Trends in numbers of migrant birds at Thunder Cape Bird Observatory ( )

Transcription

1 Trends in numbers of migrant birds at Thunder Cape Bird Observatory ( ) 2003 Annual Report Produced for Ontario Ministry of Natural Resources Wildlife Assessment Program October 2003 Debbie S. Badzinski Bird Studies Canada / Études D Oiseaux Canada P.O. Box/B.P. 160, 115 Front St., Port Rowan, ON Canada N0E 1M0

2 TCBO Population Trends Page 1 of 19 EXECUTIVE SUMMARY This report presents the results of analyses of trends in population indices for 63 species of migrant birds at Thunder Cape Bird Observatory (TCBO) from Indices were calculated from daily estimated totals of birds passing through the designated area. This report describes field methods used to collect the data, statistical methods used to obtain population indices from raw field data, and results of trend analyses, using simple linear regression. Based on spring population trends, 81% (35/43) of species declined and only 19% increased at TCBO between Six of the declining species had statistically significant population declines. The majority of fall migrants at TCBO also had decreasing population trends from Seventy-three percent (45/62) of fall migrants declined, and only 27% increased (17/62). Twelve species had statistically significant population declines, and only one species showed a significant increase. Annual population indices were also calculated for the following species that were classified as irruptive or partial migrants: American Goldfinch, Black-capped Chickadee, Blue Jay, Evening Grosbeak, Pine Siskin, Purple Finch, Red-breasted Nuthatch, and White-winged Crossbill. As expected, population indices for these species fluctuated widely among years. However, there were some interesting patterns. For example, Blue Jays numbers tended to fluctuate on a two year cycle and spring numbers were correlated with numbers from the previous fall.

3 TCBO Population Trends Page 2 of 19 Trends in numbers of migrant birds at Thunder Cape ( ) by Debbie Badzinski Bird Studies Canada, P. O. Box 160, Port Rowan, Ontario N0E 1M0 INTRODUCTION This report presents the results of analyses of trends in population indices for 63 species of migrant birds, and 8 species of irruptive or partial migrants at Thunder Cape Bird Observatory (TCBO) from These indices were calculated from daily estimated totals of birds passing through the designated area. TCBO has been operating since From there were many changes in the methods for collecting the data that may have affected the daily estimated totals. The protocol and method for calculating daily estimated totals (ET) was fully standardized in For this reason, this report includes only data from Once components of the ET are computerized for all years, it will be possible to evaluate the effects of changes in protocol, and include data collected during the first four years in trend analyses. This report describes field methods used to collect the data, statistical methods used to obtain population indices from raw field data, and results of trend analyses, using simple linear regression. Daily estimated totals METHODS Population indices are based on daily "estimated totals" (ETs) which are estimates of the number of each species of migrant migrants present, or passing through a designated area on a given day. The ETs at TCBO is the best estimate of the number of individuals detected in the count area during the standard count period. At Thunder Cape, the procedure for determining the ET is standardized to include a fixed time for banding and observations (daybreak to 7 hours after sunrise). Details of the field protocol, and justifications for using ETs, are given in McCracken et al. (1993) and Wojnowski et al. (2000). The methods adopted at Thunder Cape are consistent with the recommendations of Hussell and Ralph (1996). Abundance indices The number of birds detected in a count area on a given day reflects many different factors, including population level (the factor to be indexed), seasonal variation, weather conditions, phase of the moon, and additional (error) variation. Factors such as time in the season or certain weather variables (e.g., the passage of cold fronts in autumn) affect the number of birds migrating on a given day, whereas factors such as cloud cover, rain, or phase of the moon affect the number of birds that stop at the station.

