Who's At Four-Mile-Run Park this Summer?

At Four-Mile-Run Park, you may notice that bird sounds have quieted down with the onset of hot summer weather. By now, many migratory songbirds from the tropics, such as yellow and blackpoll warblers, have headed south after fledging single nests. They’re getting a leg up (wing up!) on their long journeys to Central and South America.

But the year-round residents are still breeding during July. Watch and listen for nests with squeaky broods of sparrow hatchlings, thumb-sized broken American robins’ blue eggshells, or fat, newly fledged grackle nestlings poking around on the ground.

If you see an extra-large egg or hatchling in a nest, you’re witnessing parasitism in action. Spot evidence of a parasite: 

Brown-headed cowbird egg in black-capped vireo nest in Oklahoma. 

Photo Credit: Christine Fallon/USFWS.

Four-Mile-Run Park is one of many places where warblers and larger birds risk getting parasitized by the brown-headed cowbird (Molothrus ater). After they’ve mated with glossy black-and-brown males, the grayish female cowbirds and drop eggs into other bird’s nests. Often, a laying female cowbird punctures and tosses out one or more of the “host” nest’s eggs before adding her own. The ~220 species of potential host birds (nationally) for the brown-headed cowbird offer her ample options for the upwards of 40 eggs she'll produce in one season. 

Brown-headed cowbird nestling in Kirtland’s warbler nest. 

Photo Credit: Cassandra Waldrop/USFWS.

Female cowbirds have evolved to be strategic, spreading out their investment by laying just one or two eggs in each nest. If the targeted host bird is savvy enough to recognize the cowbird egg, she’ll eject it, such as an American Robin did HERE. But when the parasitized female fails to detect the errant egg, she feeds and raises the oversized cowbird nestling as if it were her own, often at the expense of her offspring. 

What nest is this cowbird female at 4-Mile-Run scoping out?

Photo Credit: Matt Strachan.

Why doesn’t a cowbird just build her own nest? You’d think her large baby could be better cared for by a parent of its own species. As their name denotes, cowbirds in their native range of midwestern prairies follow cows around, benefiting from insects stirred up by the herd. Historically they followed wild, roaming bison and, thus, evolved a means to continue to reproduce along the way by coopting other birds to raise their young.

So, the brown-headed cowbirds spotted this month at Four-Mile-Run are likely up to no good, well good for them but not for other bird species. Ameliorating the impacts of cowbirds on the reproduction of native birds has long been a conservation concern, prompting the formation of a North American Cowbird Advisory Council to make recommendations for managing cowbird populations. 

Historically, cowbird impacts would have been muted by their movements with bison, but cow herds are fenced in and not migratory. These days, brown-headed cowbirds threaten other bird species as far north as Alaska and south into central Mexico. 

Brown-headed cowbird males spotted at Four-Mile-Run Park.
Photo Credit: Matt Strachan.

Who’s At Four-Mile-Run Park this Summer?

Chewed Beaver Stump. Photo Credit: Kurt Moser.

If you hang around the stream, you may notice shady evidence of lumberjacks harvesting wood along its banks. Entire trees are toppled over or encircled with telltale chunks missing from their midsections. Sharpened stumps dot the landscape like giant pencils shoved eraser-first into the ground. 

But what appear to be signs of human axes at work are traces of a mammal that has inhabited Virginia for millions of years – the North American beaver (Castor canadensis). Despite beaver hunting by Native Americans followed by intensive trapping by European colonists for their pelts, beavers persist. But thanks to legal protection beginning in the late 19^th century, beaver populations have rebounded from their extirpation in Virginia to today’s beaver presence in every VA county.

Beaver dam where Hume Spring meets Four-Mile-Run.
Photo Credit: Kurt Moser.

And the engineering talents of beavers are shaping the habitats of Four-Mile-Run stream. Peering over the red bridge just before the nature trail, you’ll see a dam that beavers have been building this summer. It impounds water from Hume Springs right before it reaches the run, creating a tranquil ponded area for beaver homes. 

With their hard enameled front teeth (a feature of all rodents), beavers harvest trees for materials to build dams and lodges. Glued together with a combination of sticks, grasses, and mud, these sturdy structures usually hold up well. Although heavy rains washed out the beaver’s dam in early June, they have been gradually rebuilding it to maintain habitat for their domed lodges.

A lodge is another engineering feat; one or more underwater entrances lead to a dry platform inside for sleeping and raising litters of 3-5 young per year. A beaver chooses a mate for life, co-parenting each offspring in the lodge for about two years before sending them off to start a colony of their own. Only if a mate dies will a beaver take up with a new partner.

Beaver caught in the act of gnawing bark off wood. Photo Credit: Todd Kiraly.

