Thursday, May 26, 2016

Marsh Birds Fragile Relationship

Station 43 in South Windsor is considered by many to be one of Connecticut’s premier birding locations. A beautiful eleven-acre marsh, Station 43 is owned by the Hartford Audubon Society and serves as one of the few breeding sites in the state for the elusive Least Bittern.

On a hot, humid day late last June, I visited the marsh with hopes of catching a glimpse of this small heron. Like many of my past outings, achieving success with bitterns often entails a lot of patience as well as a bit of good luck. 

As I eagerly anticipated the appearance of a Least Bittern, I couldn’t help but notice all the other wetland species flying about me. Swamp Sparrows, Marsh Wrens and Red-winged Blackbirds were all observed to be defending territory or actively nesting in a confined area of the marsh.  I thought to myself, how do all these birds breed in such close proximity, and still manage to be successful?

Marshes are low-lying tracts of land that hold water throughout much of the year. They are dominated by thick, rooted vegetation such as cattails, bulrushes and sedges.

Inland Marsh Habitat
Photo Credit: Paul Cianfaglione

Birds that inhabit and nest in marshes are some of the least detected species amongst birdwatchers. The difficulty of entering an overgrown wetland in the heat of summer is the likely cause for this. We know even less about the intricate relationships that play out within these lush habitats. My morning observations, though trivial, may shed light on some of those relationships.

The first species that caught my attention was the Marsh Wren. Its lively song could easily be heard from the middle of the marsh. Since I couldn’t see the bird, I decided to use a call-back recording to help it come into view. Instead, the wren flew low and directly toward me. But before it could land, a Swamp Sparrow came out of nowhere and intercepted the wren, forcing it out of its territory. An agitated male Red-winged Blackbird also flew in to secure its boundary.

Marsh Wrens typically nest in isolated pairs, for good reason. This is because wrens are notorious for destroying the eggs and young of neighboring birds such as Red-winged Blackbirds, Swamp Sparrows, Virginia Rails, Sora, Least Bitterns and even other Marsh Wrens. The theory behind these destructive habits includes creating less competition for food with other marsh nesting birds and the expansion of their own breeding territory.

Playing the Marsh Wren recording brought back memories of a time when I use to conduct call-back surveys for the Connecticut DEP in Glastonbury. I was always amazed at how fast a Virginia Rail could cross a marsh, on foot, when faced with a potential intruder. One rail I remember, parked itself at my feet for over a half-hour calling angrily. Sora are known to be even more aggressive than the Virginia Rail when it comes to defending its territory.

Photo Credit: Paul Cianfaglione

The Red-winged Blackbird is by far the most conspicuous and energetic species at Station 43. Always in defense of their territorial boundaries, a minute never goes by without a chase or an aggressive response toward another bird.

Here is nice description of the male and female blackbird’s territorial habits from the Cornell Birds of North America Online;  

     Male establishes and defends territory with clearly delineated boundaries during breeding season. Boundaries may shift within a breeding season (Dickinson and Lein 1987). All activities occur within territories, but male and female also forage, engage in sexual chases, seek extra-pair copulations, and prospect for other breeding opportunities outside territorial boundaries. Defense is based on conspicuousness, song and visual display, and aggressive responses to persistent trespassers (Nero 1956b, Orians and Christman 1968, Peek 1972).

     Existence of female territoriality is controversial. Female is aggressive toward others. Some authors (Nero 1956b, Beletsky 1983a, Hurly and Robertson 1984) assert that females defend “subterritories” within territories of males, but female display perches overlap extensively (Searcy 1986) and the primary aggressive vocalization (teer song) does not deter other females from settling (Yasukawa 1990). Nests are not overdispersed, as would be expected if females were territorial (Picman et al. 1988, Yasukawa et al. 1992a). Dominance status of resident female depends on order of settlement and distance from nest when interacting with other females on male’s territory

As for my morning search for Least Bitterns, I did finally locate a single bird clinging to an alder bush.

Least Bittern
Photo Credit: Paul Cianfaglione

Many birders, including myself, raised concerns about the absence of bitterns this spring. Were they no longer breeding at Station 43?

What I found out about this year’s late discovery is provided here, courtesy of Cornell Birds of North America Online; 

     Because Least Bitterns are highly insectivorous, delayed breeding cycle may be a response to the life cycles of aquatic prey. Emergence of aquatic insects in temperate-zone wetlands peaks in Jun (Orians 1980) when these bitterns are feeding young. Later broods not definitively known, but temporal pattern of nest initiation suggests double-brooding may occur (Kent 1951, Weller 1961). If so, late Jun to mid-Jul in Iowa, with second broods larger than first, owing to the increased availability of food later in the breeding season (Weller 1961).

