and coexistence (DuBowy, 1988; Wiens, 1989). Isolation or partitioning of common and limited resources may reduce competitive pressures among sympatric species (MacArthur, 1958). Resource partitioning facilitates coexistence when species reflect a high degree of overlap (Schoener, 1974). Potential partitioning mechanisms include dietary, spatial, temporal, and behavioral variation. For species that exist in sympatry during only a portion of their annual cycle, such as migration, resource partitioning may be less pronounced because of reduced time to adequately develop such relationships.
Numerous waterbirds migrating across the Great Plains of North America converge on south-central Nebraska, using wetlands in the Rainwater Basin and along the central Platte River (U.S. Fish and Wildlife Service, 1981; Pederson et al., 1989; Gersib et al., 1992). Midcontinent greater white-fronted geese (Anser albifrons frontalis, hereafter white-fronted geese) make extensive use of this region during spring and historically were able to acquire substantial lipid stores while staging (Krapu et al., 1995). More recently, millions of midcontinent lesser snow geese (Chen caerulescens caerulescens), which formerly used the Missouri River Basin for spring staging, expanded their distribution west into the Rainwater Basin (Davis et al., 1989; Vrtiska and Sullivan, 2009). Previous studies of each species in allopatry suggests use of similar resources during spring [e.g., roost wetlands and waste corn (Zea mays); Davis et al., 1989; Alisauskas et al., 1992; Krapu et al., 1995; Gawlik and Slack, 1996]; thus, niche overlap may be extensive when they use the same staging area. If resources are limiting in south-central Nebraska, resource partitioning may be necessary to allow coexistence of these two species of arctic-nesting geese. Past research on other species of geese suggest that species adjust use of resources in the presence of closely related species, especially when one species occurs in greater numbers (Madsen, 1985; Carriere et al., 1999; Kristiansen and Jarrett, 2002; Fox et al., 2007, 2009). Our objectives were to quantify and compare various dimensions of spring-migration niches of sympatric white-fronted geese and snow geese during spring in south-central Nebraska. We used these comparisons to determine level of overlap and identify mechanisms of resource partitioning including dietary, habitat use, movement patterns among commonly used habitats, and habitat-specific time budgets.
The Rainwater Basin includes 1.7 million ha in south-central Nebraska south of the Platte River between Lexington and Columbus, Nebraska. The region was characterized historically as a mixed-grass prairie with wetlands interspersed throughout (Gersib et al., 1992). Spring-staging geese typically used semipermanent basins as nocturnal and diurnal roost sites and agricultural fields for feeding (Pederson et al., 1989; Krapu et al., 1995). Dominant land use during the study period was annual crop agriculture (74% of landscape during late 1990s), with corn and soybeans (Glycine max) representing 75% and 18% of cultivated lands, respectively (U.S. Department of Agriculture, 2009). Wheat (Triticum aestivum) and grain sorghum (Sorghum bicolor) each accounted for 3% of cultivated lands during the late 1990s. The Rainwater Basin is bordered on the north by the central Platte River Valley (CPRV), which supported large numbers of sandhill cranes (Grus canadensis) and waterfowl during spring migration and has been described by U.S. Fish and Wildlife Service (1981). Land use of the CPRV was dominated by row crops (Pearse et al., 2010). Spring hunting of snow geese was open in portions of the Rainwater Basin in both years of study between 11 Feb.-10 Mar. Initial arrivals of white-fronted geese varied annually but generally occurred during early Feb. and peak numbers occurred in early Mar. during the late 1990s (Krapu et al., 2005). Snow geese generally arrived in the study area during early Feb., with peak numbers occurring in mid-Mar. (Vrtiska and Sullivan, 2009).
