Kodiak National Wildlife Refuge 2004 Bald Eagle Nesting and Productivity Survey

Transcription

1 Kodiak National Wildlife Refuge 2004 Bald Eagle Nesting and Productivity Survey ANNUAL REPORT by Denny Zwiefelhofer Key Words: Bald Eagle Nesting Productivity Kodiak Island Kodiak National Wildlife Refuge 1390 Buskin River Road Kodiak, Alaska November 2004 Executive Summary

2 2 Kodiak National Wildlife Refuge (KNWR) personnel surveyed refuge bald eagle (Haliaeetus leucocephalus) nesting and productivity on 32 randomly selected stratified plots during May and August Nest occupancy was only 38% in the plots surveyed during This was down from 2003 (49%) and was the lowest nest occupancy since the record low set in 2000 of 23%. Of the 129 nests occupied in May, 63 (49%) nests were successful in producing a total of 94 young. Nest productivity results during 2004 (0.73 fledglings/occupied nest; 1.49 fledglings/successful nest) were lower than 2003 (1.1 fledglings/occupied nest; 1.64 fledglings/successful nest) and 2002 (0.87 fledglings/occupied nest; 1.63 fledglings/successful nest) but were similar to the 2001 production results (.8 fledglings/occupied nest; 1.4 fledglings/july successful nest). As in 2003, the 2004 Very High stratum sample size was increased (4 to 8 plots) in an effort to improve the accuracy of the survey. While the 129 total active/occupied nests tallied during the 2004 survey effort were less than 135 total active/occupied nests in 2003 (from 32 survey plots), it is more than the mean active/occupied nests (n=76) found in 1991, and from 28 random plots. KNWR=s increasing bald eagle population may be nearing the saturation point of available habitat and carrying capacity (Zwiefelhofer 2002). Expanded occupied nest estimates from KNWR=s 2004 bald eagle random plot surveys appear to indicate the Kodiak nesting population may be decreasing. Production parameter results dropped from the 2003 results and the record 2002 production totals. The 2004 nest success (49%) and production of young per occupied nest (0.73) is very near the minimum 50% nest success level and production value of 0.7 young per occupied nest described by Sprunt et al. (1973) as necessary in maintaining a stable bald eagle nesting population. The 2004 s production of 0.73 young per occupied nest is the lowest recorded since However, the lower production rates in 2004 maybe due in part to the later (August vs. July) production survey dates. As a result of the later survey dates, some young eagles may have attained flight capabilities and left the nest prior to completion of the survey. Introduction The monitoring of bald eagle nesting activity on the Kodiak National Wildlife Refuge (Figure 1) started in 1959 with boat surveys along the shores of Karluk Lake (Chrest 1964). Aerial surveys of all Kodiak refuge lands began in The historic KNWR bald eagle nesting population has been previously described (Hensel and Troyer 1964, Troyer and Hensel 1965, Zwiefelhofer 1990) and compared to other North American bald eagle populations (Sprunt et al ) The KNWR=s Migratory Bird Management Plan directs that all refuge lands are to be surveyed for bald eagle nesting activity at five year intervals. The next complete KNWR bald eagle nesting survey coverage is scheduled for FY07. During intervening years, stratified random plots consisting of 5 degree longitude-latitude blocks are utilized to monitor bald eagle nesting and productivity (Figure 2). Stratification of the plots was based on the number of active nest sites from 1963 to 1982 historic survey data. KNWR nest plot strata were defined as Low (mean <1 active nest/plot), Medium (mean 1 to <2 active nests/plot), High (mean 2 to <4 active nests/plot), and Very High (mean = or >4 active nests/plot). The total number of refuge plots containing bald eagle nesting activity by strata during 2004

