Excluding March, the campus surveys recorded 124 species (another 19 were recorded in Dearborn off-campus). This is lower than the previous ten-year average of 130 species. The peak day was 3 May with 79 species. This is a similar to our usual peak day total of 80 species, but usually the peak occurs much later in the month (around May 16-20). Both the low species totals and early peak are at least in part attributable to the extremely early leaf-out, which made visual detection of mid- and late-season migrants much more difficult.
This leaf-out -- in which leaf and flower phenology was some five weeks ahead of schedule by April -- was due to the unprecedented hot weather in March. The National Weather Service characterized March 2012 as the warmest on record and the most unusual month in Detroit's history. The average temperature for March was 50.7 F, an incredible 15.6 degrees above the 1874-2011 mean average of 35.1F! Fortunately, the weather did cool down, with April temps right at normal, but May was the 3rd warmest on record. Thus, spring 2012 finished the warmest in southeast Michigan history.
Generally, warm periods in spring that accelerate plant growth also prompt early emergence of insects. This can result in a mis-match between insect resources and the timing of bird migration, especially for birds that are coming from wintering grounds in the tropics. These birds are prompted to migrate by changes in day length, and do not "know" that spring is advanced in the north. The same weather systems that bring warmth to the north may push migrants which have already arrived in the United States northward a little faster. This year, long-distance migrants that had yet to arrive during the early part of the season arrived right about their usual time (see Arrival dates, below).
There were some real concerns that had the hot weather continued, things would have gotten seriously out-of-whack -- you can read my blog post What does summer in winter mean for birds? for a discussion of potential ramifications. Fortunately, cooler weather in April slowed phenology down a bit. While we did have freezing overnight temperatures after the March hot spell, they did not last for many hours in the Detroit area, so insect life was not seriously diminished and it appears that migrants were able to find adequate food resources.
Spring 2012 was also quite dry...especially when compared to last year! In 2011, the precipitation total for Detroit from March through May was over 14 inches. This spring, it was less than half of that, with both April and May having below-average moisture.
While migrants dodged a resource bullet, it seems this combination of advanced plant and insect phenology and dry conditions is likely to have had some impact on breeding birds. This probably depends on how reliant various birds are on particular stages or species of insects as well as the impact weather may have had on a very small landscape scale at locations where birds nested. Perhaps we will detect some changes in the ratio of young to adult birds during fall banding that may shed some light on this. Finally, because they flowered early and pollinators were out and about, many fruiting plants also set fruit early. Fruit is a critical part of the diet of many fall migrants; we'll see if there are any changes in what birds are eating this fall.
Many of the less-common migrants we have come to expect on a nearly annual basis were not recorded this year. This includes Golden-winged Warbler, Prothonotary Warbler, Kentucky Warbler, and Summer Tanager. Acadian and Olive-sided Flycatchers and Philadelphia Vireo were also missed. The most interesting highlight was the number of White-eyed Vireo sightings: five from mid-April to mid-May. Leaf-out was so advanced by mid-May, that many birds, especially non-singing females, may have gone undetected.
I'll add a lowlight on a species I follow and mention frequently: American Crow. Of the 61 days in April and May, crows were recorded in the entire city of Dearborn on only 15 days. Reports were always of one or two birds. Crows were seen more often in March, as observations include migrating birds. In March, crows were reported on 14 days, mostly in ones or twos, with a high number of 11 birds in a flock on 13 March.
The March hot spell created a flood of reports of early migrants. Many of these are probably best viewed with caution (see my post Early Neotropical migrants for more discussion). Of the 43 migrant species for which we have adequate reliable arrival data, 31 did arrive earlier than the average date calculated through 2011. Both the median and the mode for the number of days early for these species was 2 days. Thirteen species arrived four or more days earlier than average. A few short-distance migrants had record-early dates for Dearborn:
- Tree Swallow on 18 March, previous early date 22 March
- Field Sparrow on 18 March, previous early 24 March
- Chipping Sparrow on 26 March, previous early 30 March
- Nashville Warbler on 16 April, previous early 20 April
- Canada Warbler on 4 May, previous early 5 May