4 TCBO Population Trends Page 3 of 19 By modelling variation in these additional factors, variation in the counts can be reduced, which results in indices that more closely reflect true population changes for each species (see Hussell et al. 1992). This may increase the sensitivity of the analyses for detecting changes in the populations. For the present analyses, we used multiple regression to estimate relationships between daily estimated totals and various external factors, such as weather and time of year, and then used these relationships to adjust the counts and produce a population index. Factors included in the equation include date (within a season), cloud cover, wind speed and direction, temperature, and phase of the moon. For some of these variables only linear terms were used, while for others second or higher order polynomial terms were included as well. All variables were assumed to interact additively on a log scale (multiplicatively on the original scale). Indices were calculated separately for spring and fall data. For each species, analyses were restricted to the period when most of that species migrates (migration window). Migration windows were determined by excluding isolated occurrences of the species (early or late records separated by more than 4 days from the next record), then determining the middle 98% of occurrence days. In addition, for species with breeding populations at Thunder Cape, the late spring cut-off and the early fall cut-off were selected to exclude periods when more than half of individuals detected were probably resident in the area. This involved a judgement call in many cases, but analyses of the data using a variety of different cut-offs suggest that the results were not strongly affected by the precise cut-off dates. Weather data Weather data for analyses at Thunder Cape were obtained from the weather station at the Thunder Bay airport. Hourly weather variables available for analyses include ceiling height, horizontal visibility, sea level pressure, wind speed and direction, temperature, relative humidity, and cloud cover. To date, relative humidity, station pressure and ceiling height have not been used in the standard Canadian Migration Monitoring Network trend analyses, but it would be worthwhile to determine whether they may be useful predictors of bird abundance at TCBO. BSC presently has hourly weather data from Thunder Bay for the months of April to November inclusive for the years Population trends Trends in abundance indices of migrant birds at TCBO were calculated with log-linear regression for the period The slope of the regression represents an estimate of the annual percentage increase or decrease in the population, assuming the population had changed by a constant proportion each year. Trends were calculated separately for spring and fall, and then jointly for both periods using analysis of covariance (ANCOVA), assuming that changes were parallel in the two time periods. The latter assumption was tested by re-running the ANCOVA, including a separate interaction term in the equation and testing the significance of the interaction term. In the table (Appendix 1), the joint trend estimate is presented only if the seasons were not significantly different. Species were included in trend analyses if an average of at least 10 individuals were recorded per

5 TCBO Population Trends Page 4 of 19 season per year on an average of at least 5 days, and at least one individual of a species was recorded in every year of coverage. All analyses were performed using SAS computer programs. RESULTS Estimated spring, fall and combined linear trends for migrant birds at TCBO for the period are presented in Appendix 1. Graphs showing annual population indices for all migratory species are presented in Appendix 2. Population indices of species that are considered irruptive or partial migrants are shown in Appendix 3. These species are not included in population trend analyses because the whole population is not sampled every year. Rather, only the portion of the population that moves in a particular year is being monitored. These graphs are included for interest and can help track movements of irruptive species in Ontario. Spring population trends ( ). Based on spring population trends, 81% (35/43) of species have declined and only 19% have increased at TCBO between (Appendix 1). Six of the declining species (Alder Flycatcher, Black-and-White Warbler, Canada Warbler, Cape May Warbler, Mourning Warbler, Philadelphia Vireo, Tennessee Warbler) had statistically significant population declines. No species showed significant increases in spring over this period. Tennessee Warbler and Philadelphia Vireo showed the largest population declines (- 22.6%/year and -26.8%/year respectively) from Fall population trends ( ). The majority of fall migrants at TCBO also had decreasing population trends from Seventy-three percent (45/62) of fall migrants declined, and only 27% increased (17/62) (Appendix 1). In addition, 12 species had statistically significant population declines, and only one species (Black-throated Blue Warbler) showed a significant increase. Cape May Warbler and Swainson s Thrush had the largest population declines (-27.4% and -20.9% respectively). Combined spring/fall population trends ( ). Combined spring/fall trends were calculated for 39 species at TCBO. Eighty-one percent (31/38) of species at TCBO declined and only 18% (7/38) of species had positive trends from (Appendix 1). Fourteen species exhibited statistically significant (P<0.10) declines during this time period and only one species (Black-throated Blue Warbler) increased significantly. Cape May Warbler and Philadelphia Vireo showed the largest declines based on combined spring/fall indices (-24.6%, -23.6% respectively). Spring and fall trends for species that are monitored in both seasons at TCBO are highly correlated (N=47, r=0.41, p<0.01). Irruptive or partial migrants Annual population indices were calculated for the following species that were classified as irruptive or partial migrants: American Goldfinch, Black-capped Chickadee, Blue Jay, Evening Grosbeak, Pine Siskin, Purple Finch, Red-breasted Nuthatch, and White-winged Crossbill. As expected, population indices for these species fluctuated widely among years. However, there are some interesting patterns to note. Blue Jays numbers tend to fluctuate on a two year cycle and spring numbers appear to be correlated with numbers from the previous fall. Black-capped Chickadees show a similar fall pattern as do Blue Jays, but small sample sizes of chickadees in spring in most years preclude any meaningful comparisons.