We collected juvenile and adult white-fronted geese and adult snow geese by shooting individuals from passing flocks with shotguns as they returned to randomly selected wetland roosts located throughout the Rainwater Basin and CPRV on public and private lands, 10 Feb.-2 Apr. 1998-1999. We did not use decoys or bait during collections to avoid potential biases (Sheeley and Smith, 1989). Ninety-six percent of snow geese and 90% of white-fronted geese were collected in the evening (>16:00). We immediately removed the esophagus from birds and stored them and contents in an 80% ethanol solution for diet analysis. We identified and categorized plant seeds and foliage to genus. After sorting ingested materials, we dried items to a constant mass at 55 C and weighed each to the nearest 0.0001 g. Collections were conducted under Nebraska Game and Parks Commission scientific collecting permits 6, 7, 49, and 50, in concurrence with U.S. Fish and Wildlife Service personnel and with methods approved by the Northern Prairie Wildlife Research Center Animal Care and Use Committee.
We monitored white-fronted and snow geese during 8 Feb.-4 Apr. 1998-1999 to ascertain habitat use, local movements, and time budgets. Geese foraged twice daily, leaving roosting wetlands for croplands near sunrise and again during mid-afternoon (Owen, 1972; Frederick and Klaas, 1982; Gauthier et al., 1988). To coincide with this general pattern, we conducted observation sessions during morning (sunrise-1300; [bar.x] = 5.4 h) and afternoon (1300-sunset; [bar.x] = 5.3 h). Each session began by field technicians locating a predetermined roosting wetland and following a random (determined by dice roll) group (i.e., aggregation of geese [greater than or equal to] 25 birds containing the species of interest for that observation period) to their initial destination (Alisauskas and Ankney, 1992). Technicians recorded location, habitat used (i.e., wetland, cornfield, soybean field, or other), enumerated focal flock size at each location, and recorded species and numbers of other waterfowl present. Technicians followed flocks when over half of birds moved to a new location during the observation session. We calculated Euclidean distance between successive locations as a measure of distance moved and elapsed time between the flock leaving one location and arriving at another to calculate time in flight.
When flocks remained at a location for [greater than or equal to] 5 min, observers recorded behaviors of focal birds (Altmann, 1974). A focal bird was selected randomly by moving a spotting scope across the flock of geese and selecting for observation an individual nearest the center of the field of view. Observations were made from a vehicle using spotting scopes with 20 x magnification and behaviors recorded every 10 s for up to 30 min or until the bird left the area with the flock. If a focal bird became obscured and lost within the flock, a new bird was chosen using the same procedure as described above and activity monitoring resumed. Activities recorded included foraging or drinking, resting, walking or swimming, alert, flying, and other (e.g., agonistic, courtship; Paulus, 1988). For agonistic interactions, technicians reported which bird initiated the interaction (focal bird or otherwise), the species involved, and the individual that appeared to dominate the interaction based on displays and postures (Boyd, 1953; Raveling, 1970). When reporting other species, we used the classification system that existed at the time (American Ornithologists' Union, 1998), which listed Canada geese (Branta canadensis) as a single species. Smaller subspecies have since been designated as a separate species, cackling geese (Branta hutchinsii; Banks et al., 2004). References to Canada geese hereafter include both species.
Food items consumed by geese were expressed as percentage of dry mass averaged across birds (i.e., aggregate percent dry mass) by species (Swanson et al., 1974). We determined habitat use (i.e., % of time at each habitat) of flocks under observation for each monitoring session and calculated average habitat use by time of day and species. We summed time spent conducting specific behaviors by focal birds within a flock and calculated time budgets (i.e., % of each behavior) by dividing by total time the flock was under observation. Time budgets were then summarized by species and major habitat categories (i.e., wetlands and croplands). Finally, we summed number of local movements, distance flown (km), and time in flight (min) for each observation session; we calculated mean and total movement parameters by species and time of day. We compared proportional diet, habitat use, and time budgets between species of geese by calculating indices of overlap; a value of 0.0 indicated complete separation and 1.0 reflected complete overlap in diet, habitat use, or behavior ([R.sub.0]; Horn, 1966; Krebs, 1999:471).
