Species Accounts

American Tree Sparrow

By in Species Accounts

Sparrows are some of the most challenging birds to identify.  Many of them have muted, dark plumages, offering few obvious identification features.  Add the secretive nature of many sparrows and clinching an identification can be tough.

The different genera of sparrows usually have a distinctive shape and silhouette, allowing a birder to quickly home in on the right part of the field guide.  Today, we’ll concentrate on the genus Spizella.  Members of this genus are small, slim sparrows with long notched tails and a gregarious nature.

The Chipping Sparrow and the American Tree Sparrow are the two most common species of the four Spizella in our state.  Field Sparrows are far more common in the southern third of the state but seem to be gradually expanding northward in their breeding range.  Clay-colored Sparrows are vagrants that occur annually but uncommonly.

We are about to see the changing of the Spizella guard.  American Tree Sparrows are winter residents in Maine, arriving from their northerly breeding grounds in late October.  They depart in early April, northbound with nesting on their minds.  They are replaced by Chipping Sparrows, a very common breeder throughout the state that spend their winter in the southeastern and Gulf Coast states.

For a brief time in April, both species may be present at Maine.  These two species are quite similar with the usual unstreaked breast of a Spizella sparrow.  Both species have a rufous cap and a strong, dark line through the eye.  The Chipping Sparrow has a sharply defined white stripe (called the supercilium) above the eye; that area of the face is gray and less prominent in the American Tree Sparrow.  Look for a distinctive central breast in the American Tree Sparrow, lacking in Chipping Sparrow.  Sometimes ruffling of the breast feathers can obscure that spot, so checking out the bill will confirm an identification.  The bill of the Chipping Sparrow is a single color (dark in adult breeding birds, pinkish in the non-breeding season) but in American Tree Sparrow is bicolored with the upper mandible being dark and the lower mandible being light.

The breast of the American Tree Sparrow is buff-colored contrasting with the gray breast of a Chipping Sparrow.  The upper parts of American Tree Sparrow is a more vibrant brown than the dull brown uppersides of the Chipping Sparrow.

I hope this column will prove timely in allowing you to experience a vocal treat.  As the American Tree Sparrows prepare to migrate, they start to sing their understated, yet glorious, song.  It’s a clear, sweet, soft warble.  The song starts with a few clear notes and then proceeds as a mix of warbled, descending notes.  You can hear a recording of the song here: http://tinyurl.com/bnp9z4d

Hearing the song in Maine is a special treat because we only have a couple of weeks to hear it before the American Tree Sparrows are on their way north.  Give a listen around your feeder during the first two weeks of April for this delightful song.

The common name for the American Tree Sparrow is a misnomer.  This species mostly breeds north of the latitudinal treeline on the arctic tundra.  Obviously no trees are available for perching and nesting there.  The reason for the common name is that this species is superficially similar to the Eurasian Tree Sparrow (a relative of the House Sparrow), in a different family from American Tree Sparrow.

On the breeding grounds, males court females by singing a single song type.  Interestingly, males in a local area may share that particular song.  Females build a nest on the ground with no assistance from the mate.  Four to six eggs are laid and are incubated by the female.  The male does pitch in with the feeding of the young once they hatch.

[Originally published on March 31, 2013]

Red-winged Blackbird Arrival on the Breeding Grounds

By in Behavior, Reproduction, Species Accounts

Have you been awakened yet this March by a loud Con-ka-ree? The song of the male Red-winged Blackbirds is ringing through the air again.  Male redwings have been arriving back in Maine since the first of March.  Females will not appear for a month or more.  The reason for this striking difference in arrival between males and females can be understood as the result of an unequal partnership.

To understand what I mean, let’s start with a human example.  Susan and James have decided to open a business together.  They know that they will need $100,000 to get the business launched. Susan has $90,000 she can contribute to the start-up and James can only contribute $10,000.  Their partnership is essential to create the business but Susan is clearly the partner with the most to lose.  Rightly, she should have a greater voice in the operation of the business.

Now, let’s switch back to birds.  The business of birds is to make more birds.  The egg of a female is a huge investment relative to the modest cost to a male of making sperm.  Unequal partners again.  Both partners are necessary for the production of viable eggs and ultimately new generations.  The female, however, because of her far greater investment in a fertilized egg, runs the show.  She gets to choose her mate.  The males may have to sing, display, dance and even bring food the female to entice her to choose him as her business partner.  In some species, the female chooses a male as her mate based on the qualities of his territory.

