Feeding Habitat Characteristics of the Great Blue Heron and Great Egret Nesting Along the Upper Mississippi River, 1995-1998 CHRISTINE M. CUSTER, SARAH A. SUÁREZ AND DOUGLAS A. OLSEN USGS, Upper Midwest Environmental Sciences Center, 2630 Fanta Reed Road, La Crosse, WI 54603, USA Internet:
[email protected] Abstract.—The Great Blue Heron (Ardea herodias) and Great Egret (Ardea alba) nested in eight colonies along the Upper Mississippi River, USA, and individual birds were followed by airplane to feeding sites during the nesting seasons in 1995-1998. Both species used braided channel/backwater habitats for feeding more than expected, based on availability, and open pool and main navigation channel less than expected. Most individuals of both species fed 10 km away. Habitat and distance need to be considered simultaneously when assessing habitat quality for herons and egrets. The Great Blue Heron flew farther to feeding sites during the care-of-young period than during incubation and the Great Egret showed the opposite pattern. The Great Blue Heron tended to feed solitarily; only 10% of the feeding flights ended at a location where another heron was already present. About one-third of Great Egret feeding flights ended at a location with another egret already present. Colony placement on the landscape seemed to be a function of the feeding radius of each colony. Received 16 March 2004, accepted 22 July 2004. Key words.—Great Blue Heron, Ardea herodias, Great Egret, Ardea alba, feeding habitat, Upper Mississippi River. Waterbirds 27(4): 454-468, 2004
Numbers of the Great Blue Heron (Ardea herodias) and Great Egret (Ardea alba) have declined in Wisconsin (Volkert 1992), Illinois (Graber et al. 1978), and the Upper Midwest of the USA as a whole (Thompson 1977, 1978a). The declines have been most pronounced where habitats were degraded or lost. Thompson (1978a) reported an inverse relationship between wading bird numbers in the Upper Midwest, human numbers, and the amount of land planted with agricultural crops in the same area. Late nest initiation and reduced clutch size associated with record floods in 1993 along the Upper Mississippi River indicated that feeding habitat in the floodplain could be limiting under certain conditions (Custer et al. 1996). The Upper Mississippi River has been designated a Globally Important Bird Area by the American Bird Conservancy (Chipley et al. 2003). It is also a regulated river with locks and dams to facilitate commercial boat traffic, and is heavily used by recreational boaters. The locks and dams have altered the types of habitats present in the system. With the advent of locks and dams, large lake-like features were created by water backing up behind each dam. Numerous sloughs, ponds, shallow-water marshes and swamps, more typical of the original habitat, persist below
each dam. The impacts of these habitat modifications on nesting herons and egrets are unknown, but probably influence the location and size of breeding colonies. The location and size of breeding colonies can be governed by the amount of quality feeding habitat in proximity to the breeding rookeries (Werschkul et al. 1977; Gibbs 1991), but see Kelly et al. (1993) for an opposite viewpoint. Quality feeding habitat is defined by physical, biological, and anthropogenic parameters such as water depth (Custer and Osborn 1978a; Powell 1987; Maccarone and Brzorad 1998; Bancroft et al. 2002), turbidity (Krebs 1974), vegetation characteristics (Thompson 1978b), topographic relief (Bancroft et al. 2002), food abundance or food size (Butler 1993; Maccarone and Parsons 1994), distance from breeding sites (Gibbs 1991; Gibbs and Kinkel 1997), and distance from human disturbance (Miller 1943; Rogers and Smith 1997). Alteration of feeding habitat can cause a shift in feeding locations (Maccarone and Brzorad 1995, 1998). Most feeding habitat selection studies of nesting Great Blue Herons and Great Egrets have been conducted in coastal or estuarine habitats (Custer and Osborn 1978a, b; Simpson et al. 1987; Maccarone and Parsons 1994;
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Maccarone and Brzorad 1998) or in lake situations (Maccarone and Parsons 1994; Smith 1995a, b; Custer and Galli 2002). In coastal situations, tidal fluctuations are the dominant factor in defining feeding habitat for many wading bird species (Custer and Osborn 1978a), whereas hydrologic regimes (Smith 1995a; Maccarone and Brzorad 1998) and water body size (Custer and Galli 2002) are important in lake situations. Few heron and egret habitat selection studies have been made in riverine systems (Thompson 1978b; Dowd and Flake 1985). In these studies, the rivers were much smaller and the types of habitats within the riverine system were either more limited than on the Upper Mississippi River or were not differentiated. Information on specific feeding habitat associations is needed to guide management decisions regarding critical habitats so that appropriate habitat conservation measures can be implemented where needed. The objectives of this study were to: (1) identify feeding habitats in a large riverine system for the Great Blue Heron and Great Egret, (2) determine whether or not habitats were being used in proportion to availability, (3) determine what attributes were associated with feeding sites, and (4) assess habitat partitioning between the two species. METHODS Individual Great Blue Herons and Great Egrets were followed from their nesting colony to their first feeding location using a fixed-winged aircraft, and utilizing techniques developed by Custer and Osborn (1978a) and Custer and Bunck (1992). Flights were made between early April and mid-August in 1995 through 1998, with birds from each colony followed approximately once per week, weather permitting. Data were collected from eight colonies along a 500-km section of the Upper Mississippi River (Fig. 1). The colony names, closest city, and Universal Transverse Mercator coordinates were Butler (Harpers Ferry, Iowa, 652595 E, 4785136 N) and Lansing (Lansing, Iowa, 646719 E, 4803952 N) flown in 1995 and 1996. Mertes (Winona, Minnesota, 608437 E, 4880656 N) and Root (La Crosse, Wisconsin, 640845 E, 4847894 N) were flown in 1996. Catfish (Dubuque, Iowa, 695768 E, 4703351 N) and Kellers (Savanna, Illinois, 734115 E, 4665136 N) were flown in 1997, and Smith (La Crosse, Wisconsin, 638701 E, 4855616 N) and Vermillion (Hastings, Minnesota, 515807 E, 4952728 N) were flown in 1998. One or two additional flights were made following Great Blue Herons from Butler in 1997, Mertes in 1995, and Root in