We collected 190 white-fronted geese and 203 snow geese during springs 1998-1999 in south-central Nebraska. Forty-eight percent of snow geese were females and 52% of white-fronted geese were females. Ingesta-free body mass of white-fronted geese averaged 2.27 kg (SE = 0.02, 95% CI: 2.24-2.31) and was 0.21 kg (SE = 0.02, 95% CI: 0.16-0.26) greater than average mass of snow geese ([bar.x] = 2.06, SE = 0.02, 95% CI: 2.03--2.09). Corn dominated diets of white-fronted and snow geese, representing 97-98% of aggregate dry mass (Table 1). Plant foliage represented 1-2% of diet dry mass. Diet overlap was nearly complete ([R.sub.0] = 0.994).
We quantified habitat use for 53 groups of geese in the morning and 43 groups in the afternoon during 514 h of observation. Flock size averaged 1285 (SD = 2787, minimum = 25, maximum = 20,000) for white-fronted geese and 50,389 (SD = 94,366, minimum = 50, maximum = 750,000) for snow geese. Both species used cornfields most during the morning (54-55% of observation time) and spent the greatest percentage of time on wetlands during the afternoon (51-65%; Table 2). Cornfields used by goose flocks reflected various post harvest practices in the region, including disked (42%), standing stubble (37%), grazed by cattle (11%), shredded (9%), or other post harvest manipulations (1%). Soybean fields were used infrequently by both species (0-10%). We found a high degree of overlap in habitat use during the morning ([R.sub.0] = 0.984) and afternoon ([R.sub.0] = 0.977).
White-fronted geese occupied locations without other species present only 4% of the time in croplands (n = 77 groups) and never in wetlands (n = 49 groups). White-fronted geese occurred with snow geese in croplands during 38% of observations and 31% of observations on wetlands. Snow geese were observed without other species present during 8% of observations in croplands (n = 84 groups) and never on wetlands (n = 50 groups). Snow goose observations with white-fronted geese present occurred 87% and 92% of the time in croplands and wetlands, respectively. When white-fronted geese and snow geese occurred together in croplands, we observed 57 times (SE = 11) more snow geese compared to white-fronted geese on average; white-fronted geese outnumbered snow geese in mixed flocks on 13% of occasions (n = 131 groups). While roosting on wetlands, we observed 28 times (SE = 9) more snow geese than white-fronted geese on average; white-fronted geese outnumbered snow geese during 21% of observations (n = 84 groups).
White-fronted geese made an average of 4.1 (95% CI: 3.3-4.9) moves and snow geese made 3.8 (95% CI: 3.0-4.6) moves during the morning. In the afternoon, white-fronted geese made 2.9 (95% CI: 1.7-4.1) moves on average, compared to 3.4 (95% CI: 2.6-4.2) moves for snow geese. Spring-staging geese generally flew greater distances and spent more time flying in the morning than afternoon period (Fig. 1). White-fronted geese spent an average of 102 min in flight each day, whereas snow geese flew for an average of 139 min each day. Total daily distance traveled averaged 19.9 km for white-fronted geese and 28.8 km for snow geese (Fig. 1). Geese left wetland roost sites in the morning for croplands near sunrise (white-fronted geese: [bar.x] = 5 min after sunrise, SE = 3, n = 26 groups; snow geese: [bar.x] = 5 min after sunrise, SE = 4, n = 19 groups). Both species left croplands for wetland roosts at the same time (white-fronted geese: [bar.x] = 3.6 h after sunrise, SE = 0.2, n = 16 groups; snow geese: [bar.x] = 3.6 h after sunrise, SE = 0.3, n = 17 groups). White-fronted geese left afternoon wetland roost sites for croplands 1.7 h (SE = 0.3, n = 11 groups) before sunset; snow geese left afternoon roost sites for croplands 1.9 h (SE = 0.3, n = 16 groups) before sunset.
