Maine Birds

Bird Orientation, Navigation and Smuggling by Pigeons

October 15, 2017 · No Comments

The fall migration is on the decline now with most of our flycatchers, swallows and warblers gone for the next seven months. All of these birds depend on insects for their sustenance, a resource in short supply now.

Sparrows and other seed-eaters have a more leisurely migration. They can find seeds, at least until the first snows arrive. Even so, by the end of the month most of our sparrows will be gone to more moderate southern areas.

As I discussed in the last column, we know that the majority of migratory bird species have an innate knowledge of where they should go to spend the winter. It boggles the mind to realize that many first-year birds find their way unaided by adults to their wintering habitat they have never seen. Travel instructions are encoded in their genes.

Considering how migratory birds find their way, we need to recognize two different abilities of birds. First, the birds have a well-developed sense of navigation. In other words, they can set a course and follow it, barring the intervention of hurricanes or other weather phenomena.

Second, some birds have well-developed abilities of orientation. A bird endowed with good orientation abilities knows where it is. Most migratory birds can navigate well but fewer can orient.

A famous experiment done with European Starlings in eastern Europe nicely distinguishes navigation and orientation. Some starlings were captured and placed in a cage in the spring. This particular population of starlings is migratory. In the spring, the caged birds attempted to depart on a northwesterly vector to reach their breeding grounds.

Some birds were transported several hundred miles to the west. Again, the direction that the captive birds chose was recorded. The transplanted birds again tried to migrate to the northwest. They were unable to correct for the fact that they had been moved westward. The starlings showed a good sense of navigation but a poor sense of orientation.

Contrast that result with the abilities of White-crowned Sparrows. A wintering population of birds in southern California migrates each spring to Alaskan breeding grounds. Wintering birds that were either flown to New Orleans or to Maryland ultimately found their way to their Alaskan breeding grounds. These birds were able to compensate for their eastward displacement by biologists. These birds are great at both navigation and orientation.

The abilities to orient and navigate are not restricted to migratory birds.  During the nesting season, birds need to be able to find their way to their nests. The need is particularly acute for birds like Bald Eagles that maintain huge territories or Ospreys or albatrosses that may fish miles away from their nests.

Domestic pigeons have been the subjects of the most illuminating studies on navigation and orientation. Pigeons can home to their roosts from distances as far as 1100 miles.

They use multiple cues for navigation. Pigeons have an internal clock that allows them to determine direction from the position of the sun in the sky. This so-called sun compass is the most important cue. They also can sense the earth’s magnetic field. On cloudy days, magnetic cues become important. We even have evidence that pigeons can smell their home over the last few yards.

Pigeons are able to fly steadily at 50 miles per hour. It’s not surprising that  competitive homing pigeons beat their owners home from a release point.

Homing pigeons played important roles in carrying messages in World War I and World War II. The messenger pigeons were particularly important in the Normandy invasion because the Allies did not want to use radio signals to keep the D-Day invasion a secret.

Pigeons can be used for nefarious purposes as well. Recently, a pigeon whose roost is in an Argentinian prison was caught smuggling 8 grams of marijuana and a memory stick.

[First published on October 1, 2017]

 

→ No CommentsCategories: Behavior · Migration

The Impact of Hurricanes on Birds

September 30, 2017 · No Comments

The southeastern United States and the West Indies have suffered the fury of the Hurricanes Harvey and Irma in the past few weeks. We soon know the impact on humans and buildings after the storms pass. But what is the effect on birds? How do birds weather the ferocity of a hurricane?

Our understanding of the behavior of birds during a hurricane is necessarily incomplete. No ornithologist is going to risk life and limb to try to make observations in the torrential wind and strong winds of a Type V hurricane!

Let’s first consider land birds. A land bird caught up in the winds of a hurricane is very likely to perish. Such a bird certainly cannot buck the wind to find safety. As a storm builds, land birds seek out shelter. Perhaps a tree cavity, a small hole below a tree root. A dense thicket, a stout tree. These microhabitats provide effect refuges from the wind and rain.