Red-winged Blackbirds are good examples of birds in which the female chooses a mate by inspection of the quality of his territory.  In anticipation of the arrival of the females, males vie for territories in marshes, fields and even roadside ditches throughout the state.  Territories are relatively small, averaging about half an acre.

Not all territories are equal, however. Male and female Red-winged Blackbirds forage extensively outside the territory.  The territory is used primarily for nesting.  A high-quality territory therefore will have dense vegetation allowing nests to be effectively hidden from predators.

For the next few weeks, territorial skirmishes will be frequent among the male Red-winged Blackbirds.  Each male tries to claim the best territory he can defend.  Some males are stronger than others; the dominant birds are usually older, experienced males.  The dominants will claim the best territories and the weakest birds may have no territory at all.

Females only arrive after the territorial battles have finished.  There is no reason for them to come early so they can dally a little longer in their more hospitable wintering areas to the south.

When a female does arrive on the breeding ground, she assesses the habitats of the various territorial males.  The first female will usually choose the male with the best territory.  Subsequent females then have interesting choices to make.   Is it better to be the second female on a high quality territory or the only female on a lower quality territory?

Territory quality is so variable that it often is better for a female to bond with an already paired male who commands a particularly fine territory.  The high variability of territories gives rise to this mating system in which a male may have multiple mates.  The scientific term of this phenomenon is polygyny, meaning many females.  One dominant male may have up to a dozen females on his territory while other males may have only one mate and yet other males may be too subordinate to even have a territory and therefore have no mates.

Twelve mates or no mates?  You can see how a junior partner seeks to insure his success by arriving early on the breeding grounds.

[Originally published on March 17, 2013]

Pine Grosbeaks

By in Species Accounts Tags:

Perhaps this winter will be the winter of the grosbeaks. In the last column, I wrote about the irruption of evening grosbeaks into central and southern Maine. Sightings continue throughout the state. Many people are seeing evening grosbeaks at their feeders for the first time in a decade or more.

We seem to have a nice influx of pine grosbeaks into the state as well this fall. Pine grosbeaks do irrupt into central and southern Maine more regularly than evening grosbeaks, but they scarcely put in an appearance in some winters.

Both of these species are aptly named grosbeaks because their beaks are indeed large, impressive tools capable of generating significant force. Just ask any bird-bander who has extracted an evening grosbeak from a mist net. But the name grosbeak is really a descriptive term rather than a taxonomic unit. Pine grosbeaks and evening grosbeaks belong to the finch family while a summer favorite, the rose-breasted grosbeak, belongs to the cardinal family.

An adult male pine grosbeak is hard to misidentify. The head, breast, sides, back and rump are a bright carmine red. The wings are mostly blackish with two conspicuous white wing bars. The belly region is gray. A pine grosbeak is much larger than a house finch or purple finch.

Females are more subtle in their coloration, but strikingly beautiful as well.

Some females are mostly gray, again with the two prominent wing bars. However, some have russet feathers on their head and rumps. Some of these birds are females, but others may be first-winter males. It is not safe to identify the gender of a russet-colored pine grosbeak.

The beak of a pine grosbeak differs from that of an evening grosbeak. The pine grosbeak bill is shorter and curved. The overall shape is more rounded than the massive conical bill of an evening grosbeak.

As with most birds, the bill and the diet fit like a hand and glove. Rather than crushing large seeds for sustenance, a pine grosbeak feeds on a variety of buds (conifers, elms and maples among others) as well as small seeds like those of box elders and ashes. Pine grosbeaks will often feed on fruits during the winter; mountain ash berries are a favorite. Apples or cherries hanging on a tree are used by pine grosbeaks. The many ornamental cherry trees on the Colby College campus are a magnet for these birds.

Like most finches, pine grosbeaks are social during the winter. Tight-knit groups of five to 10 birds form a cohesive group during the winter, often mingling with other such groups. Ornithologists suspect that each of these small groups is a family unit that has migrated together.

Pine grosbeaks are usually detected by sound rather than sight. They give a distinctive three-note flight call, sometimes rendered as tee-tee-tew. This call is strongly reminiscent of the call of a greater yellowlegs, but with a much sweeter, less harsh tone.