In addition to the increased availability of food, I’d like to suggest another benefit to delayed breeding. A delay could help bitterns avoid other nesting birds at a time when they are the most aggressive. A toned down wren or blackbird may be preoccupied with feeding nestlings, rather than territory expansion. This might be enough to reduce the chances of egg loss. 

Despite all the known conflicts associated with nesting in a marsh, there are a number of important advantages. First, situating nests in and above submerged vegetation helps protect eggs and nestlings from land-dwelling predators like raccoons. Nesting among Red-winged Blackbirds provides additional protection for birds against avian predators such as Blue Jays and Common Grackles.The abundance of aquatic prey that surrounds the nest site is essential to feeding fast growing young.

If I had to pick one word to describe the relationship between marsh nesting birds, it would have to be tenuous. Even though I singled out the aggressive breeding nature of both the Marsh Wren and Red-winged Blackbird, it is important to realize that each and every wetland species is fully capable of defending its territory, invading nests and destroying eggs to further their own breeding success.

Saturday, May 7, 2016

Deinonychus antirrhopus, sizing up their prey

115 million years ago, in what is today central Montana, an eight-meter long herbivore called a Tenontosaurus, peacefully forages along the edge of a forest clearing. Little does it realize, that off in the distance, a pack of theropod dinosaurs called Deinonychus is carefully watching it.

With stealth and calculating behavior, the raptors advance, quickly surrounding the unfortunate animal. The predators circle the two-ton adult, biting at its hind legs, looking for the right opportunity to attack. At a mere two-meters in length, the dominant male scales the defenseless dinosaur, slashing at its unprotected abdomen with hands and feet.
Image credit
Alain Bencteau
The others quickly climb on, using their long feathered forearms and stiffened tail as a counterbalance during the final takedown.

The discovery of several partial Deinonychus skeletons, together with the complete skeleton of the much larger Tenontosaurus, led paleontologist John Ostrom to conclude that these small raptors were cunning, swift-moving pack hunters. Before this, dinosaurs like Deinonychus were often thought of as lumbering, dim-witted lizards.

Deinonychus was without question a swift and clever animal in its day. But was it clever enough to avoid a tussle with a two-ton Tenontosaurus? With its light hollow bones and relatively small size, the risk of serious injury to this animal would have been extremely high. One misstep during an attack could have rendered it unable to support or defend himself. Instead, Deinonychus would have likely chosen a more manageable target. Since the fossil record shows a close anatomical connection between theropods and modern birds, I think it’s only fitting that we should look to our local hawks and eagles to better understand how these feathered dinosaurs might have behaved.  

Where I live in the northeastern part of the United States, Red-tailed Hawks are a commonly found along our roadways hunting for meadow voles. In fact, in all my years of watching birds, I have never seen this species capture anything larger than a gray squirrel or city pigeon.

Red-tailed Hawk with captured Rock Dove
Photo Credit: Paul Cianfaglione

Research on Red-tailed Hawks supports this type of prey selection of small to medium-sized mammals, medium-sized birds (largest pheasant) and reptiles. Interestingly, during struggles with mammalian prey, Red-tailed Hawks often bear the scars from bites to the toes and legs.

Cornell’s Birds of North America provides further information here; the Red-tailed Hawk is an opportunistic predator, focusing on prey up to jackrabbit size (Brown and Amadon 1968). Experimental evidence suggests that body size, activity, and “formidability” of potential prey may influence prey selection (Snyder 1975).

Another well-known raptor in North America is the Bald Eagle. Despite its very large size, the Bald Eagle is typically seen scavenging on dead prey and stealing food from other eagles, birds and mammals. When it does obtain food from direct capture, Bald Eagles prefer fish throughout range wherever and whenever available. But in the Yellowstone ecosystem in Wyoming, eagles used proportionately more birds than reported range wide; prey items and pellets comprised 43% birds of 23 species, 43% fish of 6 species and 14% of small mammals of 11 species. 

The Golden Eagle, on the contrary, is a more aggressive predator. Although capable of killing large prey such as cranes, deer and domestic livestock, this species subsists primarily on rabbits, hares, ground squirrels and prairie dogs. Of items identified in 63 eagle stomachs collected between November and March from 15 states throughout North America, 59% were rabbits, the rest were suspected wild ungulate and jackrabbit carrion. Winter diet in central Utah consisted almost entirely of black-tailed jackrabbits.

Videos shared online offer evidence of Golden Eagles capturing mammals such as deer, foxes and even a wolf. Close scrutiny of these videos suggest that many of the animals had been previously injured and released by humans. In other words, these are canned-hunts for the sole purpose of showcasing Golden Eagle kills. One video displays a bird-handler releasing an eagle to chase a small deer across a field. The eagle’s force during its capture causes the two to tumble violently onto the ground, nearly breaking the raptors neck and wing.

