Northern Prairie Wildlife Research Center
Human disturbance from traffic on I-80 probably was the principal cause of reduced use of habitat north of riverine roosting sites and for the high percentage of peripheral roosts. All nocturnal roosts were within 3 km of I-80. Krapu (pers. obs.) observed that low-flying cranes which flew north from roosts frequently hesitated, turned back, or gained altitude when trying to cross the highway. Birds with central roosts could be expected to travel less and conserve more energy than those with peripheral roosts (Wittenberger and Dollinger 1984), but we found no difference in travel distance between roost types. For cranes, waste corn was highly available during spring, 1978 and 1979, (Reinecke and Krapu 1986), and may have reduced the importance of roost type on foraging efficiency.
Foraging as a factor of crane staging. We observed that cranes which roosted near each other typically departed in the morning in the same general direction but did not go to the same feeding sites as observed among Common Terns (Sterna hirundo) and Ospreys (Pandion haliaetus) (Waltz 1987, Hagan and Walters 1990). Rather, flocks of cranes upon reaching the feeding grounds, tended to join existing groups of cranes already on the ground. As a result, large foraging aggregations of cranes became common by mid-morning through local enhancement (Hinde 1961, Wittenberger and Hunt 1985). Use of occupied fields probably increases the foraging efficiency of inexperienced birds because density of waste corn varies much more among fields, depending on post-harvest land use (Baldassarre and Bolen 1984, Reinecke and Krapu 1986) than within a field (Frederick et al. 1984). As a result, cranes that settle into occupied fields are likely to be more successful foragers.
Norling et al. (1991) showed that the variability in duration of departures from nocturnal roosts to foraging sites and percent of cranes leaving roosts after sunrise increased with date and population buildup. Cranes also left roosts later during fog or precipitation. These responses suggest that cranes rely heavily on visual cues to find food. The increased percentage of delayed departures as the number of cranes increased and food availability declined is consistent with our premise of the importance of local enhancement to enhance foraging efficiency. New migrants into the staging area and cranes that have recently switched activity ranges are likely to benefit most by using conspecifics to locate suitable foraging sites.
The availability of macroinvertebrates to cranes is much less predictable and distribution more clumped than that of corn and depend on soil temperature, escape mechanisms of the organisms, and local abundances (Richter 1958, Edwards and Lofty 1977). Foraging efficiency among cranes seeking macroinvertebrates is more likely to be enhanced by cranes flying or walking to specific sites where concentrated foraging activity is in progress or signs of recent foraging activity are present. Intense probing by Sandhill Cranes in areas with high concentrations of soil invertebrates disturbs the soil surface (G. Krapu, pers. obs.) and may provide visual cues to other cranes seeking animal foods even after the initial foragers have departed. Cranes spent as much time foraging to obtain the 3% of the diet comprised of invertebrates as the 97% formed by corn (Reinecke and Krapu 1986), reflecting the disparate rates of foraging success on invertebrates and corn.
Cranes probably improve their foraging efficiency and reduce energy costs by remaining in the vicinity of their feeding grounds throughout the day. Only 1.7% of the locations of radio-marked cranes between 08:00 and 18:00 h were at communal roosts; most diurnal use of communal roosts occurred during periods when cranes failed to depart at dawn because visibility was poor. Some species are thought to establish centrally located diurnal activity centers (DAC) from which they base their foraging expeditions (Caccamise and Morrison, 1986, 1988; Caccamise 1993). These DACs are proximal to feeding sites and reduce energy spent in flying to different areas. Congregations of cranes in native grasslands and planted haylands near water and feeding grounds during mid-day are suggestive of DACs. Among the activity ranges that we followed, 29 had data that could be inspected for the presence of a DAC. Twelve had sites with concentrated observations on two or more days which are consistent with a DAC, 10 had possible sites but inadequate data, and seven showed no evidence for a DAC. Among European Starlings (Sturnus vulgaris), communal roost location is determined primarily by food distribution, and birds are more faithful to their feeding sites than to communal roosts (Morrison and Caccamise 1985), leading Caccamise (1991) to suggest that the information center hypothesis did not adequately explain starling behavior. Crane distribution, however, is determined primarily by availability of suitable communal roosting habitat (Krapu et al. 1984) which is more restricted than is food availability (Krapu et al. 1985). Cranes frequently change communal roosts and foraging sites and both communal roosts and DACs serve as information exchange centers for improving foraging efficiency. The theories of information centers and DACs are not mutually exclusive (Tye 1993), and further research should be conducted to determine the contributions each makes to foraging efficiency in Sandhill Cranes.