How well do land birds survive hurricanes? The records from a bird banding station started in 2002 at a state park north of Orlando provide some heartening data. In 2004, three hurricanes hit central Florida (Charley, Francis and Jeanne) between August 13 and September 25 with winds up to 105 mph.

The banders compared the capture rates of five species (White-eyed Vireo, Tufted Titmouse, Carolina Wren, Northern Cardinal, Eastern Towhee) for the fall of 2003, 2004 and 2005. They found no differences in capture between 2003 and 2004. A slight decrease was noted for all species except Eastern Towhee in 2005. The conclusion is that the bird populations at this site did not suffer any severe impacts. Pretty good for a triple punch.

A banding station in Puerto Rico provided baseline data to assess the impact of Hurricane Andrew in 1991. The project was designed to assess the abundance and population dynamics of understory birds in a tropical forest.

After Andrew passed, the capture rate of birds was higher than before the hurricane! The explanation is that many of the birds that frequented the canopy were forced to feed on the forest floor because of canopy damage.

Bird censuses in Jamaica in a number of habitats provided a serendipitous baseline for assessing the impacts of Hurricane Gilbert in 1988. Censuses taken shortly after Gilbert hit showed that total species and the species abundances had held steady. There was some inter-habitat movement related to selective destruction of habitats.

Our understanding of the impact of major storms on land birds is that most land birds weather the storms well by finding appropriate cover. The threat to bird survival is post-hurricane because of all the vegetation damage that hurricanes wreak. Some birds may perish because their altered habitats no longer provide the food they need. Others may disperse in search of suitable habitat. Such dispersal may explain the arrival in Florida of a number of Bahamian and Cuban birds this past April following last year’s damage to these islands by Hurricane Matthew.

For migratory birds, we need to keep the connectedness of wintering, migratory and breeding habitat in mind. Tropical hurricanes can reduce the density of migratory breeding birds here in Maine.

Seabirds and coastal birds have no places to hide during a hurricane. Most are expert fliers but have little recourse except to be carried along by hurricane winds. As a result, pelagic birds often end up in strange places after a hurricane.

Hurricane Sandy in 2012 followed an unusual track, moving from the eastern seaboard northwest to Lake Ontario. Birders there following the hurricane must have thought they had been transplanted to the ocean. Here’s a list of some of their vagrant species: Wilson’s Storm-Petrel, Leach’ Storm-Petrel, Black Kittiwake, Pomarine Jaeger, Long-tailed Jaeger and Razorbill.

It’s always worth birding after a storm to see what the winds may have brought in.

[First published on September 17, 2017]

 

 

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Shorebird Foraging Behavior

September 25, 2017 · No Comments

Shorebird migration is in full swing. Many of these migrants have nested on the arctic tundra and are heading for Central or South America to overwinter. These trips demand lots of fuel so our shorebirds must feed voraciously.

Most shorebirds are fairly confiding birds so it’s easy to get close enough to watch their feeding behaviors. Our shorebirds show a diversity of foraging techniques.

Let’s start with the plovers. Semipalmated Plovers, Black-bellied Plovers and American Golden-Plovers, all common fall migrants in Maine. Plovers rely on their eyes to find food on intertidal mudflats. The plovers practice a type of feeding that animal behaviorists called run-and-peck. A plover will stand in one spot on the intertidal surface and keep an eye out for movement at the surface of the sediment. The plover will run over and grab the unsuspecting arthropod or marine worm.

Keep an eye out for a clever behavior by these plovers. Several species of marine worms called polychaetes maintain burrows in the sediment. Two of these, the sand worm and the blood worm, are dug for bait. When a small crustacean or other potential prey organism walks across the surface of the sediment, vibrations are created and the worm will shoot its proboscis out to capture lunch in an ambush.  Plovers turn the table on these worms.  A plover will place one foot right above at surface of the mud and then cause the foot to tremble. The vibrations from the trembling foot mimic the vibrations of a small invertebrate, causing a sand worm or blood worm to come to the sediment surface in search of a meal. That marine worm becomes a meal for the plover. Foot-trembling is common in plovers so keep an eye out for it.