Evening Grosbeaks Revisited

And now another installment from the small-world department. In the last column, I wrote about the cursory observations of evening grosbeaks by the geologist Major Joseph Delafield in 1823 that led to the ill-fitting common name of this finch species. I received an email from Joseph Delafield of South Portland, the great-great-grandson and namesake of Major Delafield. Joseph has a portrait of his great-great-grandfather in his house as well as the diary from which the quotation I used was taken!

Joseph filled me in on the details of Major Delafield’s career. He served as agent for the U.S. commission charged with determining the boundary between Canada and the United States. He recognized the potential mining value of rocks at the western end of Lake Superior (the Mesabi iron ore range) and negotiated the inclusion of that property as American soil.

[First published on November 27, 2012]

Red Crossbills; World Bird Listing

By in Species Accounts

One of the great challenges for biologists is recognizing the limits of a species.  Humans provide a nice example of a highly variable species.  As a demonstration of this variability, I like to show my students a portrait of the basketball great, Wilt Chamberlain and the equally famous jockey, Willie Shoemaker.  The dark-skinned Chamberlain towers more than two feet over the white-skinned Shoemaker.  That’s variation!  But we know that humans around the world are capable of producing fertile offspring, one definition of a species.

On the other hand, some species are morphologically nearly identical to other species.  We call such similar forms sibling species.  Traill’s Flycatcher is a case in point.  A painting of this “species” appeared in Audubon’s Birds of America.

Field work in the 1950’s and 1960’s showed that some males consistently sang a sneezy “fitz-bew” song and others sang a “fee-be-o” song.  Furthermore, females were only attracted to one of the two song types.  Based on this research, the AOU Check-list Committee split Traill’s Flycatcher into Willow Flycatcher and Alder Flycatcher.  Good luck distinguishing one from the other in the field if they are not singing.

A more recent example is the 2010 split of the Winter Wren into two species, the western Pacific Wren and our eastern form, still called Winter Wren.  Research showed that the two species do not interbreed where their ranges overlap in the west.

Red Crossbills present an even more complicated and controversial situation.  In 1988, Jeff Groth published an extensive monograph showing that ten types of Red Crossbills occur across North America.  These types are best identified by their flight calls, but subtle morphological differences occur as well.  Groth showed that different types have preferences for different types of conifer cones.

Matthew Young has recently produced a nice update of our knowledge of these crossbill types.  He also has made recordings of the different flight calls.  All can be downloaded at: http://ebird.org/content/ebird/news/red-crossbill-types

Click on “Add to Cart”.  You will be taken to a Checkout page but the price is right: $0.00.  Submit your order and you will get two pdf documents on Red Crossbills and 11 sound files of flight calls.

Much remains to be learned about the distribution of these 10 types of Red Crossbills.  Types 1, 2 and 10 occur regularly in New England and New York and a couple more appear on occasion.  Type 8 is found exclusively in Newfoundland.

What do we do with this information?  Should we recognize ten species of Red Crossbills instead of recognizing a single species?  I suspect that much more work will need to be done, particularly examining the ability of different types to interbreed, before the AOU Check-list Committee would be willing to split Red Crossbill into multiple species.

Nevertheless, it is fun to try to identify the type of Red Crossbill you hear when are birding in crossbill country.  Downloading the sound files will give you the resources you need to train yourself to give type identification a try.

World Life Listing

Many birders enjoy competitive birding as they seek to build the largest year list, state list or list for any other area of interest.  A milestone in world listing was recently passed by Tom Gullick, an expatriate Englishman living in Spain. Gullick is a seasoned world-traveler and bird tour leader.

Last summer at the age of 81, Gullick visited the remote Indonesian island of Yamdena where he saw Wallace’s Fruit Dove.  That species represented his 9,000th bird!  Gullick is the first birder to ever reach that milestone.

About 1,500 species remain in the world for Gullick to see but he indicates he does not have the 10,000 species milestone on his radar.  I’m sure those remaining birds are either extraordinarily rare or highly restricted in distribution.

[First published on October 28, 2012]

Common Loon Adaptations

By in Morphology, Species Accounts

Last weekend, I took my Marine Ecology students on a trip to Cobscook Bay.  One of our activities was a whale watch off Head Harbour on Campobello Island.  Birds were abundant with lots of Northern Gannets, Sooty Shearwaters, a few Manx Shearwaters, Black-legged Kittiwakes, Common Loons, many Bonaparte’s Gulls and even a few Atlantic Puffins.  And yes, we saw marine mammals.  A couple of Minke Whales and groups of Harbor Porpoises tantalized us by breaking the surface of the water briefly.

