Anne Mertens parks her Subaru in the gravel lot at the north end of Plum Island, Massachusetts, at 5:47 a.m. on a Sunday in late May. The tide is going out. She has fifty minutes of useful counting time before the mudflat she watches will be too far from shore for her scope to reach reliably.
She has been counting this site for the International Shorebird Survey since 1991.
The ISS was established in 1974 by Brian Harrington at the Manomet Center for Conservation Sciences in Massachusetts. The premise was straightforward and, at the time, unproven. If a network of trained volunteers counted shorebirds at the same sites on the same schedule across the Western Hemisphere, the resulting dataset would document the migrations of species that no professional survey could practically track.
Shorebirds present a particular methodological challenge. Most species nest at high latitudes — Arctic Canada, Alaska, the Russian tundra. Many winter in the southern hemisphere. The longest-distance migrants, including red knots, Hudsonian godwits, and white-rumped sandpipers, cover continental distances on each leg of their migration. Tracking them with conventional ornithological methods requires sampling the entire hemisphere.
The ISS protocol asks volunteers to count their assigned site every ten days during the migration windows — broadly, late April through early June for spring, and mid-July through mid-October for fall. They identify every shorebird they can to species, count or estimate numbers, and note environmental conditions.
Anne Mertens has done this approximately thirty-five times per year for thirty-five years. Her count log fills four manila folders in her kitchen.
The ISS dataset is now the most comprehensive long-term record of shorebird abundance in the Western Hemisphere. It includes data from approximately nine hundred sites in twenty-five countries, contributed by over four thousand volunteers across the project's history. The active volunteer base in any given year is closer to twelve hundred.
What the data have shown is a continent-wide decline in most shorebird species. A 2019 analysis led by Paul Smith at Environment and Climate Change Canada, using ISS data combined with the Maritimes Shorebird Survey and the Arctic Program for Regional and International Shorebird Monitoring, estimated that North American shorebird populations have declined by approximately thirty-seven percent since 1980.
Some species have done worse. Red knot, the rufa subspecies that stages at Delaware Bay and migrates from Tierra del Fuego, declined by approximately seventy-five percent between 1980 and 2010. The decline tracks closely with the collapse of horseshoe crab populations in Delaware Bay, on whose eggs the knots refuel during spring migration. The ISS counts at Delaware Bay's count sites, conducted patiently every ten days by volunteers, documented the decline as it happened.
The collapse triggered a quota system on horseshoe crab harvest in 2008, which has stabilized the crab population. The red knot population has recovered partially. The ISS counts continue.
Anne Mertens's site on Plum Island is not particularly distinguished. It does not stage continental concentrations of any species. The largest count she has ever made at the site was 1,840 semipalmated sandpipers on August 18, 2003. Most counts are in the low hundreds.
What makes her site valuable is its consistency. The same mudflat. The same observer. The same protocol. For thirty-five years.
The ISS database treats every count site as a point in a continental network. Anne's counts on Plum Island, combined with Janet Trujillo's counts at the Salton Sea in California, Mauricio Gonzalez's counts at Bahía Lomas in Tierra del Fuego, and the counts of twelve hundred other observers across the hemisphere, produce an estimate of how many semipalmated sandpipers passed through the network in any given migration window.
The model is necessarily imperfect. Sites are not randomly distributed. Detection probabilities vary with habitat, weather, and observer skill. Counts of large flocks are estimates. The Manomet team and collaborators at universities across the hemisphere have spent decades developing statistical corrections for these issues.
The corrected estimates are now sufficiently rigorous that the US Shorebird Conservation Plan, the equivalent Canadian plan, and most state and provincial wildlife agencies use them as the basis for shorebird population assessments.
Volunteers like Anne Mertens are, in a literal sense, the basis for the conservation of these species in the hemisphere.
There is an obvious problem with this. The ISS depends on volunteers showing up reliably for decades. The current active volunteer base is graying. Anne Mertens is sixty-seven. Janet Trujillo at the Salton Sea is seventy-two. Across the network, the median active volunteer is in their early sixties.
Manomet has been trying to recruit younger observers. The Shorebird Sister Schools Program connects elementary classrooms across the flyway. The Migratory Shorebird Project trains volunteers at specific high-priority sites. The ISS itself has begun offering virtual training sessions, which has lowered the entry barrier for younger participants.
Whether these efforts will sustain the network into 2050 is the open question that Manomet's program staff lose sleep over.
Anne Mertens is not thinking about this on the Sunday morning I walk with her at Plum Island. She is thinking about the small flock of black-bellied plovers that has just landed at the far edge of her count area. She raises her scope, counts them — eleven adults in breeding plumage — and writes the number in her field notebook with a pencil she has owned for an unknown number of years.
She also notes two ruddy turnstones, one whimbrel, and an estimated three hundred and forty semipalmated sandpipers feeding along the receding tideline.
These observations will be entered into the ISS database when Anne gets home. She does this within forty-eight hours of every count, a discipline she has maintained since 1991.
The database, queried decades from now by a researcher Anne will never meet, will include those numbers. The researcher will probably be more interested in the continental trend than in the specific morning. But the morning is what produced the data.
Anne packs her scope at 6:38 a.m. She has another count scheduled for ten days from now, weather permitting.
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