Differences in flight distances to specific habitats can indicate the importance of these habitats, presuming that birds will spend greater energy traveling to more important sites. The higher than expected use of native grasslands and planted haylands relative to their availability reflects the limited distribution of these habitats, together with their importance in supplying macroinvertebrates. Consumption of animal foods high in protein compensates for the low protein content of corn (Reinecke and Krapu 1986). Biomass of macroinvertebrates is relatively low, particularly in wet meadows (Davis 1991), presumably adding to the search time cranes require to satisfy dietary needs.
Influence of predation and behavior on group size. Models of flocking behavior and predation predict that the proportion of time spent watching for predators should decline with group size and reach an asymptote when the likelihood of spotting a predator no longer increases with group size. We found that cranes spent little time in alert behavior and that the proportion of time spent alert or feeding did not vary consistently with flock size. Similarly, Tacha (1988) did not find a relation between percentage of time in feeding or in alert with flock size and reported that only 0.5% of 1619 alert responses with known causes were in response to predators. The fact that alert did not vary with flock size as predicted also suggests limited vulnerability of cranes to predators that currently exist in the Platte River Valley. Raptors may kill a few cranes (Lewis 1974, Lingle and Krapu 1986), but most data are circumstantial. Only two of 170 Sandhill Cranes (1%) examined by Windingstad (1988) were apparently killed by predators during the nonbreeding season.
At night, cranes roost in riparian sites and semipermanent wetlands (Krapu et al. 1984, Folk and Tacha 1990), where they are protected by a water barrier and open canopy from most predators. Cranes spent less time in alert at night than during the day (8.6% versus 14.4%, G. Krapu, unpubl. data).
Other influences. Mate finding and behavioral synchronization also have been identified as benefits of communal roosting and flocking (Moynihan 1968). Courtship was rare in our study compared with other behaviors, but it occurs most frequently in spring (Tacha 1988). Pair-bonds in young cranes are typically ephemeral, and several pairings may occur before mates are finally chosen (Nesbitt and Shapiro-Wenner 1987). Staging could facilitate mate selection by attracting numerous birds.
Epizootics have been reported frequently in staging waterfowl (Wobeser 1981). However, disease was not an observed problem in our study, and epizootics have not been reported in Sandhill Cranes. Reported occurrences of botulism, avian tuberculosis, Salmonella (Lewis 1974), and of avian cholera (Krapu and Pearson 1981, Kauffeld 1987, Windingstad 1988) are infrequent.
Conservation concerns. Our findings indicate that staging behavior of Sandhill Cranes is strongly influenced by the massive anthropogenic alterations that have taken place in the Platte River Valley. To date, habitat loss has caused a major redistribution of cranes (Krapu et al. 1982), but sufficient remaining roosting and foraging habitat continues to attract and support most of the midcontinent population for several weeks each spring. So far, cranes have successfully avoided the energetic consequences and associated displacement that would have resulted from massive habitat loss had an abundant supply of high energy waste corn and adequate sources of protein in native grassland not been available. The status of this important Sandhill Crane staging area remains precarious, however, because of continuing degradation and loss of open river channels and native pastures (Sidle et al. 1989). With continuing habitat loss, crowding will increase and available food supplies may prove inadequate. Human presence in the Platte River Valley has risen dramatically over the past decade due to increased public awareness of cranes (Lingle 1991). The effects of this increased activity on foraging behavior and activity range characteristics have not been measured.