Northern Prairie Wildlife Research Center
Research findings summarized in this Report underscore the need to prevent further deterioration of crane habitat and thereby reduce the pressures that have already caused the abandonment by sandhill cranes of about two-thirds of the original channel habitat available in the Platte and North Platte River Valleys. With about 80% of the continental population of sandhill cranes staging along approximately 70 miles of the Platte and North Platte Rivers each spring, the crane population already is confined to a relatively small area.
Crowding of the crane population into a smaller land and water base poses several potential hazards. First, as the habitat base continues to shrink, the probability of catastrophic losses to natural forces such as ice storms, hail storms, or tornadoes increases. Such phenomena are often not very widespread, but can have a devastating impact on wildlife populations. Cranes are known to be vulnerable to severe hailstorms (Merrill 1961; Higgins and Johnson 1978; Heflebower and Klett 1980). The occurrence of hailstorms in Nebraska during March and April of the past 25 years (U.S. Environmental Data Service 1959-80) indicates the potential for heavy losses in view of the high concentrations of birds present in the Platte Valley.
Crowding also increases the likelihood of food shortages if present land use practices change and access to corn diminishes substantially. The present supply of waste corn on the study area during the spring staging period is more than adequate to meet maintenance requirements and to build fat reserves for migration and reproduction. The crane population could have difficulty meeting its caloric requirements if corn availability declines due to increased fall tillage, conversion of cropland from corn to a less energy-rich and digestible crop, or for some other reason. The continuing loss of native grasslands poses the threat of shortage of the invertebrate foods that provide much of the protein and calcium in the diet. Habitat deterioration in the river channel and the resultant crowding is leading to intensive use of the grassland tracts that remain accessible to the crane population.
Lessening vulnerability to disease represents the third reason for maintaining a dispersed population of cranes. If conditions along the Platte continue to deteriorate, cranes will shift into areas of the River still containing suitable habitat, thus increasing the density of cranes on these sites. Beyond some threshold level of habitat deterioration cranes will seek roosting sites elsewhere; the Rainwater Basin Area to the south of staging areas 1 and 2 is a probable alternative site. Sandhill cranes already occasionally gather there during the spring staging period but in relatively small numbers (Appendix K). Whooping cranes, which may be more sensitive than sandhill cranes to habitat deterioration, have been sighted in this area regularly in recent years. Greater use of the Rainwater Basin Area by cranes would increase the probability of mortality due to avian cholera and other diseases. Because less than a hundred whooping cranes exist in the population, loss of a few individuals would have a major impact.
The native grasslands of the Platte and North Platte River Valleys function as a primary source of protein and calcium for sandhill cranes during the spring staging period. Although corn is the principal energy source, it contains inadequate levels of calcium and certain amino acids. Native grasslands probably serve a similar function in supplying protein and minerals for whooping cranes (Appendix R). Therefore, preservation of native grassland tracts at strategic locations should be an integral part of the overall habitat management plan for meeting the needs of the crane populations.
Native grassland habitat should be preserved in a distribution that will ensure access by all segments of the staging crane population in order to meet nutritional requirements and maintain the current dispersal pattern. The first priority in preservation of grassland habitat should be to protect those remaining large grassland tracts which lie adjacent to the river channel in areas where major roosts are located.
It is preferable to maintain the grassland tracts in close proximity to roosting habitat. Close proximity of grassland tracts to the channel reduces daily crane movements and the energy cost associated with acquiring adequate protein and calcium. Prolonged flights to obtain these nutrients would require a substantial energy expenditure. This strategy will help to lessen the impact of any future alterations of the present corn-livestock economy that would reduce crane access to high energy foods. Second, cranes using grassland tracts adjacent to the channel are less subject to various types of human disturbance. Disturbance is less of a problem near the river channel in most areas because major highways do not intersect flight lanes and land ownership patterns and agricultural practices discourage travel there. Loss of key tracts of grassland habitat near the River would likely result in movement to areas less protected from disturbance.
Based on the present distribution of sandhill cranes and habitat requirements of the population, the most appropriate habitat maintenance plan would be to protect, through purchase in fee title, a series of tracts near the River ranging in size from a few hundred to several thousand acres. These reserves, which would be managed specifically to satisfy crane requirements, would consist principally of grassland and channel habitat but would also contain limited cropland. In some areas, it may be necessary to restore grassland habitat to increase crane access to invertebrate foods. Each unit would be centered on a major roosting area and should preferably include several km of river channel both upstream and downstream to allow intensive management of roosting habitat. Some areas should include a 200+ m wide channel to accommodate whooping cranes. Between the principal management units, grassland tracts existing on the staging areas and within 3.2 km (2 miles) of the channel should also receive protection, possibly through easements.
The Lillian Annette Rowe Sanctuary of the National Audubon Society and the Terry tract on Mormon Island acquired by the Whooping Crane Critical Habitat Maintenance Trust Fund in 1980 and managed by the Nature Conservancy are indicative of the type of reserves needed at several strategic sites. Both are principally native grassland and channel habitat situated in areas supporting high numbers of cranes. These reserves, if expanded, would sustain major segments of the staging crane population during spring in the Platte Valley.
