Bald Eagle Occupancy and Productivity Surveys on Vancouver Island

Transcription

1 Bald Eagle Occupancy and Productivity Surveys on Vancouver Island By: Christina L. Pendergast Prepared For: Nestucca Oil Spill Trust Fund June 2004

2 Table of Contents List of Tables.i List of Figures ii Acknowledgements iii Introduction 1 Study Area.2 Methods..2 Scoring Nest Activity and Productivity Results-Activity Survey..5 Results-Productivity Survey...5 Discussion...7 Recommendations..10 Literature Cited..11

3 List of Tables TABLE 1. The number of active, old and unclassified bald eagle nests per km of shoreline for each of the five study areas on Vancouver Island.6 TABLE 2. Number of active versus successful bald eagle nests found during the activity and productivity surveys for the five study areas on Vancouver Island.7

4 List of Figures FIGURE 1. Areas of bald eagle surveys, Vancouver Island, BC, Canada.3 FIGURE 2. Total nests relative to active and successful nests (per km shoreline) in the 5 study areas..8 FIGURE 3. Number of successful nests relative to number of chicks (per km shoreline) in the 5 study areas..9 FIGURE 4. Brood size frequency of 1 chick, 2 chicks and 3 chicks in each of the five study areas 11

5 Acknowledgements This study was funded by the Nestucca Trust Fund and coordinated by the BC Ministry of Water, Land and Air Protection, and the BC Conservation Foundation (BCCF). A special thank you to Don Doyle of WLAP for securing funds for this study, for conducting all of the eagle surveys, and for creating the maps of the study areas. Thank you to Bruce Pendergast for reviewing the early drafts of the paper, and to Erica McClaren for reviewing the final drafts and for providing all of the statistical analyses. Also thanks to West Coast Helicopter for all of their flying time. I would like to thank all the volunteers who helped in surveying for the eagle nests: Paul Levesque, Erica McClaren, Sue McDonald, and Sean Pendergast.

6 Introduction Bald eagles (Haliaeetus leucocephalus) are found throughout North America along the coasts, major rivers, lakes, and reservoirs (Stalmaster 2002, Bowman 1999, Steidl et al. 1997). The highest densities of bald eagles are along the Alaskan and British Columbian coasts, with bald eagle populations on Vancouver Island thought to be on the increase (Bowman 1999, Hansen 1987). The best nesting habitat for bald eagles in British Columbia is along the marine coast, especially at or near estuaries (Blood and Anweiler 1994). The bald eagle is a top predator and availability of prey is one of the key factors it uses to choose a nest territory (Moul 1998, Stalmaster 2002, Elliott et al. 1998). Adequate food supply is a critical factor influencing breeding success. If food is limited during the early nesting stages, that nest has an increased likelihood of failure (Hunt, et al. 2002, Bowman 1999). Suitable nesting habitat is also important for breeding success and includes tall old-growth (large-diameter) trees for nesting, close to water supporting abundant prey. It is rare to find nesting bald eagles farther than 350 m from the shoreline, with most birds being within 50 m of it. (Leighton et al.1979, Hancock 1964). Bald eagle populations in North America were depressed and some were classified as endangered in the 1970s largely from the effects of DDT. Eagle numbers have increased since the ban of DDT although some populations in the lower forty-eight states were still considered threatened in 1995 (Bowman 1999). Due to the bald eagle s reliance on shore zones for breeding and foraging habitat, any disturbance near the shorelines could be detrimental to eagle populations. Potential disturbances include over-fishing, water contamination such as oil spills and logging in which nesting habitat is reduced (Moul 1998). Water contamination could result in eagles becoming ill from consuming contaminated fish or seabirds (their main food source). Human disturbance, such as building of houses and development of land may disrupt eagle nesting habitat, posing a risk of habitat fragmentation and or loss. Regular inventory of bald eagles is needed to determine if populations on Vancouver Island are stable and to provide a baseline of eagle numbers to indicate when populations are in jeopardy.