Perhaps you feel a bit frustrated like me watching these marine vertebrates because a good bit of their lives is lived under the water out of our vision.  Thanks to underwater cameras, small submersibles and SCUBA equipment, we do have some understanding of life under the water surface for marine birds and mammals.

It is not surprising that these animals have particular adaptations that permit them to make a living by diving underwater.  Some of these adaptations are morphological and others are physiological.  Let’s explore some of these adaptations using the Common Loon.

Common Loons are excellent divers, capable or reaching depths of about 200 feet.  A loon can stay under water for as long as 15 minutes.

Loons swim underwater using only their feet.  The wings of loons are relatively short and are held tightly against the body during a dive.

The legs of a loon are set far back on the body.  The legs are splayed out laterally when the bird swims. When a loon is first diving from the surface, it breaks the surface by alternating strokes with the left and right leg.  Once underwater, the legs beat synchronously.

The lateral placement of the legs makes for hydrodynamic efficiency.  If the legs were close together, the turbulent eddies created by one leg would interfere with smooth movement through the water of the other leg.  The lateral arrangement allows a loon to generate maximum thrust while minimizing hydrodynamic drag.

The feet of loons are large and webbed.  The real power in swimming is generated by the rearward movement of those webbed feet against the water.  When the loon moves its feet forward during the recovery stroke, the toes are brought together causing the web to collapse and minimizing the effort needed to get the foot ready for the next power stroke.

Loons have a peculiar yet elegantly adapted leg.  Unlike most birds, the major lower bone (the tibiotarsus or the large drumstick bone on a chicken) of a loon has a prominent extension, called the cnemial crest, that extends well beyond the joint where the upper leg bone (the femur) joins.  This cnemial crest provides a broad attachment for the large muscles of the upper leg.  The massive thigh muscles generate the huge force that allows loons to dive so deeply and so quickly.

The long bones of birds are not hollow as seen in most species of birds.  These heavier bones make it easier for a loon to dive.  Just before a dive, a loon compresses its body, driving out the air trapped within its feathers.  Air trapped in the feathers would increase the buoyancy of the loon and make it harder to dive.

On to a physiological adaptation.  Loons, like other diving birds and marine mammals, have the ability to store large quantities of oxygen in their blood.  Even so, staying underwater for 15 minutes is no easy task.  During diving, a loon undergoes a physiological change called the diving reflex.  Oxygen flow to most body parts is greatly reduced except to the heart and nervous system.

This reflex causes the heart rate to slow, decreasing the amount of oxygen used by the heart.  Muscles and other parts of the body have to perform anaerobic (oxygen-free) metabolism until the bird surfaces.

[First published on October 14, 2012]

Rock Pigeon

By in Species Accounts

I have been reading great reports of interesting fall migrants on Monhegan Island and at other migration hotspots.  Any Maine birder would be thrilled to see a White-eyed Vireo or a Lark Sparrow, both of which have been reported this autumn.  But there is pleasure in seeing the everyday birds as well, even pigeons.

The standardized name for our pigeon is Rock Pigeon.  They are well known to birders and non-birders alike.   How can you avoid seeing pigeons in any city?

Wild Rock Doves were likely found from Scandinavia south through most of Europe, and further south to equatorial West Africa.  Wild populations likely occurred in Russia south to Pakistan and India in Asia.  We will never know the full range of wild Rock Doves because these birds were first domesticated 5,000 years ago.  Captive pigeons were transported around the world.  Domesticated birds readily become feral, hence the pigeon populations in your local park or on farms.

Wild Rock Doves nest in crevices and caves on rocky cliffs near short, shrub vegetation.  Feral pigeons in North America also occur in similar types of habitats, where tall buildings substitute for cliffs.

In North America, the first domesticated pigeons were brought to Atlantic coastal villages in the early 17th century.  These domesticated pigeons gave rise to feral populations, which have spread across the continent.  Rock Doves now occur throughout all of the 49 continental states, the southern portion of all the Canadian provinces and throughout Central America and the West Indies.

Among wild populations, nine subspecies have been described.  The differences among subspecies are based on color and size.  However, these differences in color bear no relationship to the feral pigeons we see.  Through domestication, pigeon fanciers have bred Rock Doves for particular colors and plumage anomalies.  These variants, called sports, were prized by upper crust British gentlemen in Victorian England.  Charles Darwin was a pigeon fancier.