Many of our sandpipers probe the sediment for food with rapid bill movements called stitching. Examples include Semipalmated Sandpipers, Least Sandpipers, Short-billed Dowitchers and Stilt Sandpipers. These birds use touch to locate prey organisms beneath the surface of the sediment. We also know that taste is important. If a sandpiper cannot taste the presence of a preferred prey after a few probes into the mud, it will take a few steps and try a new spot.

Sandpipers use their bill as forceps to capture invertebrates beneath the sediment surface. Now, thanks to the work of Margaret Rubega at the University of Connecticut, we know how some sandpipers use the surface tension of water to get a food item into the mouth.

Sandpipers are able to spread the tips of their bill while keeping the rest of the bill tightly shut, a neat little trick called rhynchokinesis. In Least Sandpipers and Western Sandpipers, Rubega used high-speed video to show that a sandpiper will capture a prey organism at the tip of its bill along with a droplet of water. The water sticks to the upper and lower part of the opened tip of the bill. Then the sandpiper slowly opens the rest of the bill. The water droplet sticks to the bill by surface tension and is stretched out with the prey organism inside. Essentially, surface tension provides an escalator to get the prey item into the mouth.

In the Bay of Fundy, Semipalmated Sandpipers feed on a small crustacean called Corophium. I found that I could see the struggling Corophium extracted when a sandpiper made a successful probe in the mud. The sandpiper also visibly gulped when it swallowed the Corophium.

I used these observations to publish a paper on the success rate of probing by the sandpipers in different areas of the mudflat. You should be able to see prey organisms in the bills of sandpipers too.

The stout, slightly upturned bill of a Ruddy Turnstone is used to flip over shells and other debris to expose the invertebrates hiding underneath.

 

[First published on August 20, 2017]

 

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Taxonomy Above the Species Level

September 23, 2017 · No Comments

Humans have a penchant for organizing. We like order. This need for organization certainly drove Karl Linnaeus, a Swedish naturalist, to published the first catalog of life, the Systema Naturae, in 1735. He devised the framework we still use in our taxonomy.

In the last post, we explored the challenges of recognizing species. New knowledge forces us to re-examine our understanding of the limits of variation of species. We regularly gain or lose species on our life lists as former species are divided into two or more new species or others combined into a single species.

Taxonomists do have methods for defining a species. The problem is that there is more than one method and the different approaches do not always get to the same conclusion!

Similar species are placed in a genus. Genera (the plural of genus) that are similar are placed in the same family. And on we go upward to order, then class, then phylum, then kingdom. Thus, the tree of life is organized.

Linnaeus based his taxonomy on similarity of form. In the next century, Charles Darwin saw that Linnaeus’ system could reflect relatedness. Species in the same genus had a more recent ancestor than two species in different genera or families. He said that our classification of life should be a genealogy.

But how does one decide how large a genus or order should be? Surprisingly, the answer is that it is arbitrary, depending on the preferences of the taxonomist. Some genera have a single species like the genus Icteria, containing only the Yellow-breasted Chat. On the other hand, the snail genus Conus contains 750 species and the sedge genus Carex has nearly 1800 species.

Ultimately, the size of the genus or other taxonomic group is not important as long as it can be defended as a natural grouping. Any taxonomic group should be monophyletic (one branch), containing species more closely related to each other than to any species in other groups. Darwin’s desire to have our taxonomy to be a genealogy is really a desire for our classifications to contain only monophyletic groups.

The job of erecting and revising a taxonomic system for any group of organisms had to rely on similarity of structure until the turn of the 21st century. Now, our ability to rapidly sequence and compare the DNA of organisms gives us a second powerful way to assess relatedness.

Some genes change through mutations quite rapidly so DNA comparisons of these genes are useful for exploring closely related, recently separated species. Other genes mutate very slowly so can be used to assess the relationship between distant groups like phyla or classes. Some genes change at intermediate rates so can be used to assess the relatedness of orders and families.

DNA comparisons have shaken the foundation of our bird taxonomy. Such comparisons allow us to avoid the twin pitfalls of species from a common ancestor diverging strongly and species in different groups converging to similar shapes.

