Last week I found a large, speckled egg in the West Meadow at Gilsland Farm. It was an empty Wild Turkey egg, and its location in the middle of a meadow, rather than in the woods at the base of a tree, where turkeys nest, suggested that it had been someone’s dinner. To any bird, this is not an uncommon occurrence.
Many species have multiple broods per season to accommodate the risks of predation, disease, unfavorable environmental conditions, or any of the other litany of things that can go wrong in a nest. Parenting is tough! Turkeys are ground-nesting species, so this turkey embryo could have fallen prey to any number of mammals or birds that roam the forest floor in search of protein.
Why then, would any bird evolve to nest on the ground? Why does any animal not at the top of the food chain spend their lives on or in the ground? The simplest answer is that the ground is space, and nature doesn’t waste space. The more complex story is that animals have evolved fascinating preferences, abilities, and physical features that allow them to exploit the many advantages of life on the ground.
Land is such a precious resource to humans in part because we, ourselves, are ground-nesting apes. Early hominids probably transitioned to sleeping and raising young on the ground once they learned to light fires that kept nocturnal predators away. The switch from trees came with other benefits. First of all, it’s stable and safe, in a way. Herring Gulls, which have taken the opposite route lately by nesting on rooftops instead of rocky islands, may deal with fewer predators, but chicks risk falling from lethal heights. Any flightless animal nesting high up lives with that danger. If you’ve ever been startled awake by the feeling of falling, you may be experiencing an instinct leftover from our arboreal sleeping habits. Getting deeper, less interrupted sleep may have helped us evolve into the clever mammals we are today.
Humans are habitat generalists. We can live and raise young in forests, deserts, tundra, rainforest, and just about everywhere in between. For many other species, location is critically important to their success. Piping Plovers, Willets, and American Woodcocks are all technically shorebirds, yet Piping Plovers nest on sandy beaches, Willets hide their eggs in dune grass, and Woodcocks incubate on leaf litter in or near young forests. All are vulnerable to predators like foxes, but all are also close to the birds’ food sources (marine worms and crustaceans, aquatic insects, and earthworms, respectively). The young of these birds are precocial, which means that they’re up and at ’em immediately after hatching (unlike altricial chicks, like those of American Robins, which are born featherless and are entirely reliant on their parents).
This is both an adaptation to a vulnerable ground-nesting lifestyle and a reason to nest close to a food source, so that chicks don’t have to walk far to forage for themselves. Belted Kingfishers are less picky about substrate, but almost always create burrows in earthen banks near water. These locations offer proximity to food and some protection from predators.
Many rodents like Eastern Chipmunks spend much of their time underground, where temperatures are warmer in the winter and cooler in the summer. It can seem like they’re everywhere, but their burrows are located in dry, elevated areas that are safe from flooding.
If a realtor’s favorite phrase is “location, location, location,” a gardener’s might be “soil composition, soil composition, soil composition” (rolls right off the tongue!). Seventy percent of bee species native to Maine are ground-nesters, as are many species of native wasps and ants, and different species seek out different subterranean microhabitats. The 56 species of miner bees in Maine prefer loamy soils which are well-draining, yet hold their structure when pocketed with tunnels and galleries. Allegheny Mound Ants need a higher clay content to construct their large, conical mounds, which gather solar heat to incubate eggs. Great Golden Digger Wasps, an important pollinator that is not nearly as fearsome as it looks, excavate vertical nests in well-draining sandy soil. Now, when that voice in your head tells you to keep throwing grass seed in that sandy patch on your lawn, you can tell it that that’s actually great pollinator habitat!*
Taking advantage of these areas requires adaptations for dealing with the predators that will inevitably get too close for comfort. Bees and wasps use bold stripes to communicate that they’re not to be messed with (whether they can actually sting or not). Ground-nesting birds that lack adequate defences rely on camouflage. Good luck spotting a Piping Plover sitting still on dry beach sand. Willets can disappear into marsh or dune vegetation (but will make a huge commotion if you get too close to a nest). The lights and darks of American Woodcock plumage perfectly mimic the shadows and highlights of leaf litter on a dappled forest floor. This strategy extends to eggs; while cavity-nesters tend to lay white eggs (which are easier to see in a dark hole), ground-nesting birds often hatch from patterned eggs, like the white and brown speckled turkey egg that I found. These speckles and squiggles break up the outline of the egg on a textured background, making them harder to spot. Studies have shown that individual birds seem to have some awareness of their own personal camouflage and that of their eggs, and choose nesting and resting spots accordingly.
Lastly, the morphology of ground-nesting animals is influenced by their lifestyle. Woodcocks are an extreme example. They have a 360º field of vision, the largest of any terrestrial vertebrate, thanks to huge eyes that are set so high and far back on their head that parts of their brain are upside down to accommodate them. This means they don’t have to sacrifice foraging or incubating time to look out for predators. Woodcock wings are also quite short and wide. This is known as a low aspect ratio. This shape gives the birds greater maneuverability and vertical lift— good for rapid takeoff to escape predators. Think of the silhouette of a pigeon in flight and of how quickly it is in the air after you step into its square of sidewalk. Ground-dwelling mammals like chipmunks also tend to be more compact, with short limbs that are good for digging. Tree squirrels, on the other hand, are longer limbed with bushy tails that help them balance.
The list of animals that live and reproduce on the ground goes on and on, from White-tailed Deer to Ovenbirds to Red-backed Salamanders (the most numerous vertebrates in New England!). This spring, perhaps as a counter to warbler neck, spend some time looking down! You may spot an endangered Rusty-patched Bumble Bee (Bombus affinis) emerging from leaf litter, a fawn curled up in the woods, or, if you’re very lucky, an American Woodcock keeping an eye on you from the back of its head.
*This joke was borrowed from Maine State Apiarist Jennifer Lund’s excellent webinar on native wasps for the Maine Entomological Society.
