2008 (with notes on spring 2007)
Introduction and overview
Spring 2008 was characterized by reasonably “normal” weather. Leaf-out, blooming, and insect hatch occurred pretty much on schedule. There were no severe events that damaged plant growth or diminished insect numbers — there appeared to be plenty of food for migrants. While spring was generally cooler and drier than average, this year did not have the extremes we have seen here in past years. In 2004, for example, it was the rainiest May on record, with over 8 inches recorded. In 2006, a hot late April was followed by a cool May with stalled weather systems, which ended with an abrupt move into summer heat at the end of the month. Last year had the most extreme and widespread weather in recent memory, however, and it is likely that it was 2007’s climatic events that influenced the abundance of birds this spring, rather than the current weather.
In 2007, much of the eastern part of the U.S. had record warmth in March, prompting early leaf-out of trees (and subsequent insect hatch). It was followed by a region-wide cold spell lasting much of April that included deep freezes in the southern U.S., where early migrants were already passing through. Many thousands of aerial insect foragers such as swallows and swifts perished. When later migrants arrived, they found the deciduous trees that had leafed out in March without green foliage, which had died from freezing temperatures. Canopy foliage in many southern states was reduced 50 to 80%, and complete leaf kill occurred on many trees and shrubs in some places.
Birds, therefore, had far fewer (or no) insects to feed on during their northward migration. This could have resulted in either direct mortality, delayed migration and consequently not obtaining a quality breeding territory, or poor physical condition on the nesting grounds which might have led to a failure breed successfully, or at all. This freeze also killed blooms, so seed, fruit, and nut crops were not produced. At the time, I felt we might see effects for years to come. Last April I wrote that
“[These conditions are] likely to have a profound and long-lasting impact on bird populations. [Reduced insect prey] could have serious consequences for migrants if they are unable successfully refuel during the journey north. If trees used up reserves producing the spring flush of leaves, this foliage reduction may persist throughout the summer; the concomitant reduction in insects available for nesting birds will surely impact reproductive success. For tree species in which even woody growth dies back due to a prolonged freeze, reduced foliage growth or flowering could persist for the next few years. … Birds that manage to make it through the migration and breeding seasons will not have overcome their final obstacles. Over much of the eastern U.S., the fruit crop will be significantly diminished. … Climate events like this are not unprecedented in history, of course, and bird populations have had to deal with them before. But so many species are facing other pressures that they are not adapted to deal with — large-scale habitat loss and fragmentation in particular — that it may be that the spring chill of 2007 could have very long-lasting effects.”
The RRBO fall banding summary for 2007 noted record-low numbers for nearly every species. Local weather over the breeding season was very hot and dry, and there were few autumn weather systems that might have resulted in fallouts of migrants here. But the ratio of adult to young birds in many species, including those that breed far north of here, indicate that there was low reproductive success for many species. This was also noted by other regional banding stations.
In order to most accurately census birds in spring 2008, RRBO focused on surveys rather than banding (although other logistical and practical considerations went into this decision as well). Birds were monitored using a daily standardized census route; most days also included one or two add-on routes so that the entire campus natural area was covered. The survey period went from 1 April through 15 June, with surveys being conducted on 62 of the 76 days, including every day between 14 April and 5 June.
A total of 143 species were recorded on campus this spring, with an additional 13 species recorded only off campus. The table below provides the highlights. Italics indicates observations elsewhere in Dearborn, off campus.