The grebe order was formerly placed close to the loon order. DNA comparisons now tell us that the closest relatives of grebes are . . . . . . . flamingoes! Here we have a case where species have strongly diverged from their common ancestor. Another cool example is that the flightless penguins are most closely related to the albatrosses and shearwaters, masters of long-distant flight.

On the other hand, the hawks and the falcons were formerly lumped together in the same order. DNA tells us that convergence to high-speed, sharp-taloned predators has occurred. The two types of raptors are placed in different orders now. The closest relatives of the falcons are the parrots and perching birds. The New World vultures share a common ancestor with the hawks.

[Originally published on August 13, 2017]

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What Is a Species?

September 21, 2017 · No Comments

This post is a follow-up to the previous one in which I discussed some of the recent decisions by the North American Checklist Committee. This committee of ornithologists makes decisions on whether some species should be split and others should be combined into a single species.

Many were surprised that the Committee did not vote to split the Yellow-rumped Warbler into three species. The votes on this issue was not unanimous. Today, we will discuss three different definitions of species, each of which has its champions.

The first efforts to classify life on earth hinged on a morphological definition of species. Individuals that look alike are combined into a single species. This definition works pretty well for most birds. However, pitfalls lurk.

Consider the bird once called the Traill’s Flycatcher, a common breeding bird in Maine. Careful observations revealed that some male Traill’s Flycatcher give a three-noted song, “three-bee-o” and others gave a sneezy, two-noted “fitz-bew”.  Females respond to only one of the two song types. We now recognize two species, Alder Flycatcher and Willow Flycatcher. But morphologically, the two species are nearly identical. Such similar species are called sibling species. A morphological definition of a species fails us here.

The other side of the coin involves species that are highly variable. The point is made by a wonderful portrait of the 7-foot, 2-inch basketball player Wilt Chamberlain and the 4-foot, 11-inch jockey, Willie Shoemaker (http://bit.ly/2vhc6gu). With the differences in height and skin color, one could easily imagine alien visitors regarding the two men as belonging to different species rather than rightly placing them in a single, highly variable species.

One of the great biologists of the 20th century, Ernst Mayr, devised an alternative definition of species, his biological species concept. A biological species is a group of interbreeding individuals that do not breed with any other species.

The ability to produce offspring is Mayr’s standard for defining a species. The Willow Flycatcher and Alder Flycatcher example above pose no problem as the song types of the two species prevent interbreeding.

Revisiting the Yellow-rumped Warbler question, the Committee decided to not recognize the western Audubon’s Warbler and the eastern Myrtle Warbler because they interbreed freely in a very narrow zone of overlap.

But the biological concept species has some shortcomings. First, what do you do when a species shows a patchy distribution? In western North America, a species once called the Scrub Jay, occupies the role of our Blue Jays in the east. But Scrub Jays also occur in southern Florida. Are the Florida and western Scrub Jays the same species? They never come into contact so we do not know if they can interbreed.

Once considered a separate species, the Committee voted years ago to recognize them as two species, the Western Scrub-Jay and Florida Scrub-Jay, based on differences in social behavior.

The other shortcoming is that interbreeding occurs frequently in many species. Ducks are perhaps the best example. American Black Ducks and Mallards freely interbreed. Hybrids show no loss of vigor or fertility. Mallards interbreed with many other species too (http://bit.ly/2u8w1c6). But there is no outcry for merging Mallards with other hybridizing ducks.

The most recent definition of a species is called the phylogenetic species. Proponents of this concept seek to identify groups that all stem from a common ancestor.  A phylogenetic species is defined on a feature uniquely found in all individuals. A distinctive song, behavior or structure might suffice. Sometimes, the unique feature is a particular section of DNA.  In such a case, field identification is problematic.

Reasonable people can disagree and our understanding of bird relationships and species limits is always subject to revision. Science by nature is provisional. Rather than being frustrated by decisions of the Committee, I take pleasure in the complexity of nature.

[First published on July 30, 2017]

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AOS Checklist Report

September 19, 2017 · No Comments

In early July, many birders eagerly await the annual report of the American Ornithological Society’s Committee on Classification and Nomenclature of North and Middle American Birds. This committee of professional ornithologists is responsible for making decisions on the splitting or lumping of species, changes in common and scientific names of birds and changes in the order in which birds appear in official checklists.

