Bald Eagles Productivity Summary 1994-1996 Lake Clark National Park and Preserve Cook Inlet Coastline Introduction: Although the bald eagle (Haliaeetus leucocephalus)is not listed as endangered or threatened in Alaska, it is protected by the Bald and Golden Eagle protection Act, which states that "no person shall take...any bald eagle,...alive or dead, or any part, nest or egg thereof...". It goes on to define take as "to pursue, shoot at, poison, wound kill, capture, trap, collect, molest, or disturb...". Eagles along the Lake Clark coastline are subject to disturbance and molestation through oil development and logging and mining activities on private and Native Corporation inholdings. Bald eagles are known to be susceptible to human disturbance on their nesting grounds (Gerrard and Bortolotti 1988). Bald eagles can, to some degree, tolerate the increased energy expenditure that may result from human disturbance; however, high levels of disturbance will cause reproductive failure and changes in behavioral patterns (Stalmaster 1987). By monitoring eagle productivity, park managers can determine the reproductive status of the park's coastal population and determine if the population is increasing or decreasing. If eagle productivity declined over several years park managers could use this information to investigate possible causes. The objectives of this study were to: 1. Obtain baseline data on the bald eagle population along the Lake Clark National Park coastline. 2. Monitor long term changes in the reproductive status of bald eagles along the Lake Clark National Park coastline. 3. Assess the effects of natural or human-related environmental changes on bald eagle reproduction. 4. Locate and obtain geographic coordinates on previously unknown bald eagle nests along the Lake Clark National Park coastline.
Methods: Aerial surveys assessing bald eagle productivity were conducted from 1994-1996 along the Lake Clark National Park coastline. An "Early Occupancy Survey" (EOS) was flown after most of the eggs were laid and before the majority of nests failed (late May-early June). A "Late Productivity Survey" (LPS) was flown when young were large enough to be seen from the air, but before they had fledged (5-9 weeks)(late July). Only those nests that were considered active or occupied during the EOS were surveyed during the LPS. The survey route followed 192 km of shoreline along western Cook Inlet (Fig. 1). All data were collected and recorded following United States Fish and Wildlife protocols (Bowman 1992) and entered into the Alaska state-wide database. Terminology for productivity followed Bowman (1992) and Postupalsky (1974)(Table 1). Surveys were conducted on clear days with winds <16 kph. The aircraft position along the survey route was continuously recorded during each survey using a Trimble 2000A Global Positioning System (GPS) navigator. Raptor nest locations (known and previously unknown) were recorded using a GPS and manually plotted on United States Geological Survey 1:63,360 topographic maps. Productivity was expressed as percent nest success (number of successful nests per total number of active and occupied nests), percent of active nests successful, number of young per occupied nest, and no of young per successful nest. Only those nests with data from both surveys were used for productivity measurements. Results and Discussion: Survey dates for the EOS and LPS varied between 1994-1996 depending on yearly variations in bald eagle breeding phenology, localized weather conditions, and availability of pilots and observers (Table 2). The same pilot and observer flew all surveys in 1994 and 1995 and during the 1996 EOS survey. The 1996 LPS survey was flown by the same pilot as previous and a different observer. The pilot and all observers were experienced in bald eagle nest surveys. Early Occupancy Surveys in 1994 and 1996 were conducted during required dates of mid to late May. However, the EOS in 1995 was conducted later in the breeding season and may have resulted in
Fig. 1. Lake Clark National Park coastline surveyed (dotted line shows survey route) during bald eagle productivity surveys, 1992-1996.
Table 1. Terminology used for estimating bald eagle productivity during the Early Occupancy Survey (EOS) and Late Productivity Survey (LPS) on the Lake Clark National Park and Preserve coastline, 1994-1996 1. Terminology Survey Type Definition Occupied EOS A nest with fresh nesting material or 2 adults near or at the nest. Active EOS A nest with eggs, young, or an adult in an incubating posture observed. Empty EOS A nest that is without an adult incubating or with only 1 adult near the nest. S1 LPS A successful nest with 1 young. S2 LPS A successful nest with 2 young. S3 LPS A successful nest with 3 young Failed LPS A previously occupied or active nest that is empty or eaglets have died. Not Found EOS, LPS A nest that is not found during the survey. Not Checked EOS, LPS A nest that is not checked during the survey. Terminology derived from Bowman (1992) and Postupalsky (1974).
Table 2. Early Occupancy Survey (EOS) and Late Productivity Survey (LPS) dates for coastline of Lake Clark National Park and Preserve, 1994-1996. Survey Type Year Date of Survey Pilot/Observer EOS 1994 May 19-26 Alsworth/Bennett LPS 1994 July 26-27 Alsworth/Bennett EOS 1995 June 1-2 Alsworth/Bennett LPS 1995 July 25-27 Alsworth/Bennett EOS 1996 May 10 Alsworth/Bennett LPS 1996 August 7 Alsworth/Kralovec an underestimate of occupied and active nests (See A. Bennett, NPS, 1995 Progress Report). Nests that were previously occupied or active may have failed by this date and adults moved away from the nest. Results from this survey should not be used to estimate 1995 bald eagle productivity. However, this information can be used to look at broad changes in productivity and bald eagle presence/absence at particular nest sites. Kralovec (1994) determined that bald eagle presence and attendance at the nest site did not decline after nesting failure. Although, adults that failed at nesting tended to perch further from the nest trees than those that succeeded (Kralovec 1994). The 1996 LPS survey, conducted in early August, may have occurred after some young had fledged. Fledgling bald eagles are more difficult to count as they move away from the nest and blend in with the surrounding vegetation. Several nests (45%) that were either active or occupied during the 1996 EOS were found empty during the 1996 LPS. Conversely, in 1994 and 1995 50% and 35%, respectively, of nests also were found empty. LPS surveys during 1994 and 1995 were conducted in late July before most young in southwestern Alaska have fledged (Bowman 1992). Only with continued observation of bald eagle breeding phenology will exact fledgling dates be determined for the Lake Clark National Park coastline. Forty-five, 48, and 50 nests were known in 1994, 1995, and 1996, respectively. Of these, 3 of 45 (7.0%) in 1994, 1 of 48 (2.0%) in 1995, and 4 of 50 (8.0%) in 1996 were not found during the EOS. Nineteen, 14, and 22 nests in 1994, 1995, 1996, respectively, were found empty during the EOS and not checked during the LPS.
