The Mount Tabor Invasive Plant Control and Revegetation Project and its affects on birds
INTRODUCTION
In order to reestablish a healthier native forest environment and improve the health of the watershed, a multi-year project is underway at Mt. Tabor Park to remove invasive plants. Actual removal of non-native species, the planting of native species, and other tasks began in September 2010. So as to monitor how these changes would affect the native bird species on Mt. Tabor, breeding bird and area search surveys were begun in 2009.
BES created the protocol for the point counts. Herrera Environmental Consultants, Inc. set up 24 avian point count stations and conducted the initial year of surveys. Station 10 was later deemed unsuitable and data was not collected at this location. During years two and three Audubon Society of Portland conducted the surveys (2010 and 2011). Due to the large number of point count stations that had to be surveyed between dawn and midmorning, each survey was conducted over a two-day span. Full surveys were conducted three times per year during the breeding season (May 15-June 31). The following results were based on birds recorded within 50 meters of each Point Count Station.
The BES Watershed Revegetation group designated 14 different units to be treated.
Revegetation Unit Map:
BES 3.14.12 Page 1 Point Count Station Location Map:
The above map was created by Herrera and shows where the point count stations are located. Point Count Stations are fairly evenly dispersed around Mt. Tabor in a variety of habitats.
BES 3.14.12 Page 2 RESULTS
Sixty-three species were recorded over the three-year period.
American Crow Mallard American Goldfinch MacGillivray’s Warbler American Robin Nashville Warbler Anna’s Hummingbird Northern Flicker Barn Swallow Orange-crowned Warbler Black-capped Chickadee Dark-eyed Junco “Oregon ssp” Bewick’s Wren Olive-sided Flycatcher Brown-headed Cowbird Pine Siskin Black-headed Grosbeak Pacific-slope Flycatcher Brown Creeper Red-breasted Nuthatch Band-tailed Pigeon Red-breasted Sapsucker Black-throated Gray Warbler Red Crossbill Bullock’s Oriole Rock Wren Bushtit Red-shafted Flicker Canada Goose Red-tailed Hawk Cassin’s Vireo Rufous Hummingbird Chestnut-backed Chickadee Song Sparrow Cedar Waxwing Spotted Towhee Chipping Sparrow Steller’s Jay Cooper’s Hawk Swainson’s Thrush Downy Woodpecker Townsend’s Warbler Dusky Flycatcher Tree Swallow European Starling Vaux’s Swift Evening Grosbeak Violet-green Swallow Golden-crowned Kinglet Warbling Vireo Golden-crowned Sparrow Western Scrub-Jay Hammond’s Flycatcher Western Tanager Hermit Warbler Western Wood-Pewee House Finch Wilson’s Warbler Hutton’s Vireo Pacific Wren Lazuli Bunting Yellow-rumped Warbler Lesser Goldfinch
The cumulative number of individuals of all species recorded during the surveys.
2009 2010 2011 943 870 985
BES 3.14.12 Page 3 Most Abundant Species found during 2009-2011 Surveys
6.0
5.0
4.0
3.0 Average/Site Visit
2.0
1.0
0.0
h w h w c per in in nco htit k u s ee is Finc J S e n Bu e Goldf n r Pi Hous rego own Cr Song Sparrose O American Cro Br American Robin Les Spotted Towhee
Below is a table depicting which point counts are within each Treatment Unit and what treatments have been applied in each unit between September 2010 and August 2011.
Point Reveg Count Blackberry Herb Tree Initial Seed Unit # Stations Cutting Appl Removal Planting Appl 1 1 X X X 2 15 X X 3 4 X X 4 8 X X X 5 none X 6 6 X X 7 11 X 8 12, 13, 20 X X X 9 21 X 10 22, 23 11 none X X X 12 17, 18. 24 X X X X 13 19 X X X X 14 16
Nine of the 23 point count stations were located outside the fourteen vegetation units, or in areas where restoration work will occur at a future date depending on funding (example: Unit 10 is where Point Count Stations 22 and 23 are located). Some stations are located in areas where no herbicide can be used, such as around the reservoirs. During 2011 the following count stations were located outside the 14 established Revegetation Units:
Stations: 2, 3, 5, 7, 9, 14, 16, 22 and 23
BES 3.14.12 Page 4 Comparison of Most Common Species Before and after Reveg Work Occurred 45.0
40.0
35.0
30.0
25.0
20.0
15.0
10.0 Average # of Individuals # Average 5.0
0.0
h n w h e e c i e n b he d w a Fi Ro Junco Cro k e n n n oldfinc Bushtit To ic s a Sparrow o a Creeper u c g g c G n ri ri ed Ch Ho tt d Son Ore ser o e Ame s p Brow p Ame S p Le a -c ck Pre-treatment Bird Species Bla Post-treatment
Post-treatment there is a change in which five species were the most commonly recorded, with numbers of Lesser Goldfinch increasing more so than the other species.