We recorded behaviors for 244 h from 1427 individual geese representing 79 flocks using wetlands and 88 flocks using croplands. We monitored an average of 8.4 focal birds per flock (range 1-28) for an average of 10 min per focal bird. Geese most commonly rested while using wetlands (63-70% of observed behaviors) and commonly foraged when using croplands (39-44%; Table 3). Time budgets of white-fronted geese and snow geese overlapped greatly when observed on a wetland ([R.sub.0] = 0.991) and in croplands ([R.sub.0] = 0.970). We recorded 111 interspecific agonistic interactions while observing staging geese (67% in wetlands and 33% in croplands). Thirty-two interactions (29%) occurred between white-fronted and snow geese. White-fronted geese initiated 19 (59%) interactions and dominated 16 (3 undetermined outcomes). Snow geese initiated 13 interactions and dominated all initiated except one undetermined interaction. Interactions with Canada geese were more numerous for both species. We observed 52 interactions between white-fronted geese and Canada geese; white-fronted geese initiated 67% of interactions and dominated 83% with the balance undetermined. Of interactions initiated by Canada geese, white-fronted geese were dominated during 88% of occurrences, but during one interaction, a white-fronted goose dominated its aggressor. We observed 20 interactions between snow geese and Canada geese. Snow geese initiated 70% of interactions and dominated all. Of six interactions initiated by Canada geese, snow geese were submissive three times, dominated their aggressor once and had two undetermined interactions.
Before agriculture dominated landscapes used by arctic-nesting geese during winter and migration, geese consumed foods found in coastal and freshwater wetlands (Prevett et al., 1985; Abraham et al., 2005). During the late 1970s, white-fronted geese staging in the Rainwater Basin consumed primarily agricultural foods including waste corn and shoots of winter wheat (Krapu et al., 1995). Spring diets of snow geese varied by location and year during past studies with greater variety at southerly staging areas and dominated by corn at more northerly sites like Nebraska (Alisauskas and Ankney, 1999). Corn was the dominant food item consumed by geese staging in Nebraska during 1998-1999, with no other specific food item representing >1% of diet. Patterns of habitat use corresponded with diet analyses; geese used cornfields most frequently compared with other upland habitats. Use of cornfields by white-fronted geese was similar in the 1970s and 1990s (Krapu et al., 1995), and snow geese used cornfields in a similar proportion to other spring-staging areas in the Great Plains (Alisauskas and Ankney, 1992). Use of soybean fields and other habitats including pastures and wheat fields occurred infrequently. Use of soybean fields also was rare for both species in past studies, but availability of soybean fields had increased greatly since the 1970s and 1980s (Krapu et al., 2004; Pearse et al., 9010). Use of wheat fields was more frequent in the past than we observed; wheat was more common historically compared to the 1990s (Taylor et al., 1978). Although we did not conduct an analysis of selection for specific terrestrial habitats, croplands other than cornfields and grasslands existed in greater availability than were used by both species of geese, suggesting selection for cornfields over other potential upland sites (Krapu et al., 2005).
Foraging was a common behavior for both species in croplands, and geese foraged for a greater percentage of time than had been reported previously. White-fronted geese exhibited foraging behavior twice as much compared to spring staging behavior in Nebraska during the 1970s (Krapu et al., 1995), and snow geese foraged 30% more compared to estimates from southeastern South Dakota during the early 1980s (Alisauskas and Ankney, 1992). These behavioral differences suggest birds foraged more intensively in fields during the late 1990s compared to past decades to acquire food potentially due to reduced food availability. Field studies support this contention; spring availability of waste corn was 2068% less in the late 1990s compared to the late 1970s along the Platte River in Nebraska (Pearse et al., 2010). Availability of waste corn may continue to decrease with greater improvements in harvest practices potentially creating situations where corn may become limiting in certain years (Krapu et al., 2004).