3 3 were: Low stratum (L) - 70 plots; Medium stratum (M) - 69 plots; High stratum (H) - 47 plots; and Very High stratum (VH) - 16 plots; for a grand total of 202 bald eagle nest survey plots on KNWR. A random eight plot sub-sample was taken from each of the four strata for the 2004 survey resulting in a total sample of 32 plots (Figure 3). Methods Surveys were accomplished utilizing Office of Aircraft Safety A1-B Husky N28HY. The nest survey flights were flown by Pilot J. Patterson with RO G. Wilker as observer. The production survey flights were also flown by Pilot J. Patterson with WB D. Zwiefelhofer as observer. The initial occupancy survey flights to determine nest location and status were completed on 6, 7, and 12 May. The follow-up productivity surveys to determine the status of nests found during the May survey were completed on 3 and 7 August. All active or occupied nests within the sample plots were re-located and checked during the follow-up productivity survey in August. The surveys were flown at approximately 200 foot AGL with the passenger as the primary observer and the pilot acting as a secondary observer. Nest sites were determined to be active by the presence of an adult in an incubating stance or eggs in the nest or with an adult standing in the nest. Occupied nests had one or more of the following criteria: fresh nesting materials present, two adults actively defending the nest, or 2 adults perched in the nest or nest tree but no eggs or young visible. Empty nests contained no evidence of use and no indication of added nest material or fresh construction activity (Bowman 1990). All observations were placed on U.S. Geological Survey 1:63,360 scale topographic maps. Data are then digitized into the refuge=s GIS database. Nest locations, survey dates, nest type and status are sent to Juneau Migratory Bird Management (Raptor Management) for inclusion in the state wide database. Results A total of 244 bald eagle nests including 128 active, 1 occupied, and 115 empty bald eagle nests were located in the 32 plots surveyed resulting in a 49% May nest occupancy rate. Tree nests comprised 84% (n=109) of the active/occupied nests with ground, shrub, or cliff nests comprising the remaining 16% (n=20) of active/occupied nests from all strata. The results of the nest occupancy survey can be found in Table 1a. The expanded estimate for the number of spring occupied bald eagle nests refuge-wide is presented in Table 1b. The 129 active or occupied nests located in the random survey plots during May were rechecked in August to ascertain the number of young produced. A total of 63 nests were successful in producing 94 young for an overall nesting success of 49%, with 0.73 young per occupied nest (May) or 1.49 young per successful nest (August). Lower production rates in 2004 maybe due in part to the later (August vs. July) production survey dates. As a result of the survey delay, young eagles may have fledged from occupied nests prior to completion of the survey. Table 2a summarizes the 2004 productivity survey results by strata. Table 2b displays the expanded (90% C.I.) 2004 bald eagle nesting production estimates for KNWR. Table 3

4 4 presents KNWR bald eagle nesting and production data from 1963 to Discussion After more than 20 years of increase, the predicted stabilizing of KNWR s bald eagle nesting population, maybe close at hand. Previous annual refuge survey reports (Zwiefelhofer 1998, 1999, 2000, 2001, 2002, 2003) have suggested KNWR=s bald eagle population may be reaching carrying capacity following the same trend as Southeast Alaska=s bald eagle population which peaked in 1992 (Jacobson and Hodges, 1999). The results from the 2002 refuge wide nest census (Zwiefelhofer 2002) however, revealed the KNWR bald eagle nesting population had continued to increase by over 18% since the previous nest census conducted in 1997 (Figure 4). The 2003 survey results seemed again to indicate the start of an apparent downward trend (Figure 4) with lower nest occupancy than had been seen for several years (Zwiefelhofer 2003). The downward trend in nest occupancy continued in 2004 with the lowest rate of occupancy (38%) since the record low of 23% was set in However, other 2003 nest and production parameters were near or above average. All nest and production parameters decreased in 2004 (Table 3 and Figure 4). The 2004 nest success (49%) and production of young per occupied nest (0.73) is very near the minimum 50% nest success level and production value of 0.7 young per occupied nest described by Sprunt et al. (1973) as necessary in maintaining a stable bald eagle nesting population. The 2004 s production of 0.73 young per occupied nest is the lowest recorded since However, the lower production rates in 2004 maybe due in part to the later (August vs. July) production survey dates. As a result of the later survey dates, some young eagles may have fledged prior to completion of the survey. Data from a coastal small boat survey completed approximately week after the bald eagle production surveys were flown indicated a minimum of 8 newly fledged bald eagles in the vicinity of a sample plot in which all but 2 of the active nests appeared to fail. It is unknown if any or all of the 8 young birds had been hatched from nests in the plot. The observation of these 8 young eagles does add some credence to the speculation of pre-production survey nest departures by young in some of the survey plots and the low overall productivity results. Considering the influence of this factor could have had on the 2004 survey results, KNWR s bald eagle nesting population growth may not have stabilized and may still be slowly increasing. Monitoring of KNWR bald eagle nesting will continue in FY05 as part of the refuge=s annual inventory surveys. Literature Cited Bowman, T.D Guidelines for standardization of bald eagle productivity surveys, U.S. Fish and Wildl. Serv., Admin. Memo., Cordova, Alas. Unpubl. 8pp. Chrest, H.R Nesting of the bald eagle on the Karluk Lake Drainage Kodiak Island, Alaska. M.S. Thesis. Colorado State University, Fort Collins, CO. 72pp. Hensel, R.J., and W. A. Troyer Nesting studies of the bald eagle in Alaska. The Condor. 66(4):