Careful study of pigeons can be a rewarding experience.  A number of interesting behaviors are associated with nesting.  Pigeons mate for life.  Eggs may be laid as early as February and as late as October.  From a thorough study of pigeons in Kansas, we know some birds have over six nests per year.  That’s some serious dedication to family life!

The formation of the life-long pair bond occurs through a series of displays.  This courtship begins with a bout of bowing and cooing, in which the male stands erect, inflates his crop, fans his tail and struts around in a circle.  He bows his head and neck while giving a coo call.

The next stage in courtship is heteropreening (literally preening the other) or nibbling.  The male preens the female first and then the female returns the favor.  Next, the female solicits food from the male.  The male obliges by regurgitating seeds from his crop.

The female then accepts the advances of the male.  She crouches with her wings half-raised and mating takes place, lasting only a second or two.

After mating, the male gives a display in which he stands erect and walks a few steps.  He then launches into a display flight in which the wings are clapped together twice above the back.

Aggressive interactions also take place with characteristic displays.  When a male approaches another male’s mate, particularly if the female is receptive, the female’s mate may crouch and shift the position of its wings.  This behavior conveys aggression.  The intruder may be pecked on the head.  Other intruders may be driven, a behavior involving a type of chasing in which the intruder is literally pushed away.

The male may also give a threat display by standing horizontally, inflating its crop and walking around in circles.  As the male walks, it gives a display call and raises its wings.

[First published on September 30, 2012]

Black Scoters

By in Species Accounts

I got fooled again.  My wife and I were recently looking out over Johnson Bay, a small embayment in Lubec, that is a part of Cobscook Bay.  I looked out over the water and saw a long string of dark material.  I thought it was a line of algal wrack, carried offshore by the powerful tides in Cobscook Bay and caught at the interface between two currents.  But no, the wrack started to disappear before my eyes.  I was looking at a flock of about 150 Black Scoters.

Black Scoters outside of the breeding season are often gregarious, sticking close together on the water either in a snake-like conga line or in a tight circle.  I don’t know of any other ducks that engage in this behavior.  Perhaps you have seen Black Scoters in tight formations as well.  This linear arrangement is seen in flight as well.  A line of scoters will fly just above the sea, rising and falling as they pass over waves.

Black Scoters are one of three scoters that occur regularly in Maine from the fall until the late spring.  Surf Scoters and White-winged Scoters are the other two species.   Black Scoters were once called Common Scoters in North American field guides.  As a budding birder in North Carolina, I was perplexed by the name because Surf Scoters and White-winged Scoters were far more “common” than the Common Scoter.  I like Black Scoter better.  By the way, if you are wondering about the derivation of the word “scoter”, we don’t know; its origin is unknown.

Black Scoters are pretty easy to identify.  Adults males are entirely black except for a yellow-orange protuberance on top of the bill.  That protuberance can be seen from great distances through binoculars.  Females are dark brown except for a distinctive two-toned head (dark cap and whitish cheeks).   Both sexes are fairly to distinguish from the other two scoters.

Some controversy exists over whether the Black Scoters we have in North America are the same species as the ones occurring in Europe and Asia.  The Old World form (subspecies or species) is called the Common Scoter.

Black Scoters winter along both coasts of North America, from the Aleutians to  the Baja Peninsula on the west coast and Newfoundland to the central Texas coast in the east.  Despite the broad distribution, Black Scoters are one of our least known ducks.  The main reason for this lack of information stems from the highly disjunct and limited breeding range.  These scoters nest on lakes in northwestern Alaska and in northern Quebec.  Lots remains to be learned about their breeding behavior and ecology.

Black Scoters dive for their food but are not chasing fish as many birders assume.  Rather, they are bottom feeders.  In freshwater, Black Scoters feed mainly on aquatic insects with a particular fondness for caddisfly larvae.  In marine waters, the diet is mostly bivalves (clams and mussels) and snails.  Scoters usually feed in waters less than 30 feet deep.  Once at the bottom, they may take mussels or snails on the surface or excavate the sediment to extract buried clams.  It’s no surprise they need to be in fairly shallow water.

Black Scoters are powerful divers.  On the surface of the water with their wings closed, they seem to leap above the water and neatly slice into the water, disappearing in the blink of an eye.  Sometimes they propel themselves with their feet and, in some populations, they will use half-extended wings to help propel them to the bottom.

