Location and Habitat Size of Sandhill Lakes Used by Swans

Scattered among the vast grassland expanse of the Nebraska Sandhills are numerous lakes and marshes that are breeding habitat for an interesting diversity of nesting waterbirds, including the Trumpeter Swan, the largest of the North American waterfowl.

Before the turn of the century this swan was reported as a breeding bird on Watt's Lake, southwest of Valentine in Cherry Co. (Bates 1900), and was considered to have probably once bred at many lakes throughout the Sandhills region (Bruner 1904).

Trumpeter Swans as nesting birds were eventually extirpated from the lakes and marshes, due to indiscriminate shooting and disturbance, as the Sandhills were settled. Reintroduction efforts undertaken during the 1960's were required to return breeding Swans to Nebraska. Birds released at Lacreek National Wildlife Refuge in South Dakota eventually became established and a portion of this growing population entered the state to once again nest (Monnie 1966).

Since that time the population status of the Trumpeter Swan, as well as other breeding ducks and geese, has been evaluated during an annual waterfowl survey carried out to compare yearly trends (pers. comm., Rolf Kraft, U.S. Fish and Wildlife Service, and John Sweet, Nebraska Game and Parks Commission). During these aerial surveys the lakes where Swans are observed and possibly breeding are recorded.

In order to assess the characteristics of the different lakes where Swans were present during the breeding season, an analysis of satellite imagery was undertaken to evaluate the relative amount of water and marsh habitat.

The habitat maps used to figure the amount of open water or marsh are available at the Remote Sensing Analysis Laboratory of the University of Nebraska at Omaha. This laboratory has prepared an inventory of the location and size of wetlands for the Omaha District of the U.S. Army Corps of Engineers. The survey used picture elements from the scanner system of the Landsat Satellite to classify four wetland types: open water, marsh, subirrigated meadows, and riparian wetland. Any area inundated by surface or groundwater frequently enough to support vegetation adapted to survive in water-saturated soil was identified and mapped on a mylar overlay the size of the relevant U.S. Geologic Survey 7.5 minute quadrangle map (Turner et. al. 1980).

These overlays were used to measure the amount of open water or marsh at the Sandhills lakes where Trumpeter Swans had been observed. A measuring device connected to a Numonics Electronic Digitizer/Planimeter was traced around the elements designated by the survey. The area of open water or marsh could be determined in this fashion and the size in acres was then easily converted to hectares.

Alkalinity data was included because of the impact it has on the vegetation of the marsh plant community. This information is from a conibrehensive study of the lakes in the region (McCarraher 1977).

Results

The 23 lakes utilized by Trumpeter Swans occurred primarily in Cherry, Sheridan, and Garden counties (Figure 1, Table 1). They ranged in size from 12 hectare (ha) West Cottonwood Lake up to 253 ha Alkali Lake. Nineteen of the 23 were from 23 to 103 ha in size. West Cottonwood was exceptionally small. The average size was 72 ha. Omitting the lowest and the 3 highest overall size values (those over 103 ha) gave an average size of 56 ha.

Figure 1. Locations in the sandhills region where Trumpeter Swans have been observed during the breeding season.

All of the lakes, except for Hoover, had greater than 56% open water. The range was from the low value of 32% at Hoover Lake to 89% at the two Alkali Lakes in Cherry Co. The average amount of open water was 76% for all 23 lakes.

Open water habitat in the Sandhills has been classified as a submersed aquatic zone. This zone, which is one of three possible wetland zones, is characterized as open water containing floating and submerged aquatic vegetation. Representative species include common duckweed (Lemna minor), star duckweed (Lemna triscula), sago pondweed (Potamogeton pectinatus) and common bladderwort (Urticularia vulgaria) (Gilbert et. al. 1980). Yellow water lily is an additional floating aquatic plant that is present.

Marsh was much less extensive than open water at the lakes, with only about one-fourth the amount of the total surface area. The values ranged from 11% to 68%, with an average of 24%. The marsh area has two plant zones. Dominant plants of the inner marsh zone are hardstem bulrush (Scirpus acutus), duckweed (Lemna spp.), and arrowhead (Sagittaria spp.). Also found in this zone are common cattail (Typha latifolia), river bulrush (Scirpus fluviatilis) and giant burreed (Sparganium eurycarpum). The outer marsh zone has a highly variable plant composition of hydric grasses and sedges and an interspersion of mesic and zeric species from the marsh and neighboring upland plant community (Gilbert et. al. 1980).

