If you live in any close proximity to the Delaware Bay, there is one question that you will see a lot of this time of year on birding and bird photography Facebook pages: where have Red Knots been sighted lately? The short answer, at least for the moment, is “nowhere.” It will be a little while yet before they really arrive. But as those asking the question will discover in the coming weeks, the longer answer is quite complex. While most birders are probably, at the very least, tangentially aware of the imminent arrival of Red Knots on the Delaware Bay, many may not realize that exactly where they descend and for how long they remain there may differ dramatically from year to year. With so many birds, we tend to forget about the specific things that drive changes in status and distribution on a local level. Because Red Knots and other migrant shorebirds using the Delaware Bay shores are conspicuous and well-studied, we actually know quite a lot about what determines these local vagaries, from year-to-year and even within a season.
To begin with, we need to know just a bit about the basic biology and ecology at work here. Red Knots and other long-distance shorebird migrants—in particular, Semipalmated Sandpipers and populations of Ruddy Turnstones and Sanderlings that are long-distance migrants—arrive emaciated and need a lot of food quickly. They find this in the form of easy to digest horseshoe crab eggs. Because of their physical condition and feeding styles, they need eggs that do not require great energy expenditure to find. Because of this, there a some perennially favored conditions for feeding. Shallow harbors with dramatic tidal flow are excellent. Feeding is easy in these areas, as spawning horseshoe crabs tend to be dense and the long periods of rising and falling tides ensure a long period of time with exposed habitat.
However, there are many other habitat conditions that can be excellent some years and not in others. Storms easily rearrange fragile bayfront habitat, creating large sandbars and mudflats where previously none existed or, conversely, narrowing formerly wide and productive beaches. Yearly differences in tidal pull can also have outsized impacts. Some years the prevailing winds and timing of the full and new moons may conspire to keep tides from ever falling low enough to produce extensive habitats, while in other years the high tides may be so limited as to present birds with near constant supplies of food—though its significant to note that the latter condition is rarer nowadays. Then there are the crabs themselves. Horseshoe crabs may have difficulty spawning in rough waters and may delay their activities when water temperatures are cold. Therefore, lingering cold snaps and periods of long, deeply unsettled weather may create subpar conditions across the whole bay rather than just at a handful of beaches.
Birders seeking to find Arctic-bound shorebird on the Delaware Bay are thus thrust into the complicated and fickle world of local bird status. Very often it is easy to focus on what is happening on a broad scale: we know, for instance, that Black Vultures are sweeping over New England and Pine Siskins and Purple Finches continue to march north across the entirety of the Northeast and Midwest. However, the factors that influence what is happening within a very small subset of states, or even a few counties within a few states, can be just as complicated and just as rewarding to untangle.
Shorebirds are an excellent study in this, but the same could be true for many species. As spring migration peaks across much of the continent, pay attention to what is happening at your local hotspots. Where are the best migrant traps this year? The handful of traditional old spots? Or are there new areas bringing in the high counts? Then think about why. Everything from the direction of the prevailing winds to what trees are in bloom to what the local rainfall totals have been like could be at play. The more we learn about what we are encountering locally, the better equipped we become to understand what’s happening now across the continent. Good birding!