Geese roosted overnight and spent a large portion of mid-day on wetlands scattered throughout the region. Although we did not collect wetland-specific characteristics, geese generally used semi-permanent wetlands located on publically owned lands managed by the U.S. Fish and Wildlife Service (Waterfowl Production Areas) or the Nebraska Game and Parks Commission (Wildlife Management Areas; Webb et al., 2010). Resting was the main activity on wetlands, which conformed to general roosting behavior of geese during nonbreeding periods (Davis et al., 1989; Ely, 1999). We did not conduct nocturnal observations or collect geese in the morning; thus, we were unable to determine if geese consumed foods overnight at roosting wetlands (Glazener, 1946; Owen, 1972). Geese spent considerable time flying between wetlands and croplands or moving among upland sites each day. We do not have comparable data for white-fronted geese, but snow geese were in flight an average 1.5 h/d while staging in Iowa (Davis et al., 1989) and 2.3 h/d during our study. Greater flight time found in our study may relate to differences in distribution and availability of food between regions and time periods. Additionally, disturbance from spring hunting activities, which occurred during our study, may have increased flight time as have been observed in other populations of snow geese (Bechet et al., 2004).
POTENTIAL FOR COMPETITION
Extensive overlap occurred in various components of spring-staging niches for white-fronted and snow geese. Species displayed similar diets, patterns of habitat use, daily movement patterns, and behaviors. In contrast, studies of sympatric geese generally reported some level of differentiation creating opportunity for coexistence (Carriere et al., 1999; Fox and Bergersen, 2005; Fox et al., 2007, 2009; but see Giroux and Bergeron, 1996). Additionally, we found a high degree of spatial and temporal overlap (i.e., each species using habitats at the same time) and few instances where either species used an area in isolation. During observations of white-fronted and snow geese using the same area, snow geese outnumbered white-fronted geese in 84% of instances, with substantially greater numbers on average. Therefore, snow geese used a greater proportion of resources due to their overwhelming abundance. Competition may not be entirely exploitative; competitors can dominate individuals, causing them to use less suitable areas and restrict acquisition of resources, especially if one species is larger in body size than the other is, as was the case for these species (Schoener, 1983; Wiens, 1989). Based on agonistic interactions, we did not find evidence of unbalanced interference competition between white-fronted geese and snow geese during spring migration, although we did not record social status of birds which has been found to influence these outcomes of these behaviors (Gregoire and Ankney, 1990). Canada geese were included in more interactions than other species and generally were dominated by both white-fronted and snow geese, suggesting Canada geese may be periodically denied resources based on interactions with these species.
Use of common resources does not provide sufficient evidence for competition; resources must be limiting and competition should negatively influence individual fitness (Wiens, 1989). Commonly used resources that may be limiting include food (i.e., waste corn) and roost sites (i.e., wetlands). Intensive agriculture in this region has provided abundant food resources for some wildlife species (Krapu et al., 1995, 2004), yet changes in agricultural practices may have led to decreased overall food availability (Pearse et al., 2010). Wetland conditions were above average during springs 1998-1999 (M. Vrtiska, Nebraska Game and Parks Commission, personal communication), yet spring wetland conditions can be variable depending on winter and spring precipitation (Webb et al., 2010). Ultimately, we do not know the extent of potential resource limitation but suspect it may occur during certain years. With respect to negative effects on fitness, white-fronted geese did not gain lipids during the years of our study as they have in the past, which may have occurred, in part, because of the presence of large numbers of snow geese in the region (Pearse et al., 2011).