6 TCBO Population Trends Page 5 of 19 DISCUSSION From , the majority of landbirds passing through TCBO declined in both spring and fall. The reasons for the preponderance of declining trends at TCBO are unknown. However, it is possible population trends may not be representative of a longer term trend because of the relatively short time short time period (8 years). The declining trends for some species at TCBO may be related to Spruce Budworm infestations. Budworm infestations in Ontario declined dramatically over the time period in question. This has likely been at least partially responsible for the significant declines noted in the budworm specialists (e.g., Bay-breasted Warbler, Cape May Warbler, Tennessee Warbler). These three species also showed declines at Long Point Bird Observatory during the period (Badzinski 2002). The preponderance of declining trends at TCBO are similar to those observed at Delta Marsh Bird Observatory in Manitoba ( : 80% declines in spring; 91% declines in fall) and Last Mountain Lake Bird Observatory ( : 75% declines in spring; 87% declines in fall). Declining trends at these three stations, which are likely monitoring birds from similar geographic origins, may be sufficient cause for concern to prompt a more thorough investigation of the potential underlying factors, particularly related to the scale and intensity of forest management activities in the north. Philadelphia Vireo showed the highest rate of decline of any species in spring and the third highest in fall, and declines in both seasons were statistically significant. Philadelphia Vireo has also declined significantly at Delta Marsh Bird Observatory (-13.5%/year) between 1993 and 2002 ( This species is ranked as very high and high in Canadian and Ontario stewardship responsibility, respectively (Dunn 1997), so trends should be closely monitored. However, long-term population trends from Long Point Bird Observatory ( ) show no significant changes in spring or fall, and short-term trends ( ) also show little change (Badzinski 2002). The Breeding Bird Survey does not adequately monitor this species (Sauer et al. 2003). Black-throated Blue Warbler is the only species that increased significantly at Thunder Cape from This species has shown similar increases at Prince Edward Point Bird Observatory and Long Point Bird Observatory in Ontario, and at Atlantic Bird Observatory in Nova Scotia ( In addition, data collected from the 2 nd Ontario Breeding Bird Atlas data show that this species has expanded its range in Ontario over the last 20 years ( Many atlas squares in the vicinity of Thunder Cape Bird Observatory have recorded breeding evidence for Blackthroated Blue Warblers in the second atlas where they were not present in the first atlas. This range expansion is likely responsible for the observed increase in numbers at TCBO, since Black-throated Blue Warblers are at the edge of their range in the Thunder Bay region. Breeding Bird Survey data also show that Black-throated Blue Warblers increased significantly from (4.0%/year), but show a non-significant increase for the period , and a nonsignificant decrease from (Sauer et al. 2003).

7 TCBO Population Trends Page 6 of 19 TCBO is also monitoring eight species of irruptive or partial migrants. While monitoring these species during migration may not be an appropriate method for population monitoring, data collected will help track movements of these species. For example, several bird observatories have documented a two-year cycle in numbers of Blue Jays, including Prince Edward Point Bird Observatory in southern Ontario and Lesser Slave Lake in Alberta ( It would be interesting to analyze data from other CMMN stations as well as data collected from Project FeederWatch to determine whether this pattern occurs across North America. Many of the irruptive/partial migrants recorded at TCBO are also monitored in winter by Christmas Bird Count and Project FeederWatch. Future studies could examine whether population indices from CMMN stations are correlated with those from Christmas Bird Counts and Project FeederWatch. ACKNOWLEDGMENTS All analyses in these reports were carried out using programs written in SAS. These programs and their associated algorithms were mostly derived from models originally developed by David Hussell and Charles Francis. Field data were gathered by the hard work of many volunteers at TCBO. Data collection and data entry for 2002 were coordinated by John Woodcock and Jody Allair. Thunder Cape Bird Observatory is jointly operated by Thunder Bay Field Naturalists, Ontario Ministry of Natural Resources Wildlife Assessment Program, and Bird Studies Canada. Special thanks the Ontario Ministry of Natural Resources for key financial support of the station. Thanks to Thunder Bay Field Naturalists, Bryan Smith (Atmospheric Environment Branch, Environment Canada) for providing weather data for TCBO, Jon McCracken (Bird Studies Canada) for editing this report, and to Audrey Heagy for providing comments on the report. Thanks also to David J. T. Hussell, and Charles Francis for their advice and leadership in this project. LITERATURE CITED Badzinski, D.S Trends in numbers of migrant birds at Long Point Bird Observatory ( ) Annual report. Unpublished report for Ontario Ministry of Natural Resources Wildlife Assessment Program. Dunn, E.H Setting priorities for conservation, research and monitoring of Canada s landbirds. Technical Report Series No Canadian Wildlife Service, Ottawa. Hussell, D.J.T., and C.J. Ralph Recommended methods for monitoring bird populations by counting and capture of migrants. Unpubl. report, Intensive Sites Technical Committee, Migration Monitoring Council. Hussell, D. J. T., M. H. Mather, and P.M. Sinclair Trends in numbers of tropical- and temperate-wintering migrant landbirds in migration at Long Point, Ontario, Pp in Hagan, J. M. III, & D. W. Johnston [Eds], Ecology and Conservation of Neotropical Migrant Landbirds. Smithsonian Institution Press, Washington, D. C. McCracken, J. D., D. J. T. Hussell, and E. H. Dunn A manual for monitoring bird migration. Long Point Bird Observatory, Port Rowan, Ontario. 65 pp. Sauer, J.R., J.E. Hines, and J. Fallon The North American Breeding Bird Survey, Results and Analysis , Version USGS Patuxent Wildlife Research Center, Laurel MD. (