7 This study objective was to inventory the total number of bald eagle nests, the number of active nests and number of young produced within the five study areas. Determining activity and productivity estimates will help to track any trends or changes in eagle populations over the years. Comparisons of nest densities and productivity will be made between study areas. The nesting and productivity numbers collected from this study will be compared to other studies conducted in British Columbia and North America. Study Area The five study areas included (Figure 1): 1) Quatsino Sound- from Koprino Harbour to Quatsino Narrows; 2) Campbell River to Courtenay- from Comox Harbour to Discovery Passage; 3) Gulf Islands- Newcastle Island to Gabriola Island and islands in Stuart Channel (Mudge Island to Pylades Island); 4) Broken Group Islands- Broken Group Islands including Effingham Island; 5) Broughton Archipelago- and Blackfish Sound including Plumber and Pearse Islands Methods Originally, this study was going to be carried out following the routes and nests previously identified by Hodges et al. (1984) in their study of bald eagle breeding populations on coastal British Columbia. However, the surveys in the Hodges et al. (1984) study were conducted using a turbo-beaver on floats and these are thought to be less accurate than the rotor-wing aircrafts for aerial surveys (Ewins and Miller 1995). As a result, some nests could have been missed with the fixed-wing aircrafts. As well, if previously defined routes were followed, new nest locations may be missed if surveyors concentrated on locating the next nest on the maps. Due to these reasons, a standardized search was conducted without reference to previously located nests to prevent biases. Only the coastlines of the five survey areas were surveyed within 150 meters of shore (the width of the line of sight), with no inland surveys conducted. All surveys were conducted with an A-Star helicopter or Bell Jet Ranger, and each area was surveyed twice. The activity survey was done in late March/ early April to determine whether a nest found was occupied or not. The productivity survey was done

8 in early to late June to determine whether chicks were hatched from the active nests found in the activity survey. All flying and survey protocol involved the pilot and two observers. Occasionally more than one pass was made with the helicopter in activity surveys to verify nest occupancy and in productivity surveys to verify chick numbers. The length of visit at each nest was kept short to minimize disturbance. No attempts were made to scare or disturb adult eagles from nests in order to count eggs or chicks. If an adult eagle was situated near two nests, the one it was closest to was deemed the active one, and the other one as an alternate nest. If nests located in the activity survey were not relocated in the productivity survey, these data were not included in the calculation of total nest numbers. As well, nests found only during productivity surveys were not included. This was done to avoid over-estimating the number of successful nests in an area because it is much easier to identify a nest with chicks in it than one with none in it (i.e., one that was not successful). If nests found solely on the productivity survey were included, it would increase the number of successful nests but would result in a bias because during the productivity survey, the goal is to count the number of chicks not look for new nests, whether they be occupied or not. Nest locations were identified by GPS in the helicopter and later hand mapped based on the UTM co-ordinates onto 1:50,000 marine charts to assist in relocating nests for the productivity flight and future study. Shoreline kms were calculated using MWLAP ArcView GIS GOAT software. Nest locations will be digitally mapped by GIS technicians at a later date. Scoring Nest Activity and Productivity For activity surveys, nest status was defined as follows: Active: Minimum 1 eagle perched near the nest or in the nest tree; or 1 eagle perched within 50 m of a nest Incubating: Minimum 1 eagle perched on top of a nest in incubating position Old: no signs of eagles nearby an identified nest and no signs of fresh greenery material added to the nest

9 Unclassified: an unattended nest with eggs present- or a nest with fresh greenery added, no eggs and no adults present [Active and Incubating nest statuses were totalled to give the overall occupancy numbers] For productivity surveys, nest status was defined as follows: Not found: nest could not be relocated following activity flight Not active: nest not active on activity flight; no eagle seen and no chicks in the nest 0 Chicks: indicative of a possible failed nesting attempt (nest was originally defined as active/occupied) 1 Chick: one chick present in the nest 2 Chicks: two chicks present in the nest 3 Chicks: three chicks present in the nest If a nest originally defined as unoccupied or old, had chicks in it on the productivity survey, it was defined as successful. Successful nests were defined as nests successfully having one or more young at post-bandable age (approx. 51 days) during the productivity survey (Steenhof 1987). Productivity is defined as number of young fledged per occupied territory; Brood size is defined as number of young fledged per successful territory; and nest success is defined as the percent of pairs fledging 1 or more young (Steidl et al. 1997). For this study the number of chicks found in the nest during the productivity flight were estimated to be the number of chicks fledged, to assist in the productivity, brood size and nest success calculations. Statistical Analyses Chi-square analyses were used to compare the number of active nests and the number of successful nests among study areas. An ANOVA was used to compare mean productivity among the five study areas. The influence of study area on mean nest productivity was tested further using Tukey s adjustment for multiple pairwise comparisons between study areas. Means are presented and results are considered statistically significant at P<0.05.