Mean productivity from 1994-1996 for the Lake Clark National Park coastline eagle population was 0.8 young fledged/occupied nest and 1.44 young fledged/succesful nest, 56% of occupied nests were successful (success rate) and 60% of active nests produced one or more chicks. Sprunt et al. (1973) determined that a bald eagle population should have at least a 50% succes rate and 0.7 young fledged/occupied territory to remain stable. The success rate on the Lake Clark National Park coastline exceeded those figures for all 3 years studied and was similar to those of other areas in Alaska (Sprunt et al. 1973, Hansen 1987, Tetreau 1991, Kralovec and Yerxa 1995). The number of occupied and active nests increased from 1994 (n = 21) to 1995 (n = 30), but declined again in 1996 (n = 19). Nest success appeared to follow the same trend (Table 3). With more occupied and active nests successful in 1995 than either 1994 or 1996. It is likely that nest success was high in 1995 because the EOS survey was flown late in the breeding season. This, in turn, resulted in an underestimate of the number of occupied nests. A total of 56 young fledged during 1994 (n = 17), 1995 (n = 24), and 1996 (n = 15). Even though overall nest success was higher in 1995 than 1994 or 1996, the number of young fledged per occupied nest remained relatively consistent (Table 3). This can be attributed to the fact that those nests that did succeed in 1994 and 1996 produced more young/nest than nests that succeeded in 1995. Fifty-five and 50% of the successful nests in 1994 and 1996, respectively, produced 2 fledglings/nest. In 1995, only 26% of the successful nests produced 2 fledglings/nest. On the other hand, 74% of the 1995 successful nests produced 1 fledgling/nest. Several factors; including food supply, environmental conditions, and human disturbance; around individual nests could have contributed to this discrepancy between years. Hansen (1987) found that in areas of the Chilkat River where food was limited fewer nests were successful than in those areas where food was abundant. More likely it is a natural fluctuation in the breeding cycle caused by more viable eggs and young produced in 1994 and 1996. Several years of monitoring bald eagle productivity are necessary to identify any significant change in reproductive success. In Lake Clark National Park, the first aerial surveys of bald eagle reproduction began in 1983 (A. Bennett 1995). However, surveys were often sporadic and incomplete and the methods were not standardized. Since 1994, the methods used at Lake Clark National Park are those developed by the USFWS and used by other parks in Alaska. This systematic approach to monitoring productivity is needed so that results can be compared between years and geographic regions.
In addition, an understanding about bald eagle use of the Lake Clark National Park coastline is currently incomplete. Further work on food habits, identification of foraging areas, human disturbance near nest sites, and breeding phenology could contribute to a better understanding about bald eagles in the Lake Clark National Park ecosystem. Literature Cited: Bowman, T. D. 1992. Guidelines for conducting bald eagle reproductive surveys in coastal Alaska. Unpubl. Report, U.S.D.I., Fish and Wild. Serv., Cordova, Alaska. Gerrard, J.M., and G. R. Bortolotti. 1988. The bald eagle: haunts and habits of a wilderness monarch. Smithsonian Inst. Press, Washington D.C. 177pp. Hansen, A. J. 1987. Regulation of bald eagle reproductive rates in southeast Alaska. Ecology 68:1387-1392. Kralovec, M. L. 1994. Home range size and movements of bald eagles from Glacier Bay National Park and Preserve, Alaska: with an analysis of satellite telemetry. M.S. Thesis, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 164pp. Kralovec, M. L., and R. Yerxa. 1995. Bald eagle productivity in Glacier Bay National Park and Preserve, September 1995. Unpubl. Report, U.S.D.I., Natl. Park Serv. 10pp. Postupalsky, S. 1974. Raptor reproductive success: some problems with methods, criteria, and terminology. Pages 21-31 in F. N. Hammerstrom, Jr., B. E. Harrel, and R. R. Olendorff, eds. Management of Raptors. Raptor Res. Found., Vermillion, S.D. Stalmaster, M. V. 1987. The bald eagle. Universe Books, New York, 227pp. Tetreau, M. 1991. Bald eagle nest surveys in Kenai Fjords National Park, 1986-1990. Unpubl. Rep. U.S. D.I., Natl. Park Serv. 21pp.
Table 3. Year of survey, number of occupied and active nests, percent of occupied nests successful (percent nest success), percent of active nests successful, number of young/occupied nest, and number of young/successful nest for Lake Clark National Park and Preserve, 1994-1996. Year of No. Occupied % Occupied Nests % Active Nests No. Young/ No. Young/ Survey and Active Successful Successful Occupied Successful Nests Nest Nest 1994 21 52.0 52.0 0.81 1.55 1995 30 63.0 66.0 0.80 1.26 1996 19 53.0 63.0 0.79 1.50