Thursday, June 30, 2016

A Birds Last Minute Binge on Calcium

The structure of the avian egg has always held a keen interest of mine. Bird eggs come in a wide variety of shapes, sizes, and above all, a remarkable assortment of colors.

My only direct contact with wild bird eggs usually occurs during the breeding season, when the most conspicuous of nests have either been abandoned, or plundered by resident corvids and small mammals.    

These eggs, and pieces of egg, are not only cherished for their sheer beauty, but collected and catalogued for my own future comparative studies.

American Woodcock Nest
Photo Credit: Paul Cianfaglione

An egg in the hand creates a special feeling, a feeling which elicits a personal desire to want to learn more about them. So when I spotted Tim Birkhead’s new book, The Most Perfect Thing; Inside (and Outside) a Bird’s Egg (2016), on the shelf of a local bookstore, I was more than ready to dive right in!

A quick in-store look at the books contents revealed a strong focus on the eggs of wild birds, rather than barnyard chickens. This information alone sealed the deal. With chapters titled The Shape of Eggs, Colouring Eggs-How, Much Ado about Albumen and Yolk, Ovaries and Fertilisation, I was thrilled at the chance of having some of my most longstanding questions answered.   

But it was a new question, a question focused exclusively on the period prior to the formation of eggshell that really grabbed my attention.  How does a wild bird find the extra calcium carbonate to form eggshells, especially when they don’t usually consume it? How do hummingbirds and warblers find it? How about a bluebird?

A lack of calcium supplies can be detrimental to a bird’s reproductive success. This can lead to smaller eggs, thinner eggshells, smaller clutches and even slower chick growth (Wilson, Mary F. The calcium needs of wild birds. 2011).   

Below, Tim Birkhead discusses some of the difficulties birds face when having to find extra calcium at a short times notice.   

     During the twenty-four hours prior to the laying of her first egg of a clutch a female bird is both busy and stressed. Creating an egg requires lots of additional nutrients, but the calcium for the shell is the most difficult to obtain. This is partly because most birds don’t carry large reserves of spare calcium in their body, and depend on finding sufficient extra calcium at short notice. The problem is particularly acute for birds like hummingbirds, tanagers and swallows whose normal diet doesn’t include much calcium. A colleague of mine who studied Barn Swallows calculated that because there is so little calcium in their normal diet of flies, if they didn’t have an alternative source of calcium, a female would impossibly have to forage for as much as thirty-six hours simply to accumulate sufficient calcium to make a single egg.

     The amount of calcium needed differs between species and is obviously greater in species that produce a relatively thick eggshell or that produce large clutches, like the Blue Tit, which can lay sixteen or more eggs. Indeed, Blue Tits need to find more calcium for their eggshells than is present in their entire skeleton.

     Where does this extra calcium carbonate come from?

     It comes, ultimately, of course, from what the bird eats. If there’s plenty of calcium in a birds ordinary diet as there is with Lammergeiers that eat little other than bone, and as there is with birds of prey, owls and seabirds like guillemot, that swallow their animal prey whole, there’s no problem. The ingested calcium goes from the gut into the bloodstream, temporarily into the skeleton and then into the glands in the uterus, and from there on to the shell. If there isn’t enough calcium in the diet, then a female can draw on the calcium in her skeleton, but only a few birds do this. The Red Knot is one but it has only sufficient stored calcium for two of its four eggs, the rest comes from what it can find during the days it is forming eggshells.

     For several species of small sandpiper breeding on the Arctic tundra rodent bones are also their main source of dietary calcium. Egg-laying females acquire the bones and teeth of brown lemmings either from skeletons they find or from the pellets that skuas regurgitate after eating lemmings.

A lack of calcium in a Red Crossbills diet has also been reported to cause nest-building females to seek rodent bones in the feces of carnivores.

Other sources of calcium include millipedes and woodlice, those of which have exoskeletons that are rich in calcium. Many small birds seek out spiders, insect larvae, calcareous grit and even snail shells. 

Land Snails in Hungary
Photo Credit: Paul Cianfaglione
Land Snails in Connecticut
Photo Credit: Paul Cianfaglione
Snail shells are very interesting. For many birds, snail shells provide the main source of calcium during the egg laying period. Some species such as the Great Tit and Red-cockaded Woodpecker will go to roost with their gizzards crammed with fragments of snail whose calcium is extracted overnight and deposited on their eggshell (Birkhead, T. 2016).