Like any scientists, ornithologists revisit bird identification and classification as more information becomes available. Requests for consideration of thorny taxonomic problems are accepted by the Committee and sometimes the Committee decides to reconsider decisions on their own.

The Committee tends to be conservative, only making changes when the new information is strongly compelling.

Perhaps the most significant decision by the Committee this year was to merge Thayer’s Gull into Iceland Gull. In Maine, Iceland Gulls are regular winter visitors. Like many gulls, adults are white below with gray backs and upper wings. However, Iceland Gulls lack black on the wingtips. Thayer’s Gulls have a similar shape and size to Iceland Gulls but do have black wingtips.  Confusing intermediates do occur.

The Committee recognized Thayer’s Gull as a distinct species in 1973 based on research done on Baffin Island where both Thayer’s Gull and Iceland Gull nest. The researcher claimed that the two types of gulls did not interbreed. This researcher claimed that females used the color of the skin around the eye (the orbital ring) to distinguish male Iceland Gulls from male Thayer’s Gulls. By capturing males and painting the orbital ring to match the other species, the researcher claimed that 55 hybrid matings were produced. His conclusion was that the orbital ring color serves to isolate the two species and prevent interbreeding.

The Checklist-Committee accepted this two-species argument. Subsequently, researchers on Ellesmere Island found that Iceland Gulls and Thayer’s Gull freely interbreed. Furthermore, evidence has surfaced that the original study was sloppy at best and fraudulent at worst. The recent decision to merge Thayer’s Gull into Iceland Gull is warranted in my view. Unfortunately, many of us lose a species on our life list.

Many ornithologists were anticipating a split of Yellow-rumped Warbler. For much of the 20th century, we recognized the Myrtle Warbler as distinct from Audubon’s Warbler. Myrtle nests broadly across Canada and many northern states, including Maine. Audubon’s nest from the Rockies westward from New Mexico to British Columbia.

The two forms are similar but Myrtle has a white throat and Audubon’s has a yellow throat. Based on a study that showed interbreeding of the two forms in a narrow region of overlap in western Canada, the Committee decided to combine the two into a single species, the Yellow-rumped Warbler.

Since then, ornithologists have compared the DNA of different populations. The genetic differences suggest that the two forms are different enough to qualify as separate species. A third species restricted to Guatemala is supported by DNA differences as well as plumage differences.

In a surprise, the Check-list Committee turned down the proposal and continues to recognize the Yellow-rumped Warbler as a broadly distributed species.

The conservative nature of the Committee was evident in other decisions as well. Other proposed changes that were rejected included the split of Willet into two species, the split of Brown Creeper into two species and the lumping of Common Redpolls and Hoary Redpolls.

A proposal to change the common name Ring-necked Duck to Ring-billed Duck was turned down as well. The ring on the bill is much more obvious than the ring on the neck.

Based largely on DNA comparisons, we now know that Yellow-breasted Chat is not a warbler. It is now in a separate family, the Icteriidae. New World sparrows are placed in their own family, the Passerellidae, separate from the Old World Buntings, the Emberizidae.

 

[First published on July 16, 2017]

 

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June Vagrants in Maine

September 17, 2017 · No Comments

Roger Tory Peterson once quipped that birds have wings and they use them. One of the thrills of birding is seeing birds that are either passing through or lost. A vagrant bird can really spice up a daily bird list.

We are accustomed to seeking out rarities during the concentrated spring migration or the more leisurely fall migration (August into November).  In June, most birds should be on their breeding grounds.

Contrary to this logic, June brought some remarkable rare birds to Maine this year. On June 7, a Burrowing Owl was photographed near the Katahdin Inn in York. At this time of year, Burrowing Owls should  be in the western half of temperate North America, nesting in abandoned prairie dog burrows on the plains. There is also a disjunct population in Florida. Unfortunately, the owl was a one-day wonder. If accepted by the Maine Bird Records Committee (hereafter, MBRC), this bird will be only the second ever found in Maine. The first Burrowing Owl lingered for over a month in late summer in Washington County in 2006.