Which brings us back to the beginning.  Why arrange yourself in tight lines or circles?  If scoters were fish-eaters, perhaps they could herd fish into a tight ball.  But no one needs to herd sessile mussels or slow-moving snails.  We have to simply accept that fact that Black Scoters are sociable creatures.

[Originally published on May 27, 2012]

Brown-headed Cowbirds

By in Species Accounts

One of the many birds returning to Maine for the breeding season is the Brown-headed Cowbird.    The male is a striking bird with a brown head on a sleek black body.  The female is a drab gray. Cowbirds feed on items as varied as insects (especially grasshoppers), seeds and grasses.

The Brown-headed Cowbird is a relatively recent arrival in eastern North America.  Before Europeans colonized the east, forests were the general rule in the landscape.  Cowbirds were restricted to the west where they followed the huge bison herds, feeding on insects be disturbed by the bison.  Cowbirds are much more likely now to feed among flocks of cows rather than bison, hence the common name.

Cowbirds have had an insidious negative impact on many species of songbirds in North America.  Cowbirds are brood parasites, meaning that females lay eggs in the nests of other species of birds.  Host parents are tricked into accepting a cowbird egg as one of its own.

Cowbirds are called obligate brood parasites because they rely totally on other species of birds to do their parenting.  Cowbirds never form nests, brood their eggs or feed their nestlings.

The embryo inside a cowbird egg develops at a faster rate than the host’s eggs.  A female cowbird retains an egg in its uterus for about 18 hours before it is laid, giving the developing chick a head start.

Since the cowbird chick hatches before the eggs of its hosts, the cowbird chick gets all of the food brought by the host parents until the other chicks hatch.  By then, the cowbird chick has a size advantage and succeeds in getting more than its share of the food brought by the host parents.  In some cases, cowbird chicks are known to push the unhatched eggs out of the nest by instinct; the cowbird chicks are blind when they push the eggs out of the nest.

Cowbirds parasitize over 200 species of breeding birds in North America.  Sometimes, the prospective host parents detect the presence of the cowbird egg and will remove the egg or abandon the nest.  Lots of host species can’t distinguish the cowbird eggs and accept them readily.  In part, this lack of discrimination arises because cowbirds have only been present in the east for the last 150 years.  Host species have not had time to evolve the ability to discern a cowbird egg among their own eggs.

It is heart-rending to see a pair of Yellow Warblers feeding a cowbird chick that is larger than the host parents.  But, we have to remember that nature is not always kind.  Brood parasitism represents an effective way for one species to perpetuate itself at the expense of other species.

[First published on April 29, 2012]

Irruptive Finches

By in Migration, Species Accounts

In the fall, I had high hopes for a winter with tons of northern finches.  Red spruce, balsam fir, tamarack and eastern hemlock trees were producing bumper crops of cones.  These so-called mast years occur sporadically and different tree species do not necessarily have mast years in the same years.  This winter was shaping up as a marvelous smorgasbord for the finches that depend on conifer cones for their main food.

As a baseball fan, I can’t help but think of the movie Field of Dreams where Kevin Costner’s character builds a ballfield in his Iowa cornfield because a voice told him ”If you build it, he will come”.  Sure enough, the ghost of Shoeless Joe Jackson and other members of the Chicago Black Sox show up to play baseball.

But with our finches, the presence of a bumper crop of food is not enough to guarantee their presence.  If trees build large number of cones, the northern finches may or may not come. Obviously, these nomadic birds are finding sufficient food elsewhere.  The absence of all of these seed predators is good news for the trees of course because many seeds will find their way to the ground and germinate.

Pine Siskins, Common Redpolls, Pine Grosbeaks, Purple Finches, Evening Grosbeaks, White-winged Crossbills and Red Crossbills were all pretty scarce on Maine Christmas Bird Counts.  On a recent trip to the Cobscook Bay area, I found very few northern finches despite the many conifers heavily laden with cones.  However, a recent trip to the Flagstaff Lake region yielded sightings of good numbers of both crossbill species.  Perhaps we will see a late-winter irruption of some of these species into more southerly parts of our state.

Of the northern finches, the crossbills are perhaps the most erratic in their appearances in Maine.  Watching crossbills feed is to marvel at their skill in extracting conifer seeds with their peculiar bills.  The hallmark of the crossbills is the odd overlapping arrangement of the upper and lower bill. When observing a crossbill from above or below, one can see that one bill curves left and the other curves right. Sometimes, the upper bill is the left-curving one and in other birds the lower bill curves leftward. Why such a strange bill?