Great Tit
Photo Credit: Paul Cianfaglione

Calcium has also provided important evidence for paleontologists interested in the evolution of birds. Called medullary bone, it forms as a calcium-rich layer inside the hard outer bone, and is used as a calcium source to make eggshells. The presence of medullary tissue found in the fossil bones of both Tyrannosaurus Rex and Allosaurus is clear evidence that the specimens were egg-laying females. So here we have another character that directly linked birds with dinosaurs. 

Tuesday, June 14, 2016

Hostile House Wrens

As a devoted birdwatcher, springtime is the most anticipated time of the year. Warmer temperatures, the greening of colorless landscapes, and of course, the return of migrant birds.  

But spring also brings with it more work and responsibility. Whether it’s tending to the hummingbird feeder, managing invasive plant species or the placing of new native shrubs, the growing season is without question a full-time job.  

Another activity that keeps me on the go is the annual installation and monitoring of the bluebird houses. Yes, bluebirds do occasionally breed on our property, but in most seasons, these boxes end up being used by other species of birds like the graceful Tree Swallow.

Since taking over one of the boxes a four years ago, Tree Swallows have successfully bred and raised young each and every spring. That was until last May, when I made the shocking discovery of four white swallow eggs that had been pierced and dumped onto the ground.    

Tree Swallow Eggs
Photo Credit: Paul Cianfaglione

I knelt down beside the eggs, combing the area for clues and a possible motive. Was it the bluebirds? Maybe a White-breasted Nuthatch? A burst of familiar song from a clearing in the woods had suddenly revealed a possible suspect.

Buried deep in a tangle of vines, a highly agitated House Wren was keeping a close eye on my every move. Was this cute little bird the destroyer of the swallow eggs? Or did this wren, like myself, just happen to discover this unoccupied box? Seeing as I didn’t witness the carnage firsthand, I decided to give the bird the benefit of the doubt and move on to inspecting the other bluebird houses.

Toward the opposite side of the yard, I found a surprisingly similar situation. Skulking within a blackberry thicket was another House Wren, this time carrying small sticks into a freshly mounted nest box. This particular bird had quickly adopted this house as its own.

But a short time later, a White-breasted Nuthatch had the same idea, and tried to enter that very box. The nuthatch was greeted at the entrance hole by a series of loud scolding notes and a defiant squatter. The confrontation lasted until dusk, leaving the nuthatch with no option but to fly away and surrender its nest box.

House Wren in nest box
Photo Credit: Paul Cianfaglione

Today’s two House Wren observations had me wondering; is this type of behavior normal? Does this wren really deserve to be portrayed as a home wrecker, or even a murderer?

The wren’s habit of destroying the nests and eggs of other birds is well known among researchers and birdwatchers alike. Once the male lays claim to its breeding territory, a series of potential nest sites will become established. During that time, he will also actively search and eliminate any nearby competitor, either by taking over an existing nest or by simply removing their eggs. In my neck of the woods, victims of the House Wren have included the Black-capped Chickadee, Tree Swallow and Eastern Bluebird.   

L. Scott Johnson of Maryland’s Towson University, who has studied the species for over twenty-five years, states that there is no hard evidence to imply that the House Wren is causing widespread nest failure in other bird species. But he does acknowledge the wren’s participation in the destruction of nests and eggs, even open-cup nesting birds.

After mating, the female will choose a nest and lay her eggs. This is when something incredible happens. As soon as eggs become part of the male House Wrens life, the aggressive behavior will magically disappear, as if a light switch had been shut off in its mind. Interestingly, this change in aggression may have evolved as a safety mechanism to keep wrens from inadvertently destroying their own eggs (Di Silvestro, R. House (Wrecking) Wrens, 2010).  

Amazingly, this abrupt change of behavior was confirmed right before my very own eyes, where a Tufted Titmouse is now found nesting side-by-side with the wren. Both species, at the moment, are watching over and feeding young!

Tufted Titmouse Nest Hole in Tree
Photo Credit: Paul Cianfaglione
Nest Box With House Wren
Tufted Titmouse Nest Hole In Background
Photo Credit: Paul Cianfaglione

The aggressive pre-egg behavior of the Marsh Wren is also well documented (see post titled; The Real Angry Birds, Marsh Birds Fragile Relationship). Marsh Wrens conduct similar raids on neighboring wetland birds to lessen the competition for food resources, and to possibly expand their breeding territory.

Wednesday, June 1, 2016

The Avian Tongue

As I get older, I'm starting to realize, that my birdwatching habits are gradually changing. No longer do I feel the need to chase after local rarities or visit places popular with the birding masses. 