This time of year, newborn beaver kits are sheltered in the lodges while older siblings explore outside. Because beavers are nocturnal, you’ll be lucky to spot one in the daytime, but they occasionally cruise around at dawn or dusk feeding on plants and adding to their construction projects. If you suddenly hear a SMACK sound, it’s a beaver slapping its tail on the water to send an alarm signal to its family that danger – likely YOU – is lurking nearby.

In addition to using trees for building, beavers eat them. They’re wholly herbivorous, feeding on leaves, bark, and twigs of riparian saplings like aspen, willow, tulip poplar, and maple. A suite of adaptations equips beavers for a stream lifestyle: ear and nose flaps to keep water out when swimming; webbed hind feet for paddling; a rudder-like tail that stores fat; and a gland that produces water repellant for their fur. And, if you hear whines, grunts, or snuffles at dusk near Four-Mile Run, you may be listening to beavers chatting! 

Beaver eating stream vegetation. Photo Credit: Matt Strachan.

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Summer Kayak Clean-ups

Spend a few hours on the water and help keep Four Mile Run litter-free this summer at one of our regular kayak clean-up events. Reserve a kayak from our fleet or bring your own. Registration for each date opens 2-3 weeks in advance. RSVP HERE

  Sunday, June 8 @ 9am
  Tuesday, June 17 @ 5:30pm
  Wednesday, June 25 @ 5:30pm
  Tuesday, July 1 @ 5:30pm
  Saturday, July 5 @ 3:30pm
  Saturday, July 12 @ 9am
  Sunday, July 20 @ 9am
  Saturday, July 26 @ 9am
  Wednesday, July 30 @ 5pm
  Saturday, August 9 @ 9am
  Sunday, August 24 @9am

See you on the water!

Who’s At Four Mile Run Park this Summer?

While collecting water samples, Four-Mile-Run intern Guy saw a female
snapping turtle selecting her nesting site. Photo Credit: Guy Cardwell

If you walk along the stream this month, you may come across a prickly prehistoric animal doing some important business. Snapping turtles are in their egg-laying season, which in Northern Virginia tends to span the month of June. Female North American snapping turtles (Chelydra serpentina) have been tracked returning to their same general nesting area year after year. Indeed, generations of snapping turtles have been nesting on land for about 90 million years, far longer than humans have been around. So, a female snapper knows exactly where she’s going, although she might be stymied by changes to the landscape from human development, storms, or other factors.

Once the female finds what she deems a suitable spot, the hard work begins. Like all turtles (with just a couple of odd exceptions), a female snapper must excavate a nest hole using her hind legs. It’s an arduous task, likely made more so by the nonnative turf grass that compacts the soil along parts of Four-Mile-Run Park. But, loaded up with seven or more eggs, she’s compelled to complete the task of dropping the eggs, one by one, into the hole. In Virginia, the record clutch size for a snapper is 55 eggs, a startling statistic that only makes sense when you consider that North American snapping turtles can grow to almost 20 inches long (two basketballs side by side). 

Female snapping turtle ejects an egg through her cloaca (reproductive opening) into her nest. Photo Credit: Todd Kiraly

This female snapper (nicknamed “Snappy”) was seen in action earlier this week, dropping what looked like ping-pong balls into her flask-shaped nesting chamber. Typically, turtles urinate while they dig to soften up the soil, but the recent rains likely served that purpose, encouraging females to nest. Like many turtle species, a North American snapper female can store sperm for weeks or months after mating, then choose to fertilize her eggs when she anticipates favorable conditions.

Once a female ejects all her eggs into the nest, she kicks soil over the hole, then smooths it over with sweeping motions of her rear end. It’s arguably the most important part of the job, since it camouflages the nest from egg-loving predators like raccoons and foxes. The incubation period for snapping turtle eggs depends on temperature; for Virginia, it’s typically 75–95-days. So, Snappy’s eggs are likely to hatch toward the end of August. A snapping turtle hatchling is about the size of a quarter with a soft, bumpy shell that hardens gradually. Watch out for the babies later this summer as they make their way from nests to Four Mile Run stream!

Hatchling North American snapping turtle. Photo Credit: D. Diaz, National Park Service.

A snapping turtle hatchling is about the size of a quarter with a soft, bumpy shell that hardens gradually. Watch out for the babies later this summer as they make their way from nests to Four Mile Run stream!

Kayak launch construction starts!

We were pleased to officially break ground on the kayak launch project yesterday afternoon at Four Mile Run with representatives from the Department of Recreation, Parks, and Cultural Activities and general contractor Landivar and Associates. And we had a good omen and sure sign of spring earlier in the week with the return of an osprey!

See the full press release, or drop by the park to see the progress!

Photo credit: Judy Lo