10 Results Activity Survey Within the five study areas, a total of 213 nests were identified during the activity surveys. The Campbell River to Courtenay coast study area had the greatest number of total nests with 54, and Quatsino Sound had the least total number of nests with 29. The number of nests per study area was broken down to reveal the number of nests per km of shoreline (Table 1). Of the 213 nests found during the activity surveys, 130 were active and 48 (36.9%) of these had an incubating bird on the nest, 17 were old, 43 were unclassified and 23 were old and unclassified. The number of active nests per study area did not significantly differ (χ 2 = 4.2, P = 0.38). However, the Gulf Islands had the highest density of active nests with 0.29 per km shoreline and Quatsino Sound had the lowest density with 0.15 active nests per km shoreline. The Broken Islands had the greatest number of old nests and the most unclassified nests (Table 1). Table 1. The number of active, old and unclassified bald eagle nests per km of shoreline for each of the five study areas on Vancouver Island. Active Old Unclassifed Total Quatsino Sound Gulf Islands Broughton Archipelago Broken Islands Campbell River to Courtenay Productivity Survey Of the 130 active nests found during the activity survey, only 76 were found to have at least one chick in them during productivity surveys. The Campbell River to Courtenay coast had 24 productive nests corresponding with an 89% nest success rate. Conversely, the Broken Islands had a nest success rate of 23% (Table 2). The number of successful

11 nests was nearly significantly different among the five study areas (χ 2 = 12.4, P = 0.01). When nest success was standardized into nests per km of shoreline, the Broughton Archipelago had the most active and successful nests of the five study areas. The Broken Islands had the least number of successful nests per km of shoreline (Figure 2). Each study area had nests previously defined as old and or unclassified during the activity flight, be successful and produce chicks. Of the 213 originally identified nests, 11 (5.16%) were not relocated during the productivity flights. Table 2. Number of active versus successful bald eagle nests found during the activity and productivity surveys for the five study areas on Vancouver Island. Activity Survey Productivity Survey Quatsino Sound 18 9 Gulf Islands Broughton Archipelago Broken Islands 22 5 Campbell River to Courtenay The total number of chicks produced per km of shoreline was highest in the Broughton Archipelago, and lowest in the Broken Islands (Figure 3). Mean nest productivity significantly differed among the five study areas (F 4,130 = 3.66, P = 0.007). Of the five study areas, the Campbell River to Courtenay coast study area had the highest productivity with 1.30 chicks fledged per occupied territory and the Broken Islands had the lowest productivity with 0.36 chicks. The significant difference (P<0.05) between productivity in these two study areas accounted for the overall study area effect. No other two-study areas had significantly different nest productivity. The Gulf Islands had the highest brood size with 1.69 chicks fledged per successful territory and Quatsino Sound had the lowest brood size with 1.11 chicks. Nest success was highest in the Campbell River to Courtenay study area with 89% and lowest in the Broken Islands with a 23% nest success rate (Table 3).

12 Table 3. Productivity, brood size of successful nests, and nest success of bald eagles in the five study areas on Vancouver Island. Occupied Territory Young fledged/ Occupied_ Young fledged/ Successful Nest Success (%) (n) Territory (x) Territory (x) Quatsino Sound Gulf Islands Broughton Archipelago Broken Islands Campbell River to Courtenay Of the 76 total active nests, 40 of them produced only one chick, 34 produced two chicks, and 2 produced three chicks, yielding a total of 114 chicks. The Gulf Islands and Campbell River to Courtenay coast study areas were the only two areas that produced nests with three chicks (Figure 8). As well, the Broughton Archipelago and Gulf Island study areas both had a nest with an unhatched egg found in it during the productivity surveys. Discussion For a territory to be considered sustainable, 0.7 chicks per occupied territory must be produced (Elliott et al. 1998). Of the five study areas, Quatsino Sound and the Broken Islands were not considered sustainable due to low chick numbers, whereas the Broughton Archipelago, Campbell River to Courtenay and Gulf Island study areas were considered sustainable. These results are comparable with a five-year study in Clayoquot Sound (Moul 1998) in which 210 total active nests were found, however only 68 were successful yielding 0.45 chicks per occupied territory. Gebauer and Moul (2002) found productivity rates to vary between areas on Vancouver Island as well, with southeast Vancouver Island having the highest rate at 1.08 and Johnstone Strait having a productivity rate of A five-year study conducted by Elliott et al. (1998) found nest success of 0.27 chicks per active territory in Clayoquot Sound; productivity success of 0.95 chicks per active territory in Southeast Vancouver Island (Nanaimo to Crofton area)