On June 9, an apparent drake King Eider was sighted off Potts Point in South Harpswell. The written description and fuzzy photographs (taken from a long distance) support the identication. These birds breed in the high Arctic; an adult male in Maine in June is peculiar, indeed.

On June 12, a Magnificent Frigatebird was photographed while perched on Stratton Island and was seen later that day from Prouts Neck as well as Pine Point. The bird could not be relocated the following day.  The MRBC lists eight records of this tropical species in Maine, none of which have been reviewed to date.

A Magnificent Frigatebird was seen off Salisbury Beach on the North Shore of Massachusetts on June 14. Perhaps it was the same bird that was found in Maine.

In an amazing contrast, a Snowy Owl was photographed on June 13 in a driveway in Freeport, near Hedgehog Mountain Park. Snowy Owls should be nesting on the arctic tundra at this time of year. Normally, more than 2,000 miles separate frigatebirds and Snowy Owls in June.

Why not add some western vagrants to add to the mix? On June 13, a Snowy Plover was found with Piping Plovers at Reid State Park.  If accepted by the MBRC, the Snowy Plover will a new addition to the official Maine bird list. On the same day, a Townsend’s Solitaire was photographed in Whitneyville. Townsend’s Solitaires do wander regularly to eastern North America but a June record is quite unusual.

And the hits keep on coming. On June 20, a Brown Pelican was photographed off the Prouts Neck Yacht Club. The bird was seen regularly through June 23 by many birders. The bird split its time among Prouts Neck, Stratton Island, Bluffs Island and Pine Point.

The plumage and the presence of a pale stripe on the lower part of the throat pouch indicate this bird was in its second year of life.

The MBRC lists one accepted record for Brown Pelican in the state, a bird seen on June 16 in Harpswell. Four older records from 1826, 1914 (2 records) and and 1922 have yet to be reviewed by the MBRC.

Despite diligent searching on June 24, the pelican could not be located in Maine. However, a Brown Pelican was seen in Rye, New Hampshire on that day. This bird was also a second-year bird and may well have been the same bird seen in Maine. Two additional Brown Pelicans were reported from Salisbury Beach in Massachusetts.

New Hampshire birders enjoyed the first record of another tropical vagrant, a Brown Booby. This cooperative bird was found at Cobbett’s Pond in Windham. Here’s a remarkable YouTube video of this delightful bird http://bit.ly/2sk2dJE They are normally found no further north than the Caribbean.

[First published on July 2, 2017]

 

 

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Ruby-throated Hummingbirds

September 15, 2017 · No Comments

The hummingbirds are back! Who among us has not joyfully exclaimed when the first Ruby-throated Hummingbird of the year appears at our flowers or feeder? It’s hard to think of a Maine migratory breeding bird whose spring arrival is more eagerly anticipated.

The hummingbird family is restricted to the New World with most of the 328 species occurring in Central America and South America. Unsurprisingly, south Texas and southeastern Arizona have the highest hummingbird diversity in the U.S. with almost 20 species in each area. For us New Englanders, we have to be content with a single species.

But, our Ruby-throated Hummingbirds have a broad nesting distribution, occurring in the United States east of 100 degrees latitude everywhere except the southern tip of the Florida peninsula. In Canada, ruby-throats occur from the Maritime Provinces westward into Saskatchewan. No other species of hummingbird in North America has a broader geographic range.

The delights of watching Ruby-throated Hummingbirds are many. The red throat feathers (called a gorget) of a male Ruby-throated Hummingbird may seem to sparkle in the right light.  Their throat feathers refract light, giving the bird an iridescence that makes the feathers seem to shimmer as the bird moves its head.

Who isn’t amazed by a hummingbird’s ability to fly backwards? Unlike other birds that extend their wings down and forward during a powerful downstroke and then fold the wings to raise them back for the next downstroke, hummingbirds generate lift and thrust on both the downstroke and upstroke. Their wings in motion describe a figure-8 when viewed from the side. At the end of the downstroke, the wing is moved backwards and power is created by the upstroke moving forward to the head.