Grasslands will require certain management practices to ensure continued high use by cranes. Cranes prefer grassland with relatively short vegetation and largely avoid tracts of tall rank cover. Most crane use in the Valley occurs on native grasslands receiving moderate grazing pressure by livestock. Burning provides an alternative management technique to grazing that is acceptable to cranes. In situations where rank stands were burned during the staging period, cranes occupied the sites almost immediately after burning. Sandhill cranes sampled while foraging at these sites were feeding principally on earthworms. In 1980, two whooping cranes were attracted to a recent burn on the Lillian Annette Rowe Sanctuary, suggesting that burning may be useful in attracting whooping cranes to managed units along the River. Data obtained in this study suggest that both grazing and burning or a combination of both practices maintain habitat in a condition suitable for cranes during the spring-staging period. However, additional research is needed to determine grassland management practices most conducive to optimizing production of the invertebrate foods sought by cranes.
Although the high water table on lands adjacent to the river channel and associated water-logged soils have discouraged tillage in the past, there has been a gradual lowering of the water table and a concurrent encroachment by agriculture upon the remaining grasslands in recent years. Because this trend will probably continue, it is important that acceptable methods be found to protect strategic areas along the river in the near future.
This study established the importance of wide, shallow, unobstructed river channels for roosting sandhill cranes. River segments with open water at least 150 m wide supported nearly 70% of all roosting cranes in 1979, although only 25% of the segments sampled had channels of this width. Over 18% of the river segments have a maximum unobstructed channel less than or equal to 50 m wide, but less than 1% of the cranes roosted on them. Crane use of river segments in which the maximum channel width was between 51 and 150 m was less than expected based on its availability, except where average bank vegetation was less than 0.5 m high.
Before extensive upstream diversion of the North Platte and South Platte Rivers, flows were adequate to maintain a wide channel and suitable roosting conditions throughout the reach under study. River stage levels during the early and mid-summer months kept sandbars and mudflats submerged, thereby preventing the establishment of woody vegetation. Vegetation that became established later in summer probably was cleared from the channel both by the scouring action of ice and by high flows during the following spring and the concomitant shifting of alluvial sediments. Peak flows in spring are now largely retained upstream in reservoirs. Increased consumptive use of the Platte's waters has reduced average annual flows in some areas to less than 20% of levels present 100 years ago. As a result of flow reduction, permanent islands supporting woody vegetation are now a common feature of the River, channel width has been drastically reduced in many areas, and extensive woodlands are now rooted in sediments which were formerly parts of the channel. Cranes are extremely wary birds and tend to avoid roosting in areas where vegetation is tall and channel width is narrow.
The natural establishment of woody vegetation must be controlled to maintain roosting habitat for sandhill and whooping cranes. This study has quantitatively documented that seedling establishment of cottonwoods and willows, the predominant trees to invade sandbars and former channel lands of the Platte Valley, occurs primarily on sites with a high percentage soil moisture, fine textured soils (i.e., sand and fine sand), and an extensive substrate exposure period during the time when viable seed is available. Viable seeds are released from mid-May to mid-July, but individual cottonwood seeds remain viable for only 3 weeks, and willow seeds remain viable for less than 2 weeks. Under these circumstances, viable seed is available during the seed release period (mid-May to mid-July), and for approximately 1 month after seed dispersal (i.e., until the end of August). Sandbars with high soil moisture levels must be exposed for at least 1-2 weeks during this mid-May to August viability period if seeds are to germinate and seedlings become established.
Water manipulation is probably the most effective means of controlling encroachment by woody vegetation. When seedlings become established, major floods and scouring are necessary to remove them. Field observations by Shull (1944) and Dietz (1952) have shown that cottonwood and willow seedlings and saplings are quite resistant to the effects of flooding. Hosner (1958) experimentally demonstrated that 7.6 cm (3 inch) cottonwood seedlings could survive 16 days of inundation but that recovery was very slow. Willow seedlings were able to survive the same treatment for 32 days and recovered very rapidly. Experiments conducted with Platte River seedlings 15 cm tall indicated that cottonwood was able to survive 30 days of inundation, and recovered completely after the flooding. The 15-cm seedlings were chosen for the latter study because they were representative of seedlings that established on Platte River sandbars in July and August of 1978, when the substrate was exposed for 4-5 weeks. A similar but later exposure of the substrate occurred in August and September of 1979. If this water regime is typical of most years along the Platte, seedlings that become established in mid-summer would probably have ample opportunity to develop to a height of 15 cm or greater. Manipulation of river stage levels during the fall, winter, or spring, when demands on river water for irrigation are minimal, would undoubtedly remove some of the seedlings that became established during the previous summer, but this technique would probably not be very effective in removing seedlings 15 cm or taller.