Abundance of the Five Most Common Species after Treatment Began
45.0
40.0
35.0 30.0
25.0 Pre-treatment 20.0 Post-treatment
15.0
AVG # of Individuals of # AVG 10.0
5.0
0.0 American House Finch Lesser Song Oregon Robin Goldfinch Sparrow Junco Bird Species
BES 3.14.12 Page 5 To examine in more detail how the project affected birdlife we looked at the most commonly detected species, American Robin, at the stations where invasive plant species received some form of treatment after the first two years of surveys. The data suggests that the majority of stations had reduced numbers of robins detected while others showed marked increases.
American Robin Numbers at Treated Sites
12
10
8
Pre-treatment 6 Post-treatment
Average # of Individuals # of Average 4
2
0 #1 #4 #6 #8 #11 #12 #13 #15 #17 #18 #19 #20 #21 #24 Point Count Stations
American Robin Numbers at Non-treated Sites
18
16
14
12
10 Pre-treatment Post-treatment 8
Average No. of Individuals 6
4
2
0 #2 #3 #5 #7 #9 #14 #16 #22 #23 Point Count Stations
BES 3.14.12 Page 6 So that one doesn’t infer that decreases in American Robin numbers were due entirely to habitat changes, a similar chart was prepared for the stations where no treatment was performed (see above). The data indicates that there were similar downward fluctuations in the number of robins at many of these locations also.
On 20 May 2011 there was a bike race on Mt. Tabor which may have caused decreased numbers of robins being detected. Station #7 and #9 located in heavily-trafficked areas (Station #7 being at the very top of Mt. Tabor) where many people congregate. How much the bike race influenced the data is questionable. There could be weather conditions or food availability issues in the broader landscape that may have played a larger role for that season.
DISCUSSION
During the three years of data collection a total of 63 different species of birds were found during the breeding season at the 23 point counts stations on Mt. Tabor. The first two seasons of surveys were conducted prior to invasive plant removal that occurred between September 2010 and April 2011. During 2009 and 2011 similar numbers of individuals were counted cumulatively, while fewer birds were detected in the interim year of 2010 (7.74 percent decrease). In 2011 more individuals were seen than either year, increasing from 2009 by 4.5 percent and 13.2 percent over 2010.
For the first two seasons the most abundant species noted was House Finch, followed by American Robin. Lesser Goldfinch came in sixth. In the third season, after work had begun, American Robin became the most abundant while Lesser Goldfinch increased enough to become the third most common.
As mentioned above the overall most abundant species was American Robin. To compare their response to habitat changes two charts were prepared: one for the point count stations inside areas where revegetation work had occurred and those where no work had been done. The findings show that there was quite a bit of fluctuation at the treated stations, some with dramatically lower numbers, others with noted increases. But it should also be noted that at the stations where no treatment was performed there were also marked decreases and increases.
The changes in bird species composition, numbers and distribution around Mt. Tabor pose a number of questions. What were individual species response to the changes from a more monotypic habitat of blackberry, English ivy, clematis, and non-native trees to a more diverse mosaic of native vegetation? Why did Lesser Goldfinch numbers increase so much? Was it due to increased open areas as one would suspect? What about other noted increases in species that were detected in lower numbers: Black-headed Grosbeak, Chestnut-backed Chickadee, Northern Flicker, Pine Siskin, and Wilson's Warbler. Several species showed declines: American Crow, American Goldfinch, and Hutton's Vireo. What species are benefitting from the brush piles which were created by volunteers? It would be interesting to determine if changes in the vegetation caused the above species to increase and decrease. It should be noted birders have commented about Hutton's Vireos being noticeably absent elsewhere in the northern Willamette Valley during 2011 and their decline may not be an isolated incidence at Mt. Tabor.
The next steps would be to make a more detailed analysis of the vegetation cover as it changed after initial treatment began. Determine what percentage and types of invasives were removed immediately around the point count stations, particularly within the 50 meter boundary. Determine which native plant species are thriving since treatment began. For example, Thimbleberry is reported to be coming back well. Look at the natural history of the species whose populations showed noticeable changes like the Lesser Goldfinch. A number of species feed heavily on Himalayan blackberry and English ivy when they are in fruit. Which species are they and how have their numbers changed with the removal of these plants. Which native plants might they switch to and are there any additional species that might be planted that are missing from the
BES 3.14.12 Page 7 current selection? Was the end goal of invasive species removal reached and what were the results?
Lastly, we just have to wait to see how the removal of invasive species and the reestablishment of native species affects the overall composition and numbers of birds over time. Native shrubs can take many years to grow and trees even longer. In the meantime continuing annual surveys will provide a long-term view of change and normalize any annual fluctuations in observations from year to year caused by weather, migration events, or other reasons.
BES 3.14.12 Page 8