8 TCBO Population Trends Page 7 of 19 Wojnowski, J.K., G.C. Gibson, A. Heagy, and D.J.T. Hussell Field protocol for monitored bird migration at Thunder Cape Bird Observatory. Unpublished report for Ontario Ministry of Natural Resources.

9 TCBO Population Trends Page 8 of 19 Appendix 1 Estimated annual percentage change in numbers of migratory birds passing through TCBO between , based on linear regression on indices derived from daily estimated totals. The combined spring/fall trend estimates are only presented if the seasons were not significantly different. These analyses match the graphs in Appendix 2. Species Spring Fall Combined spring/fall Alder Flycatcher ** ** American Pipit -1.5 American Redstart American Robin -0.2 American Tree Sparrow -6.0 Bay-breasted Warbler * Black-and-White Warbler Black-throated Blue Warbler Black-throated Green Warbler Blackburnian Warbler Blackpoll Warbler 2.3 Brown Creeper 3.3 Canada Warbler * Cape May Warbler *** *** Chestnut-sided Warbler Chipping Sparrow Clay-colored Sparrow -5.2 Common Grackle -0.9 Common Yellowthroat Dark-eyed (Slate-colored) Junco -1.8 Eastern Bluebird Eastern Kingbird Golden-crowned Kinglet -2.3 Gray-cheeked Thrush Hermit Thrush Horned Lark 6.7 Indigo Bunting 14.1 Lapland Longspur 7.5 Least Flycatcher Lincoln's Sparrow Magnolia Warbler * Mourning Warbler * Nashville Warbler Northern (Yellow-shafted) Flicker -2.8 Northern Parula Northern Waterthrush Olive-sided Flycatcher Orange-crowned Warbler Ovenbird Palm Warbler (Western) Philadelphia Vireo * * **

10 TCBO Population Trends Page 9 of 19 Appendix 1 continued. Species Spring Fall Combined spring/fall Red-eyed Vireo Red-winged Blackbird -1.3 Rose-breasted Grosbeak Ruby-crowned Kinglet Rusty Blackbird -3.8 Ruby-throated Hummingbird Savannah Sparrow 1.5 Snow Bunting -0.1 Solitary (Blue-headed) Vireo 1.4 Song Sparrow Swainson's Thrush * Swamp Sparrow Tennessee Warbler * Veery White-crowned Sparrow White-throated Sparrow Wilson's Warbler Winter Wren 5.4 Yellow Warbler Yellow-bellied Flycatcher * Yellow-bellied Sapsucker -0.6 Yellow-rumped (Myrtle) Warbler *

11 TCBO Population Trends Page 10 of 19 Appendix 2 - Graphs showing annual population indices for spring and fall, and estimated loglinear trends for each species in the analyses for TCBO ( ). The solid circles represent spring indices, while hollow triangles represent fall indices. The smaller symbols represent seasons with reduced coverage (less than 30 station days within the species migration window). Seasons in which there were fewer than 10 station-days during the migration window for a species were excluded from the trend analyses. Solid lines represent estimated population trends for spring indices, while dotted lines represent curves for fall indices. On the graphs, indices for spring and fall have been adjusted to the same mean value, thus it is not possible to tell from the graph whether a species was more abundant in spring or in fall. Some species, although recorded in adequate numbers in one season, were rarely detected in the other season and hence are graphed only for one season. Trends were estimated based on log-linear regression for the period , for both seasons. Significance of the trends can be determined from the tables in Appendix 1.

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20 TCBO Population Trends Page 19 of 19 Appendix 3 Population indices for eight species of birds classified as irruptive or partial migrants at Thunder Cape Bird Observatory. Triangles represent spring indices and squares are fall indices.