Seed germination and seedling establishment can be most effectively controlled by raising river stage levels during the seed viability period (mid-May to August), inundating sandbars either throughout the period of seed germination or for sufficient time to prevent seed germination and seedling establishment. During the present study, seedling establishment occurred primarily on raised sandbars which, on the average, were exposed at stage levels (relative to the Overton gaging station) less than 0.70 meters (2.3 feet). Stage levels averaged 0.82 meters (2.7 feet) during the early portion of the summer (10 June to 16 July) and thus were adequate to prevent seedling establishment. From 16 July to 30 September, however, stage levels averaged only 0.47 meters (1.5 feet) and numerous sandbars were exposed and seedlings became established. Mean stage levels would have to be kept above 0.79 meters (2.6 feet) for all or part of May, June, July, and August, when viable seeds are present along the Platte to successfully prevent seedling establishment.
Alternative methods will be needed to control existing stands of woody vegetation, particularly on islands within the channel because water is only partially effective in removing seedlings after establishment. Chemical and mechanical techniques probably are the most effective tools, but are both costly and labor intensive, and require repeated use in order to ensure successful control of woody vegetation. A thorough evaluation of the effectiveness of specific chemical and mechanical techniques for control of woody vegetation is needed before guidelines for their use can be formulated.
In view of the effect of channel width and vegetation height on crane use of the river, management techniques will be needed to prevent further channel shrinkage and woody vegetation encroachment on islands. The findings of the study suggest that a long-term management goal should be to maintain unobstructed river channel width at a minimum of 150 m throughout the staging areas. Reaches of river channel within management units that are either in public ownership or otherwise managed specifically for cranes should be maintained with channels at least 200 m wide to accommodate the needs of whooping cranes, which appear less tolerant of woody vegetation.
Among the 91 0.8 km (0.5 mile) segments within staging area 1, 65 contain unobstructed channels over 150 m wide and 28 of these have channels over 200 m in width (Appendix I). Only five of the segments with wider channels were not utilized for roosting by cranes and each of these had either a bridge crossing the river or a road immediately adjacent to it. Although the quality of roosting habitat in staging area 1 has been deteriorating, most of it is still acceptable to cranes and, therefore, receives heavy use. Management of river flows to maintain unobstructed channel width at a minimum of 150 m throughout staging area 1, and at a width of 200 m in certain restricted sections, should be adequate to satisfy the habitat requirements of roosting cranes. Control of woody vegetation on some river channel islands will be necessary to prevent the vegetation from reaching a height that will inhibit roosting.
Within staging area 2 only 20 of the 50 0.8 km (0.5 mile) segments have channels over 150 m wide, and much of the western half of this staging area (segments 148-171, Appendix I) has been abandoned by cranes during recent years. Although birds continue to use this staging area, many cranes probably have recently shifted to staging area 1. Further deterioration will very likely lead to additional shifts. Channel habitat in the Overton area, a key staging site in historic times and an area formerly used by whooping cranes, is threatened and some reaches are gradually being abandoned. Habitat conditions on much of the eastern half of staging area 2, which still supports large numbers of roosting cranes, are approaching a critical condition. In contrast to staging area 1, most island vegetation in staging area 2 exceeds 8 m in height. Much of staging area 2, particularly the western half, will probably require extensive rehabilitation of channel habitat to restore suitable roosting conditions. Initial management should concentrate on maintaining open channels over 150 m wide in the eastern half of the staging area and on clearing island vegetation, particularly at western sites where abandonment by cranes appears imminent.
Staging area 3 is quite unlike staging areas 1 and 2. Less than 10% of the 0.8 km (0.5 mile) segments contain channels over 150 m wide, and in fact, 75% of them are less than 100 m in width. Low vegetation on islands and banks (< 8 m) may account for continued crane use despite the relatively narrow width of the channel. However, substantial island vegetation is in the 4 to 8 m height class, indicating that the trees are well established and will probably grow to a sufficient height to adversely affect roosting, particularly when one considers the narrow width of the river channel. The most appropriate management in staging area 3 seems to be application of procedures to prevent further establishment of woody vegetation and to remove existing woody vegetation on those river islands where the combination of channel width and vegetation height is most likely to inhibit crane use.
Along with habitat preservation, adequate facilities are needed at certain of the reserves to allow the many visitors that travel to the Platte Valley to view cranes in natural settings without having their presence disrupt the crane population. Development of areas managed specifically for the cranes and other migratory birds, and with adequate facilities to accommodate visitors, would largely eliminate the disruption to both landowners and cranes which has accompanied the influx of crane watchers in recent times.
In the absence of facilities and public lands, visitors now gather along public roads often creating traffic congestion and related problems. Opportunities also exist to attract large numbers of travelers throughout the year because of the close proximity of I-80 to the river channel and, with the aid of a well-designed visitor center, to educate the public to the unique features of the Platte River Valley and its significance in meeting the needs of midcontinent migratory bird populations. The fascinating history of the Platte River Valley and the importance of wildlife populations in the settlement period adds to the potential scope of the exhibit.