A second video that has recently surfaced features a Golden Eagle attacking a goat-antelope species called a Chamois. The authenticity of this video is still up for debate, but it clearly shows the eagle on the receiving end of a number of the goat’s dangerous head-butts. The attack concludes with both the eagle and goat tumbling down a rocky slope until they both separate. A visibly shaken eagle is slow to get up and may be injured.   

Again, like the Deinonychus and Tenontosaurus encounter, the Golden Eagles decision (willingly or not) to attack a horned-goat was reckless at best. Even if it was feeling pressured to provide for its young, a broken wing in this situation is a certain death sentence for both the adult and chicks. An attempt on a small coyote or goat may seem like a good idea to some people, but it does beg the question of why take the risk on dangerous prey, if there are safer options available. Isn’t it a species sole purpose in life to feed itself, mate and reproduce? Failure to fulfil just one of these responsibilities will cause a species to become rare or no longer exist.

Brian Switek’s book, My Beloved Brontosaurus; On the Road with Old Bones, New Science and Our Favorite Dinosaurs (2013), rekindles the Deinonychus pack hunting event, but also offers another theory.

{In 2007, the paleontologist Brian Roach and Daniel Brinkman of the Yale Peabody Museum reviewed the available evidence and determined that the case for a pack-hunting Deinonychus was not straightforward. Rather than working together, Roach and Brinkman hypothesized, the dinosaurs competed with each other, and the Deinonychus carcasses at the site were individuals killed in the struggle for the meaty Tenontosaurus. Naturalists have observed similar behavior among modern Komodo Dragons, each lizard working in its own interest, even as multiple animals gravitate toward the same carcass. In short, competition for fleshy morsels may have killed these Deinonychus, not a botched attempt to score a major meal}.

Like today’s eagles and hawks, Deinonychus was an exceptionally well-armed predator. With blade-like teeth, clawed forelimbs and strongly curved sickle claws on its feet, it would’ve had little problem seizing and securing much of the smaller prey it encountered. In 2011, Denver Fowler and colleagues suggested a new method by which Deinonychus and other dromaeosaurs may have captured and restrained prey.

{This model, known as the "raptor prey restraint" (RPR) model of predation, proposes that Deinonychus killed its prey in a manner very similar to extant accipitrid birds of prey: by leaping onto its quarry, pinning it under its body weight, and gripping it tightly with the large, sickle-shaped claws. Like accipitrids, the dromaeosaur would then begin to feed on the animal while still alive, until it eventually died from blood loss and organ failure.

Image Credit
Emily Willoughby

Please visit Emily Willoughby’s wonderful website!

This proposal is based primarily on comparisons between the morphology and proportions of the feet and legs of dromaeosaurs to several groups of extant birds of prey with known predatory behaviors. Fowler found that the feet and legs of dromaeosaurs most closely resemble those of eagles and hawks, especially in terms of having an enlarged second claw and a similar range of grasping motion. However, the short metatarsus and foot strength would have been more similar to that of owls. The RPR method of predation would be consistent with other aspects of Deinonychus's anatomy, such as their unusual jaw and arm morphology. The arms were likely covered in long feathers, and may have been used as flapping stabilizers for balance while atop a struggling prey animal, along with the stiff counterbalancing tail} (Source: Wikipedia).  

I fully agree with the idea that Deinonychus was an active and successful predator. But its continuous portrayal as a big game hunter, both in writings and in paleo-art, is difficult for me to accept. Instead, I envision Deinonychus as a solitary hunter of the thick underbrush, in an environment that was out-of-the-reach of larger predators. A niche which featured plenty of insects, small mammals and reptiles to prey upon. Deinonychus could also modify its behavior into a shameless scavenger, feeding communally when the opportunity arose.

One piece of evidence that I believe dismisses the idea that small feathered dinosaur’s routinely attacking larger prey, is the famous Mongolian fossil called “The Fighting Dinosaurs”. Discovered in the Gobi Desert in 1971, this three-dimensional fossil remarkably captures Velociraptor (a close relative of Deinonychus) and Protoceratops locked in combat.

Image Credit
Why did the turkey-sized Velociraptor attack the similar sized Protoceratops is anyone’s guess? Could it be that Velociraptor was targeting its young and was cut off in its tracks by the mother? At any rate, the fossil clearly shows the Protoceratops chomping down and breaking the right arm of Velociraptor, an outcome that no one would have ever predicted. Unless of course, you are an avid hawk watcher.