Instead, I find happiness in tranquil environments, patiently observing birds in their natural habitat. These habitats include forests, marshes and grasslands, but it also includes my own backyard, where upwards of twenty species of nesting birds call home. 

One of these species is the Ruby-throated Hummingbird, the only hummingbird native to the eastern United States. They can be spotted around my yard feeding on flowers, snapping up insects in air and drinking sap from sapsucker wells.

However its favorite source of nectar comes from my “artificial” hummingbird feeder, where two pairs of prospective breeders have claimed as their own.

Ruby-throated Hummingbird
Photo Credit: Paul Cianfaglione

The aggressive defense of my suburban yard has led to many interesting observations, including how these little sprites attain food.

Viewed best through my sugar water feeder, hummingbirds use their long tongues to extract nectar, which can be stuck out to about the same length as its bill. 

The tongue of a hummingbird is tipped with two long skinny grooves. Rather than wicking, the nectar is drawn into the tongue by the elastic expansion of the grooves after they are squeezed flat by the bill.

The tongue structure is collapsed during the time it crosses the space between the bill tip and the nectar pool, but once the tip contacts the nectar surface, the supply of fluid allows the collapsed groove to gradually recover to a relaxed cylindrical shape as the nectar fills in.

When the hummingbird squeezes nectar off its tongue during protrusion, it is collapsing the grooves and loading elastic energy into the groove walls. That energy subsequently facilitates the pumping of more nectar. (Foran, Sheila. Uconn Communications. 2015. Hummingbirds Use Tiny Pumps to Extract Nectar) Also; (Rico-Guevera and Rubega, M. 2011. Main Paper). 

With that in mind, my thoughts now turned to the tongue of other bird species. Could the tongue actually be as helpful to a species acquisition and consumption of food as its bill? If so, does the diversity of bird bills reflect the diversity of bird tongues, or do all the tongues look and behave pretty much the same way?

That’s a good question without an easy answer. Aside from woodpeckers, hummingbirds and the oddball flamingo, there really isn’t much information out there on avian tongues. In fact, it’s hard to find more than a sentence or two devoted to the subject, even in ornithological texts.

When I did look for information in ornithological texts, I found more information about the bones of the tongue, rather than the tongue itself. The bones of the tongue are collectively called the hyoid apparatus.

Common Grackle Hyoid Apparatus
Photo Credit: Paul Cianfaglione

In most birds the hyoid apparatus is fairly short. But woodpeckers have developed an exceptionally large and unusual apparatus to support their long tongues. Woodpeckers feed by probing their tongues deep into crevices in trees searching for insects.

To allow the long tongue to be extended and retracted from the mouth, the ceratobranchial and epibranchial horns of the hyoid apparatus wrap completely around the back of the skull and attach near the bones at the base of the upper mandible. (Proctor, N., P. Lynch. Manual of Ornithology: Avian Structure and Form. 1993).  

As far as the tongue goes, the woodpeckers are usually hard-tipped and pointed with barbs like a fish hook to capture insects and larvae. Another woodpecker, the Northern Flicker, uses its long, sticky tongue to lap up and eat ants. The Yellow-bellied Sapsucker has a shorter tongue, with hair-like structures for drinking sap.

An interesting side note on woodpecker tongues sees the Downy Woodpecker as an occasional visitor to hummingbird feeders to drink sugar water. How a woodpecker with a long barbed tongue can do this is quite amazing, but its diminutive size and sticky saliva make this type of feeding possible.  

Other birds such as the Cedar Waxwing, Common Grackle and Semipalmated Plover have simpler tongues that we would pretty much think of as just ordinary.

Semipalmated Plover
Photo Credit: Paul Cianfaglione
But these too have evolved some interesting features like minute backward facing barbs for holding onto insects or guiding berries down the throat.

Cedar Waxwing eating crabapple
Photo Credit: Paul Cianfaglione
Or, they may have small spines on their tongue like the Red-breasted Merganser, which help these ducks hold onto slippery fish.   

The flamingo, on the other hand, have fat tongues that they use as a tool to filter feed. Flamingos feed with their heads down, swinging back and forth, allowing water to flow through the bill.

Flamingo Feeding
Photo Credit: Paul Cianfaglione

At the same time, the tongue is moved fore and aft like a pump, straining food onto rows of horny plates that line the bill. (Ehrlich, P.R. 1988. Flamingo Feeding).

Gaining a wide-spread appreciation for the avian tongue is not something that is going to happen soon. The difficulty of observing the tongue of wild birds is obviously a big part of the problem here. But as high-speed photography becomes more commonplace in the field, we may one day see a greater interest in the study of bird tongue function.