This bill turns out to be an efficient took for extracting conifer seeds.  Everyone knows what the cone of a pine, fir, spruce or other conifer looks like. The cone has a number of scales. At the base of each scale, the seeds of the conifer can be found. Most birds have a difficult time negotiating the scales to get to the seeds deep within the cone.

Not crossbills however. A crossbill inserts its bill between two scales of a cone. The crossed points of the bill act as a wedge, increasing the distance between the two scales.  One bill pushes to the left and the other to the right. The strong tongue of the crossbill can then be inserted to the base of the scale and the nutritious seed removed and gobbled up.

Although the crossed bill serves these birds marvelously in feeding on cones, the specialization comes at a cost. Crossbills are quite awkward in handling food that other finches readily eat such as thistle or birch seeds. A crossbill beak is not a generalized tool.

Usually crossbills feed on cones that their bills can handle with greatest efficiency. The Red Crossbill has a larger and longer bill than the White-winged Crossbill. Red Crossbills feed on white pine, balsam fir and sometimes hemlock cones. White-winged Crossbills feed on smaller cones like those of larch and red spruce.

Here’s hoping the crossbills will come further south this March so we can all admire the skill with which they extract conifer seeds.

[Originally published on March 18, 2012]

Crow Roosts

By in Behavior, Species Accounts

It’s thrilling to see an unusual bird.  Who is not excited about the chance to see a Snowy Owl or an American Oystercatcher in Maine?  Yet there is the potential to experience wonder in watching our common birds.

My case in point today is the American Crow.  Here in Waterville, we are treated to an amazing spectacle late each afternoon.  Hundreds of crows fly in from all directions to converge on a patch of forest behind the shopping center at Elm City Plaza in the fading daylight.  The river of crows seems endless.  No doubt, you have seen such behavior in your neck of the woods as well.

Why do the crows congregate to spend the night together?  We don’t have a definitive answer but some intriguing suggestions have been proposed.

Aside from humans with guns, the major threat to American Crows is predation by Great Horned Owls.  Perhaps you have seen American Crows mob a roosting Great Horned Owl during the day.  The crows are brutal, swooping down on the owl and pecking it.  Usually the owl is forced to fly to find a more peaceful place to sleep.

Roosting might therefore offer protection from owl predation.   It is awfully hard for an owl to approach when a thousand or more crows are keeping a vigilant watch.

Another possible explanation comes from a somewhat controversial idea called the information-exchange hypothesis.  The premise is that crows can learn about good food sources from each other.  We have to be careful to avoid ascribing a human perspective to this information sharing.  In the natural world, behavior that appears to be altruistic turns out to be selfish behavior.  In nature, individuals that watch out for themselves and their kin fare better than altruists.  How does helping unrelated individuals survive increase the chance of getting your genes into future generations?

Honeybees through their remarkable waggle dance let other members of a hive know where good nectar sources are.  But all individuals in the colony are related to each other so sharing information indirectly benefits all colony members.

A roost of crows consists mostly of unrelated individuals.  Crows are not so kind-hearted as to somehow share the location of a food bounty with unrelated individuals.  However, it may be possible for crows to discover the whereabouts of good food.  Crows may be able to assess the nutritional status of a crow as it comes back to the roost.  Perhaps it is full of energy.  If so, a crow in need of a good meal may follow the well-fed crow in the morning.  This behavior can be seen as a type of parasitism.  The definitive study to test for this type of information sharing in crows has yet to be done.

One other explanation for crow roosts is the patch-sitting hypothesis.  This explanation entails roosting near a site where there is a reliable source of food.  The food does not need to be the most nutritious food but will provide roosting crows with a breakfast to get them going in the morning in search of more substantial food and a snack in the evening before they go to sleep.  Like the information-center hypothesis, this hypothesis needs more study as well.

Crow roosts are seasonal, occurring in the fall and winter.  Some of these aggregations must be truly spectacular.  I know of one report from Oklahoma of two million crows in a single roost!  Roosts from 100 birds to tens of thousands are much more common.

We are seeing a trend of American Crow roosts occurring in urban environments rather than in more rural or undeveloped habitats.  Some have speculated that the crows are taking advantage of the slightly warmer temperatures found in cities.  Just a few degrees of warmth can make a big difference.  Plus the lights of a city make it easier to see Great Horned Owls at night.

[Originally published on Mar 4, 2012]

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