Beginning in February 1996, a team of field ornithologists has been studying
winter bird populations on Block Island, Rhode Island, by bracketing the
traditional Christmas Bird Count (CBC) with similar counts around 10 November
(Veterans Day Count, VDC) and 20 February (Presidents Day Count, PDC). Block
Island was chosen because of its isolation from the nearest neighboring
landmasses (the RI mainland and Long Island) and because it lacks (or nearly
lacks) resident populations of many landbirds that are common year-round in
these neighboring areas. Thus, the variable occurrence of such species on Block
Island is easily interpretable as migration or dispersal, even in species for
which such behavior is difficult to measure, or even discern, on the mainland
(Downy and Hairy Woodpecker, White-breasted Nuthatch, etc.). The target date of
the November count was chosen to fall later than the bulk of stereotyped
long-distance migration by most nocturnal-migrant landbirds and many diurnal
migrants, too. Similarly, the target date of the February count was chosen as
deep into the winter as possible but still prior to the advent of stereotyped
northbound migration in most species of landbirds.
The primary original goal was to evaluate the over-winter survival or
persistence of many “half-hardy” landbird species that, although near or beyond
the normal limits of their winter ranges in southern New England, are regularly
observed there on the mid-late December CBCs. Another question was whether the
diminution of such species between December and February could be attributable,
at least in part, for some species in some years, to over-water dispersal, in
addition to mortality. Within a few years we recognized that all of the
half-hardies that are most numerous on the CBC typically persist into mid-late
February, and that their reduction in abundance is often no greater than among
the most abundant landbird species wintering in the same habitats. We also
noted a strong tendency for many species, but especially the half-hardies, to
increase in abundance between November and December, proving a general capacity
of such birds to undertake major movements long after their periods of “normal”
migration. This latter finding suggests that such movements are at least
possible later in the winter and might in fact contribute, along with
mortality, to the reduced abundance observed in February. Related to both of
these goals was the important practical question of whether the timing of the
traditional CBC is possibly too early in the winter to provide a reliable
measure of winter abundance in many species. Our conclusion (at least in the
early years of this now multi-decade project!) was a validation of the CBC for
this site: abundance on the CBC accurately predicted abundance on the PDC,
whereas abundance on the VDC varied wildly and did not predict abundance on the
CBC. Finally, the extra data and context provided by the supplementary counts
were intended to assist with studying long-term trends (for instance, related
to climate change); as hinted above, insights have been gained on this question
as well.
This year’s Presidents Day Count was conducted on 21 Feb 2022 by 17 observers
in ten parties—significantly better coverage than usual (average seven
observers in four parties). Note, however, that the 39.8 party-miles on foot
exceeded the average of 27 to a lesser extent than implied by the other
metrics, because extra effort is devoted to legwork in years with fewer
participants. An unprecedented total of 95 species was recorded, with variety
well distributed across seabirds, freshwater species, and landbirds.
All but one of the notable misses involved species that are frequently
missed on the PDC: Northern Gannet (seventh miss), American Coot (eleventh
miss), and Bonaparte’s Gull (eleventh miss). Ruddy Duck, Pied-billed Grebe,
Double-crested Cormorant, American Bittern, Great Blue Heron, and Belted
Kingfisher resemble Coot in their tendency to vanish after hard freezes, so it
was notable that all except for the grebe and kingfisher were tallied this
year—all singles, except for three Great Blue Herons. Virginia Rail is somewhat
less sensitive than the preceding, but this year’s total of six was remarkable
for the date. Freshwater ducks that can tolerate short periods of freezing were
conspicuous: Gadwall, Ring-necked Duck, and Hooded Merganser continued their
trends toward increased abundance, two rare species were found (Blue-winged
Teal and Redhead, both fourth occurrences), and various others were present in
large numbers, even though frequently missed altogether (18 Snow Goose, 36
American Wigeon, and 13 Green-winged Teal).
Had I been told ahead of time we would see four species of alcids, I
could not have imagined we’d miss Common Murre, which has been recorded on all
but two of the previous 17 PDCs. Razorbills were numerous, four Black
Guillemots was more than usual, and a trio of Dovekies and a Thick-billed Murre
were excellent finds. Horned Grebes were 22 times more numerous than on the
CBC, when just one was tallied, continuing their dismal slide toward scarcity.
A total of just 99 Red-breasted Mergansers was also consistent with their
negative trend, but the also-declining Great Cormorant hung on this year, the
total of 77 close to the long-term average of 79.