White-fronted geese and snow geese exploited the same resources in a similar manner likely because it was the most efficient means of fulfilling comparable needs while staging in south-central Nebraska. Snow geese were relatively new to the area in the late 1990s, staging in numbers vastly greater compared to white-fronted geese. Resource competition is conceivable in years with reduced availability of waste corn or dry years when numbers of roost wetlands are constrained. When both resources are available in excess of demand, presence of snow geese still may have deleterious effects for white-fronted geese due to status of snow geese as carriers of avian cholera (Samuel et al., 2005) and disturbance occurring in conjunction with hunting activities during spring to reduce the overabundant midcontinent snow geese population (U.S. Fish and Wildlife Service, 2007; Feret et al., 2003; Pearse et al., 2012). Incorporating interactions among species may be necessary for future landscape-scale conservation planning for migratory birds in this and similar regions (Gersib et al, 1992; Bishop and Vrtiska, 2008).
Acknowledgments.--We thank the Nebraska Game and Parks Commission, Rainwater Basin Joint Venture, U.S. Fish and Wildlife Service Rainwater Basin Wetland Management District and U.S. Geological Survey--Platte River Priority Ecosystems Study for funding and in-kind assistance. D. A. Brandt, R. Griebel, G. Knutsen, F. Sargeant, R. Waiters and R. Woodward provided assistance during fieldwork. T. G. LaGrange, E. B. Webb, W. C. Conway, and anonymous reviewers provided comments of earlier versions of this manuscript. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the United States Government.
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SUBMITTED 23 JUNE 2011 ACCEPTED 17 JULY 2012
AARON T. PEARSE, (1) GARY L. KRAPU AND ROBERT R. COX, JR. (2)
U.S. Geological Survey, Northern Prairie Wildlife Research Center, 8711 37th Street SE, Jamestown, North Dakota 58401
(1) Corresponding author: email: firstname.lastname@example.org
(2) Present address: P.O. Box 712, Ipswich, SD 57451
TABLE 1.--Percentage aggregate dry mass of esophageal contents from greater white-fronted geese (GWFG) and snow geese (SG) collected while returning to wetlands during spring migration in south- central Nebraska, 1998-1999 Food item GWFG (n = 190) SG (n = 203) Seeds 98 99 Corn 97 98 Sorghum < 1 1 Echinochloa < 1 < 1 Polygonum < 1 0 Foliage 2 1 Thlaspi < 1 < 1 Chenopodium < 1 < 1 Festuca < 1 0 Unknown 1 <1 TABLE 2.--Use of habitats (%) by groups of spring-staging greater white-fronted geese (GWFG) and snow geese (SG) during morning (sunrise-1300) and afternoon (1300-sunset) time periods in south- central Nebraska, 1998-1999 Morning GWFG (n= 28 (a) SG (n =25) Habitat % use SE % use SE Wetland 35 5 41 6 Cornfield 54 6 55 6 Soybean field 10 5 4 3 Other 1 <1 0 -- Afternoon GWFG (n =19) SG (n = 24) Habitat % use SE % use SE Wetland 65 7 51 7 Cornfield 34 7 45 6 Soybean field 0 -- 2 2 Other 1 <1 2 2 (a) Number of flocks observed TABLE 3.--Time budgets (%) for spring-staging greater white-fronted geese (GWFG) and snow geese (SG) using wetlands and croplands in south-central Nebraska, 1998-1999 Wetland GWFG (n= 36 (a) SG (n = 43) Behavior % SE % SE Foraging/drinking 12 2 8 1 Resting 70 3 63 3 Walking/swimming 9 2 15 3 Alert 7 1 9 1 Flying 1 1 4 1 Other 1 <1 1 <1 Cropland GWFG (n = 40 SG (n = 48) Behavior % SE % SE Foraging/drinking 39 4 43 3 Resting 45 4 32 3 Walking/swimming 5 2 5 1 Alert 8 1 13 1 Flying 2 <1 6 1 Other 1 <1 1 <1 (a) Number of flocks observed
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|Author:||Pearse, Aaron T.; Krapu, Gary L.; Cox, Robert R., Jr.|
|Publication:||The American Midland Naturalist|
|Date:||Apr 1, 2013|
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