5 5 Jacobsen, M.J. and J.I. Hodges Population trend of adult bald eagles in southeast Alaska, Journal of Raptor Research 33(4): Sprunt, A., IV., W.B. Robertson Jr., S. Postupalsky, R.J. Hensel, C.E. Knoder, and F.J. Ligas Comparative productivity of six bald eagle populations. Trans. Of 38th N. American Wildlife and Natural Resources Conf. Washington, D.C. p Troyer, W.A., and R.J. Hensel Nesting and productivity of bald eagles on the Kodiak National Wildlife Refuge, Alaska. The Auk. 82(4): Zwiefelhofer, D.C Kodiak National Wildlife Refuge 1998 Bald Eagle Nesting and Productivity Surveys. Annual refuge report. Unpubl. 13pp Kodiak National Wildlife Refuge 1999 Bald Eagle Nesting and Productivity Surveys. Annual refuge report. Unpubl. 13pp Kodiak National Wildlife Refuge 2000 Bald Eagle Nesting and Productivity Surveys. Annual refuge report. Unpubl. 14pp Kodiak National Wildlife Refuge 2001 Bald Eagle Nesting and Productivity Surveys. Annual refuge report. Unpubl. 15pp Kodiak National Wildlife Refuge 2002 Bald Eagle Nesting and Productivity Surveys. Annual refuge report. Unpubl. 19pp Kodiak National Wildlife Refuge 2003 Bald Eagle Nesting and Productivity Surveys. Annual refuge report. Unpubl. 11pp.

6 6 Table 1a. Results of Kodiak NWR bald eagle stratified random plot nest survey conducted on May 6, 7, and 12, NEST STRATA TOTAL NO. OF PLOTS NO. PLOTS SURVEYED OCC./ACT. NESTS EMPTY NESTS TOTAL NESTS Low Medium High Very High TOTAL Table 1b. combined. Mean number of occupied/active bald eagle nests for plots on the Kodiak NWR during May, 2004 with the 90% confidence intervals, standard error of the mean, and expanded estimate of active/occupied nests by stratum and all plots NEST STRATA MEAN 90% Con. Interval ACTIVE NESTS Upper Lower S.E. TOTAL NUMBER PLOTS EST. NUMBER OF NESTS Low Medium High Very High TOTAL

7 7 Table 2a. Results of Kodiak NWR bald eagle stratified random plot productivity survey completed on August 3 and 7, Nest Strata Nests W/0 Yg Nests W/1 Yg Nests W/2 Yg Nests W/3 Yg Total Young Low Medium High Very High TOTAL Table 2b. Mean number of young bald eagles produced per plot on the Kodiak NWR during 2004 nesting season with the 90% confidence intervals, standard error of the mean, and expanded estimate of young produced by stratum and all plots combined. NEST STRATA MEAN NUMBER OF YOUNG PER SAMPLE PLOT 90% Con. Interval Upper Lower S.E. TOTAL NUMBER OF PLOTS EST. NO. OF YOUNG IN ALL STRATA PLOTS Low Medium High Very High TOTAL

8 8 Table 3. Summary of Kodiak Bald Eagle Nest Survey Data 1963 to Survey Year Empty Nests Active: No Production Check Active W/O Yg Active W/1 Yg Active W/2 Yg Active W/3 Yg Yg/Occ. Nest Total Young ''' ' ''' ''' ''' ''' ' ''' ''' ''' ''' ' ''' '' '' ''' ' ''' ' ' ' ' ' ' ' ' ' '' ''' Note: = Complete KNWR survey coverage. = Includes Afognak, Shuyak, Whale, Raspberry, Ban, Amook, Uganik, and Spruce Islands plus the north and western coasts of Kodiak Island. = Random plot data only. Occupied and active nests are combined and reported as "active".

9 9 Figure 1. Location and boundaries of the Kodiak National Wildlife Refuge. KODIAK NATIONAL WILDLIFE REFUGE AFOGNAK Alaska Shelikof Strait City of Kodiak Refuge Boundary KODIAK ISLAND Pacific Ocean E Kilometers

10 10 Figure 2. Map of Kodiak National Wildlife Refuge=s 5x5 minute longitude/latitude blocks used as bald eagle nest survey sampling plots. Kodiak National Wildlife Refuge Bald Eagle Nest Survey Plots

11 11 Figure 3. Map locations of 2004 Kodiak National Wildlife Refuge random bald eagle nest survey plots.

12 12 Figure 4. Expanded or actual number of occupied/active bald eagle nests on the Kodiak National Wildlife Refuge by survey sampling strata Number of Nests Total Low Medium High Vhigh * * * Survey Year * - Years of refuge wide surveys - actual counts. Total=All Strata Low=Low Stratum Medium=Medium Stratum High=High Stratum Vhigh=Very High Stratum