Raptors were very well represented, perhaps not surprising given the
overall diversity and abundance of landbirds. Thicket-dwelling landbirds almost
always diminish in abundance from Dec to Feb, as measured by birds per
party-mile on foot. This year, uniquely, many such species maintained their
numbers or even increased in abundance, including numerous half-hardies.
Furthermore, relatively few (compared to most years) increased between Nov and
Dec. Among thicket birds seen on all three counts, only Winter Wren, Swamp
Sparrow, and White-throated Sparrow showed the classic “half-hardy bump,” an
increase from Nov to Dec followed by decline from Dec to Feb, that we’ve come
to expect for many species in most years. It must be noted that this winter’s
cycle (Nov 2021-Feb 2022) was highly unusual. The first interval (VDC-CBC) was
strikingly open, with hardly any freezing and no snow, whereas the second
interval (CBC-PDC) featured repeated bouts of severe cold (none very prolonged,
though) and two significant snow storms. If one were to describe a year when
cold-sensitive landbirds could remain numerous and widely distributed across
southern New England through year’s end, and only later experience pressure to
disperse to coastal thickets, this was the year—the only question being whether
the severity of the end-January blizzard and subsequent intense cold were
survivable at Block Island.
Apparently they were. A Pine Warbler was the first ever for the PDC and
a House Wren just the second. Six Virginia Rails made it through the bottom of
the winter, and Yellow-shafted Flicker, sensitive to snow and cold, was more
numerous than usual. Corrected for effort, the Feb abundance of Brown Thrasher,
Hermit Thrush, and Fox Sparrow equaled that of Dec. Gray Catbirds increased 22%
over Dec, whereas Eastern Towhee increased by almost 300%. This was not because
the CBC was held on a bad day. The three count days (15 Nov, 16 Dec, and 21
Feb) were all favorable for effective bird-counting; all featured westerly
breezes of comparable intensity and seasonably mild temperatures. Furthermore,
detection rates were actually slightly higher in Dec than in Feb for the highly
sedentary Carolina Wren and the highly mobile Myrtle Warbler, just slightly
lower in Dec for Black-capped Chickadee, and almost identical for Song Sparrow.
As noted above, White-throated Sparrow was more numerous in Dec than in Feb, so
it is clear that the surprising abundance of many half-hardy species in Feb was
not due to poor detection in Dec and better detection in Feb. This conclusion
also demands that serious consideration be given to the seemingly preposterous
idea that the Dec to Feb increases observed for Northern Mockingbird (65%),
House Finch (94%), and Northern Cardinal (29%) might have been driven by
large-scale, over-water dispersal in mid-late winter! The abundance of American
Robin and Red-winged Blackbird (and also Killdeer, and perhaps some of the
freshwater ducks) probably or certainly reflected northbound migration, but
there is no body of evidence, however anecdotal, suggesting that species such
as Catbird, Hermit Thrush, or Towhee migrate northward at this date.
But perhaps the last statement must be qualified with “--yet.” All of
the long-time participants in the winter counts agree that over the past 27
years fall migration has stretched further over our November count window, and
spring migration over our February window. These questions await critical
analysis, but I can offer one metric as illustration of how much the overall
winter season dynamic has changed over the course of this study. As mentioned
above, an early aim of the work was to validate (or not) the idea that the CBC
was accurately measuring winter abundance. We concluded that it was, based in
part on the relatively strong correlation in overall landbird abundance between
December and February, and the absence of such a correlation between November
and December. Over the first 14 years of the project, the Dec-Feb correlation
was 0.76, which I think is remarkable, given all of confounding variation from
count to count in personnel and weather. In contrast, abundance in November
(presumably influenced by each year’s late fall migration) failed to predict
abundance in December, just a month later (0.10). But this dynamic has broken
down in recent years. Over the past 13 years, the Dec-Feb correlation has
diminished by more than half (to 0.34) and is now exceeded by the Nov-Dec
correlation (0.56). To state it plainly, the December count is now a late fall
count, and the February count the truer measure of winter abundance here for
most species.
To conclude, I would like to express my deepest gratitude to all of the
participants on this year’s PDC, including three people new to the winter count
project. If one dates this project to the CBC preceding the first PDC, in Dec
1995, the count we just completed was the 80th, and the 75th for me. A total of
68 individuals have devoted 720 person-days to the counts, of which no fewer
than 441 have been contributed by the 17 people participating this year.
Shai Mitra
Bay Shore, NY
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