Of the 21 lakes for which alkalinity is known, 12 (57%) are slightly alkaline, 1 (5%) is moderately alkaline, and 8 (38%) are of medium alkalinity.

Table 1. Habitat characteristics of sandhill lakes where Trumpeter Swans have been observed during the breeding season.
Map Site. No.	County	Habitat Size (Hectares)					Water Chemistry
	Lake Name and Location	Marsh	%	Open Water	%	Total	Alkalinity 1
	Garden
1.	Gimlet (T21N R44W)	12	38	20	62	32	slight
1.	Black Steer (T21N R43W)	14	17	68	83	82	slight
1.	Miller (T22N R43W)	12	32	25	68	37	moderate
	Sheridan
2.	Spade Ranch (T27N R41W)	17	20	70	80	87	medium
3.	Dolly Warden (T28N R42W)	9	35	17	65	26	medium
3.	Round (T28N R42W)	7	20	28	80	35	medium
4.	Frye (T35N R41W)	11	22	40	78	51	slight
4.	Hoover (T35N R41W)	36	68	17	32	53	slight
	Cherry
5.	Alkali (T34N R40W)	20	11	157	89	177	medium
5.	South Twin (T33N R39W)	15	36	27	64	42	medium
5.	Spall (T35N R39W)	13	39	20	61	33	slight
6.	Cody (T35N R33W)	37	23	125	77	162	slight
7.	Wolf (T29N R35W)	8	22	28	78	36	slight
7.	Home Valley (T29N R37W)	13	13	84	87	97	no report
7.	West Cottonwood (T28N R38W)	3	25	9	75	12	medium
8.	Rat (T29N R39W)	12	28	31	72	43	slight
8.	Fawn (T28N R39W)	6	26	17	74	23	slight
8.	Turpin (T28N R39W)	16	30	38	70	54	slight
9.	Alkali (T26N R39W)	28	11	225	89	253	medium
9.	School Section (T26N R40W)	38	37	65	63	103	slight
9.	Castle (T25N R39W)	25	25	76	75	101	slight
10.	Twenty-one (T29N R27W)	32	43	42	57	74	slight
	Brown
11.	Amer. Game Assoc. (T27N R24W)	10	19	42	81	52	no report
	Total	394	24	1271	76	1665
1. Data from McCarraher, 1977

The higher values for the amount of open water would seem to indicate that this lake characteristic may have more of an influence on use by Trumpeter Swans than the extend of marsh vegetation. Also, Swans were recorded more often on lakes that had only slight alkalinity.

Acknowledgements

I would like to thank John Sweet and Rolf Kraft for providing the lake names and locations.

Literature Cited

Bates, J.M. 1900. Additional notes and observations on the birds of northern Nebraska. Proceedings of the Nebraska Ornithologists' Union 1: 15-18.

Bruner, L., R.H. Wolcott, and M.H. Swenk. 1904. A preliminary review of the birds of Nebraska. Klopp and Barlett Co., Omaha, Nebraska. 116 pp.

Gilbert, M.C., M.W. Freel, and A.J. Bieber. 1980. Remote sensing and field evaluation of wetlands in the sandhills of Nebraska. U.S. Army Corps of Engineers, Omaha, Nebraska.

McCarraher, D.B. 1977. Nebraska's sandhills lakes. Nebraska Game and Parks Commission, Lincoln, Nebraska. 67 pp.

Monnie, J.B. 1966. Reintroduction of the trumpeter swan to its former prairie range. Journal of Wildlife Management 30: 691-696.

Turner, J.K., D.C. Rundquist, A. Bieber, and L.C. Hamilton. 1980. Wetlands inventory of the Omaha District. Final Report, U.S. Army Corps of Engineers. Omaha, Nebraska. 110 pp.

March 1984. Location and habitat size of lakes in the Nebraska sandhills utilized by trumpeter swans. Nebraska Bird Review 52(1): 19-22.