|Day with highest number of species||77 species on 20 May. The largest influx of new migrants occurred around 26-27 April, while a big departure occurred on 22 May|
|Uncommon or rare species||Horned Grebe (2 Apr)|
Red-shouldered Hawk (pair arrived in early, with at least one through most of the survey period; may have attempted to nest, which would be a first for Dearborn)
Bald Eagle (a pair made a half-hearted nesting attempt at Greenfield Village)
Merlin (10 May)
Virginia Rail (14 Apr)
Lesser Yellowlegs (19 Apr)
Whippoorwill (26 Apr)
Red-headed Woodpecker (6 May)
Olive-sided Flycatcher (20 and 29-30 May)
Acadian Flycatcher (17-20 May)
White-eyed Vireo (8 May)
Cerulean Warbler (13 May)
Prothonotary Warbler (8 May and 3-4 Jun)
Connecticut Warbler (27 May)
Hooded Warbler (16 May)
Summer Tanager (25-26 Apr and 2 May)
Orchard Oriole (4 and 27 May)
|Record early spring arrival dates||Northern Rough-winged Swallow (9 Apr)|
Blue-gray Gnatcatcher (12 Apr)
Wood Thrush (26 Apr)
Chestnut-sided Warbler (tie, 26 Apr)
Common Yellowthroat (26 Apr)
Scarlet Tanager (26 Apr)
|Record late spring departure dates||Yellow-throated Vireo (3 Jun)|
Blue-headed Vireo (24 May)
Northern Parula (7 Jun)
Blackburnian Warbler (tie, 3 Jun)
Prothonotary Warbler (4 Jun)
Purple Finch (20 May)
Although diversity wasn’t bad, the abundance of nearly all species was very low. Kinglets are usually very common to abundant. The highest daily count for these species was very low this year (Ruby-crowned Kinglet, 6; Golden-crowned Kinglet, 23). Hermit Thrush numbers were dismal, with the most counted in one day being 6 and only 38 total counted. Other thrushes were found in good numbers. Barn Swallows and House Wrens were generally uncommon, with very few pairs remaining to nest.
Of the early warblers, an average of 9 Yellow-rumps per day (with a high count of 59) was low. High count for Black-and-White was only five, and we did not have more than 2 or 3 Common Yellowthroats on a given day. Maximum daily counts of Yellow, Black-throated Green, and Palm Warblers — usually common species — were all around five. Ovenbirds and Northern Waterthrushes were relatively slightly more numerous.
Mid-season warblers were more abundant in absolute numbers, but were many times less numerous than expected. Five species typically make up the most common warblers for much of May: Tennessee, Nashville, Chestnut-sided, and Magnolia Warblers, and American Redstart. The high number of these species was all around 12-16, except for the dramatic near-absence of Tennessee Warblers. I recall plenty of times in the past when it was hard to hear anything else except Tennessees singing from the treetops. This year, my maximum number was 4, with a total of only 29 counted.
Mid-to-late season warblers were a mixed bag. There were very few Bay-breasted and Blackpoll Warblers, less than 20 total each. But the migration caboose species of Wilson’s, Canada, and especially Mourning Warblers (22 recorded, with a high count of 4, is good for this usually elusive species) were in relatively more average numbers.
Sparrows numbers in general seemed normal, with the exception of Fox Sparrows which were slightly down. White-throated and White-crowned Sparrows were common and lingered into late May. Lincoln’s Sparrow is usually a species better censused through banding, but the total count of 16 was higher than our spring banding average of 14. Cowbird numbers were enormous, which is likely attributable to the Ford sunflower fields, as a large number now overwinter in them (around 700 last year).
These numbers do suggest a pattern. This is a fairly accurate reflection of the what species would be expected to be impacted by the freeze of spring 2007. Numbers of typically early migrating species were low, perhaps due to direct mortality or failed breeding in 2007. Mid-season migrants that are aerial feeders or canopy gleaners were also generally found in low numbers (low productivity from lack of insects on blackened foliage in the south), while those that feed close to or on the ground were more numerous (low foliage was more protected from freezes, and leaf-litter invertebrates were not as diminished by the cold).
RRBO has 15 previous years of spring survey data on paper. This is in the process of being databased, so that more accurate statistical comparison of year-to-year abundance can be examined. Time will tell if the patterns indicative of population declines linked to the spring 2007 freeze continue.
UPDATE: The June 2008 issue of the Ontario Field Ornithologists newsletter had an article entitled “Where have all the migrants gone?” by Seabrooke Leckie. It describes extremely low numbers of migrants at southern Ontario monitoring stations in spring 2008 and low numbers of young birds at station in fall 2007. This is precisely what we have observed and discussed here, and it indicates something wider than just a local phenomena. You can read this article by downloading the PDF here.
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