Figure 1. Map of the Upper Mississippi River, USA study sites, 1995-1998. 1995. Of the eight colonies, the Great Egret nested in only three (Mertes, Smith, and Butler). Flights following the Great Egret generally started 3 to 4 weeks later than those following the Great Blue Heron because Great Egrets nested later. Vermillion was only flown twice in April 1998, so summary data are presented, but no statistical analyses were done because of the more limited data. Each breeding colony was circled in a counter-clockwise direction at an altitude of approximately 300 m above the ground. A bird was selected for following as it left the colony. The location where the bird landed was mapped using global positioning system equipment and/or landmarks. Habitat categories and the type of landing substrate (see specifics below) were assigned while the airplane circled the bird’s landing site. The presence or absence of other herons and egrets within 100 m was recorded, as was the elapsed time since the
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bird left the nesting colony. After the above data were recorded, the airplane returned to the nesting colony and another bird was followed. As many birds as possible were followed during a 2-h to 3-h period in the morning and a similar period in the afternoon. Only one species was followed during a specific period (i.e., either morning or afternoon) and the order of the species followed during the next period was reversed, so that both species were followed in the morning and afternoon. The timeof-day when a flight was made, however, was probably unimportant because the Great Blue Heron (McNeil et al. 1993) and Great Egret (Powell 1987; Maccarone and Parsons 1988) feed throughout the day. We first classified feeding habitat as within or outside the Upper Mississippi River floodplain. Upper Mississippi River floodplain habitats were further subdivided into three categories: backwater/braided channels (hereafter called backwaters), open pool, and main navigation channel. Backwaters were generally downstream of a lock and dam and consist of sloughs, ponds, shallow lakes, and wetlands of various sizes. An open pool was a large lake-like habitat created when water backed up behind a lock and dam. Water depths in an open pool were generally deeper than those found elsewhere within the floodplain. The main navigation channel was the 3 m-deep channel maintained for navigation by the U.S. Army Corp of Engineers. Feeding sites off the Upper Mississippi River floodplain were characterized into four categories: rivers and creeks; ponds and lakes; marshes; and terrestrial and agricultural land. Specific landing substrates were classified as (1) shoreline [with either bare ground or dead or live vegetation]; (2) man-made structure, log, snag, or rock; or (3) open water. Sample sizes varied for each of the analyses because not every category could be completed for each bird. Elapsed time was taken for all birds, even those lost en route, but data on their feeding habitat and landing substrate were not available for those birds. Birds that flew to another heron colony were omitted from further consideration. Statistical Analyses Distance from the colony to the feeding site and elapsed time from the colony to the feeding site were first analyzed for differences between years using data from the Lansing and Butler colonies (1995 and 1996). Most other colonies were studied in only one year. The effect of nesting chronology (incubation or care-ofyoung periods) on distance and elapsed time was tested with Butler and Lansing data from 1996 for the Great Blue Heron because the nesting chronology of those two colonies was known in 1996. The effect of nesting chronology for the Great Egret from Butler in 1996 was also assessed. Median tests were used for statistical comparisons of elapsed time and distance because some individuals (6% of total number followed) were lost en route and the final time and distance could not be quantified. The median test was appropriate because it did not require the final time or distance for every bird, but only that most birds lost en route had gone farther than the median time or distance. In all, 90% of birds lost en route had flown for greater than the median time. The 2sample median test and the Brown-Mood multisample median tests were used (SAS, version 8). To examine directionality of flights, the area around each colony was divided into 45° arcs (N, NE, E, SE etc.). The amount (ha) of aquatic habitat that was
present in that arc for each distance zone (0-1, 0-5, 0-10 and 0-20 km) was quantified based on aquatic area designations (Wilcox 1993). The amount of aquatic habitat was then correlated with the percentage of birds feeding in each of the eight directions. Landing substrates and number of birds already present at feeding sites were compared among locations and between species using Chi-square and Fisher’s Exact analyses. For habitat selection analyses, two data sets were utilized. One data set was comprised of randomly generated locations (available points) and the other was the actual feeding locations (used points) of each bird followed. Approximately 500 available points per colony were generated within a 20-km radius of each colony and represented a theoretical set of possible feeding locations. These points were located a minimum of 50 m from one another to account for the minimum interbird distances of these two species. Before data analysis, each available and used point was classified into distance-from-colony category (0-1, 1-5, 5-10, and 10-20 km) and habitat category (backwaters, main channel, and open pool). The 20-km cutoff point was used because >95% of Great Blue Herons and >90% Great Egrets fed within that distance of their colony, and this range has been used in other studies (Gibbs et al. 1987; Kelly et al. 1993). Habitat classes were defined based on aquatic area designations of habitat maps (Wilcox 1993). Only data from birds feeding within the Upper Mississippi River floodplain were used in the habitat selection analyses. Selection ratios were calculated for distance from colony and habitat categories. Selection ratios are proportional to the probability of a category being utilized (i.e., selected) and do not depend on the types of habitats deemed available (Manly et al. 1993). With selection ratios the results would be the same whether a particular habitat, such as impoundments are or are not included in the analysis. A selection ratio >1 indicates that a category is used more than expected based on the availability of that category. A selection ratio 1 indicates selection. Intervals with both upper and lower bounds