13 and a productivity rate of 0.30 in Johnstone Strait. Reduced productivity in the Elliott et al. (1998) study was attributed to failure of active nests. Nest failure could be one of the reasons why in the first survey of our study, 130 nests were identified as active in comparison to the second survey where only 76 were found to have successfully produced chicks. This would suggest a possible nest failure rate of 58.5%. However this conclusion would be difficult to draw as the nests were surveyed early in the nesting phase and late in the fledgling phase due to budget constraints. If a survey was conducted corresponding with the hatching phase, a more accurate conclusion of failure numbers could be reported. There are many factors that might influence the reproductive success of an area. Geographical differences in habitat quality, human disturbance, nesting density, environmental contaminants, and prey abundance are all thought to have effects on productivity (Steidl et al. 1997). The availability of adequate food supply appears to be one of the main factors influencing productivity numbers (Elliott et al., Anthony 2001 and Warnke et al. 2002), with prey availability before and during incubation being the most critical (Steidl et al. 1997). A study by Ewins and Miller (1995) on osprey found that brood size was reduced in response to food shortage. The density of nests in an area is also thought to possibly affect productivity, as having a high density of nests could result in a decrease in food availability (Anthony 2001). Therefore an area that has a high density of nests may not necessarily be indicative of a highly productive area. The Broken Islands study area for example, had 0.23 active nests per km shoreline but had the lowest productivity of all the five areas. This corresponds with Gebauer and Moul (2002) stating that productivity numbers are lowest in wild and remote areas. A wild area such as Clayoquot Sound had a productivity of 0.36 whereas a less wild area like south-eastern Vancouver Island had a productivity of Many studies have determined eagle numbers per kilometre of shoreline, but few have determined number of active nests per kilometre of shoreline. Hodges (1984) conducted a study of the Queen Charlotte Islands and Vancouver Island, and found 0.09 active nests and 0.08 active nests per kilometre of shoreline respectively. The five areas surveyed in

14 2003 reported a significantly higher number of nests per km of shoreline ranging from 0.15 to Hodges (1984) reported that the highest densities occurred in areas of close island groupings which is consistent with the 2003 surveys where the highest number of active nests per kilometre of shoreline was found in the Gulf Islands. Greater human activity was thought to be an attributing factor to the low number of active nests found in populated areas by Hodges (1984) study. The Campbell River to Courtenay coast study area, an area with high human activity, had the second lowest concentration of nests partially supporting Hodges (1984) theory. However, the Quatsino Sound study area (relatively low human activity) had the lowest nest density per km of shoreline, suggesting factors other than human disturbance may contribute to low nest densities. Possible explanations for low nest numbers might be lack of adequate prey availability and or sustainable nesting habitat (Gende et al. 1997, Hansen 1987). High reproductive success in an area can be correlated with prey availability and habitat diversity (Hunt et al. 2002, Harmata and Montopoli 2001). A sustainable access to a variety of prey within a short distance from the nest enables more effective provisioning during the lengthy breeding season of the bald eagle (Harmata and Montopoli 2001). In areas of high productivity, sometimes three chicks are fledged from one nest, although it is considered to be rare (Warnke et al. 2002). Three chicks were fledged from one nest in the Gulf Islands and one nest in the Campbell River to Courtenay study areas, suggesting that food availability and habitat quality were able to provide for the population. Productivity may not be the best method to interpret the relative success of an area as an area with low productivity could have a brood size similar or greater than an area with high productivity (Steidl et al. 1997). The Broken Islands for example, had the lowest productivity but had the second highest brood size of the five areas and the Campbell River to Courtenay study area had the highest productivity and second lowest brood size. When this happens, nest success is the best determinant of overall productivity (Steidl et al. 1997). The Broken Islands had the lowest nest success rate of the five areas and the Campbell River to Courtenay study area had the highest.

15 Recommendations Although this study represents only one year of data, it is helpful in providing baseline data for numbers of nesting bald eagles on Vancouver Island in Bald eagles have strong nest-site fidelity which is helpful in identifying annual trends in population data (Arnett et al. 2001). Changes in occupancy and productivity may vary more readily than brood size, and therefore may be more useful in interpreting immediate responses to environmental impact(s) (Steidl et al. 1997). The Broken Islands and Quatsino Sound results suggest that eagle populations are not considered sustainable in these areas (<0.7 chicks produced), and this serves as a warning that continuous monitoring is a high priority. For future surveys, a flight during the nestling phase would be recommended to help determine the failure rate of nests as well as help with the issues of nest misclassification. Prey numbers fluctuate annually (Restani et al. 2000) therefore acrossyear comparisons would be ideal to obtain a better idea of what productivity numbers are in a given area on Vancouver Island. Frequent surveys are important to provide historical information for bald eagle populations prior to any kind of environmental catastrophe. Without historical data, no conclusions can be drawn with respect to population numbers after an environmental catastrophe has occurred.