We always like to have our personal myths of what constitutes the first true ‘sign’ of Spring. Frogs croaking in the marsh, Robins gamboling around the lawn, or Bluebirds singing from a nest box, each of these acts as a comfortable reassurance that winter is finally on the wane. But most of our signs of Spring are presaged by some subtle physical and biological changes, and it’s these changes that really herald the change of the seasons. Since I do a lot of wildlife surveys, I cue into these markers for when the seasons switch over, so that I can stop running winter surveys and start noting Spring phenomena. It makes for a richer Spring when you start to notice all of these changes and how they interlock.
Here in the midwest, there are several fundamental features in the changing landscape that allow birds and other animals to transition between Winter and Spring. The most profound are (1) Water un-freezing, (2) Ground thawing, and (3) Tree budding and flowering. Each of them has a rather dramatic impact on shaping the arrival of Spring migrants. Let’s look at each of them.
Ice Break-up. This is one of the most traditional markers of Spring in far northern latitudes, and it can still hold sway here in Ohio. Until the ice-cover starts to dwindle on lakes and ponds, fish and insects can’t become active, waterfowl can’t settle down (whether on migration or permanantly), and shore-feeding birds can’t visit the area. We’re all aware, especially after this year, of how much ice-cover influences the movement of migrating waterfowl. Ducks and Geese will wait a long time for ice to clear before moving north. This year’s delayed ice break-up probably pushed back waterfowl arrival dates by 2-3 weeks. Not only that, but many ‘early’ waterfowl like diving ducks didn’t stay long, but hurried through to get up to their nesting territories.
The ice thaw doesn’t just affect waterfowl. The ice-choked shores also couldn’t produce the insects that fatten migrant shorebirds, pipits, and sparrows. Several of our anomalous early arriving birds – Virginia Rails, Tree Swallows, and Red-winged Blackbirds – come to directly feed on many of these shore and near-shore insects. Other early arrivals like Savannah Sparrows and Meadowlarks start by feeding along thawed wetland margins and later move to their preferred drier fields. This can be seen especially where wetlands abut fields, like at Battelle-Darby wetlands or the Slate Run wetlands. And lest you think that these bugs need warmer temperatures to do their thing, there are lots of cold-hardy midges and flies that come out just as the water thaws. The running water of streams unfreezes even faster, so that bugs can start developing here even sooner. Louisiana Waterthrushes have become one of our earliest arriving warblers to tap into this resource.
Ground Thawing. This is another physical change that leads to a host of biological changes. As long as the surface of the ground is frozen, microbial and invertebrate activity there is minimal. With no bugs, many ground foraging sparrows and meadowlarks delay showing up. Several kinds of birds, most notably Longspurs, Horned Larks, and Pipits, like to catch bugs that are out early enough that they accidentally become chilled on patchy snowbanks. Horned Larks are especially good at exploiting these showy edges; think of how many times you’ve seen them flitting around mostly snow-covered fields. These ‘snow-edge’ foragers actually leave early to follow the receding snow north.
Activity also heats up under the ground. Think of those Robins gamboling over your lawn. They’re not there just because you use Tru-Green; they’re chasing down newly-arrived earthworms. Woodcocks, whose display flight is such a dramatic marker for Spring, can probe even deeper for worms that haven’t moved to the still-cold surface, but they need the ground at least softened. Lots of insects and worms start to become more mobile as the ground temperature heats up, and those birds that can get at them can often arrive a bit early to take advantage of this new food source. ‘Scratching’ birds like Fox Sparrows, Towhees, and Brown Thrashers are all relatively early migrants that tap into this resource; quite a few Fox Sparrows and Towhees even overwinter in southern Ohio where protected valleys can stay unfrozen.
Deciduous Tree Flowering. This is one of the most underappreciated changes at the dawn of the Season. I can tell you from forest transects that it completely changes the character of the forest. Before flowering, the woods have a few hardy gleaners like chickadees, nuthatches, and creepers, working the trunks and large branches. During and afterwards, everything changes. As the sap rises and buds swell, squirrels and sapsuckers start to tap the trees for sap flow, and bud-eating birds like Purple Finches start to move through the area. As the flowers open, insects congregate in the canopy, attracting larger numbers of insect-eating birds. First, it’s the permanent residents, like chickadees and titmice, that start foraging in the canopy. Then early migrants start to arrive and join them. Most of these early insects aren’t large or very active, so gleaning birds tend to have an advantage over aerial fly-catchers. This may be a reason why kinglets and vireos arrive well before most warblers and flycatchers.
In any year, these changes usually start sometime between mid-February and mid-March. Usually all have occurred by the 3rd week of March, which is a good concidence with the Vernal Equinox. By the following week the first waves of forest floor flowers start blooming, and we can start thinking about neotropical migrants.