Don’t expect to see these amazing wing movements with your naked eye. Hummingbird wings are a blur in motion; the wings beat up to 70 times a second. Slow-motion video is needed to see the figure-8 wing movements.

Ruby-throated Hummingbirds have a mating system called promiscuity. Each male tries to mate with as many females as possible. No long-lasting pair bonds are forged.

A male will court a female through complicated flight displays. When a female flies into his territory, he begins with a dive display, flying U-shaped loops starting from as high as 30 feet above the female. If the female perches, he switches to very fast, side-to-side flights, with his gorget extended, within two feet of the female. If the male is acceptable as a mate, the female will cock her tail feathers to one side and lower her wings, inviting the male to mate with her. Mating lasts only about 2-3 seconds and that is the end of the male’s contribution to the offspring.  Keep an eye out for these behaviors. I find them fascinating.

As a single mom, the female builds the nest by herself. The base is made of the down from dandelions and thistles and is attached to the upper side of a branch, much like a saddle over the back of a horse. The sides of the nest are made of plant down, bud scales and spider webs. The plant material is woven into the nest with the spider silk.

The eggs are usually two in number and, as you might imagine, are tiny. An average egg is half an inch long.  That’s barely larger than an English pea.

Incubation takes 12-14 days and the young hatch as naked, blind chicks. Feeding usually begins soon after hatching and the young fledge about 20 days after hatching.

We think of Ruby-throated Hummingbirds as depending on nectar for their nutrition. However, these birds also take spiders and insects (mosquitoes, gnats, fruit flies and small bees).  Sometimes hummingbirds steal insects caught in spider webs and may take insects attracted to oozing sap from Yellow-bellied Sapsucker wells.

[First published on June 18, 2017]

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Migration Physiology II

September 13, 2017 · No Comments

Spring bird migration has run its course. Birds are busy attracting mates, establishing territories and building nests. Before I put migration to rest, I want to add a bit on the remarkable physiological demands of bird migration and the adaptations birds have to meet those demands.

In the last column, we explored the engineering problem migrating birds have of how much fuel (fat) it bird should carry. Carrying extra fat for insurance means a bird’s mileage will be reduced because of the excess weight. Cutting the fat stores too close might mean running out of fuel and perishing.

Thanks to the work of Dr. Scott McWilliams and his students at the University of Rhode Island, we know many migrating birds have a trick up their wing to improve their flight performance.

In preparation for migration, a bird enlarges and lengthens its gut to allow it to feed more rapidly and put on weight. It’s an adaptation for gluttony. The cells of the intestines become larger and new cells are formed.

However, the physiological demands of the gut are high. So during migration, once fat stores have been loaded, a bird essentially shuts down its gut and the gut decreases in size and weight. The diversion of energy from the gut can be used to fuel the flapping of the wings for a migrating bird.

Once a bird has stopped after completing a leg of its journey, it is unable to feed efficiently because its gut has shut down. It must rebuild the gut to allow food to be digested properly.

McWilliams showed that proteins are essential to get the digestive tract functioning well. Birds that only have access to fruit at a stopover will refuel more slowly, often entailing a delay in their migration.

Let’s revisit the Semipalmated Sandpipers discussed in the last column. To fuel their four-day migration over the Atlantic Ocean from the Bay of Fundy to Suriname, the birds pig out on small crustaceans called Corophium in the upper Bay of Fundy mudflats. Dr. Jean-Michel Weber of the University of Ottawa noted that the Corophium are high in omega-3 fatty acids. He found that the efficiency of the sandpipers’ muscles increased over the two weeks or so that birds spent fattening on the mudflats.  Weber suspected that the omega-3 fatty acids might be the reason for that increase in efficiency. However, he could not rule out other reasons (hormonal changes, exercise) to explain the muscle improvement.

He resorted to some lab experiments with Bobwhite quail. These birds rarely fly and do not migrate, eliminating exercise and migration-related hormonal changes as possible factors. By supplementing the diet of the Bobwhite with omega-3 fatty acids, he found a direct increase in muscle efficiency between 58% and 90%.  These changes are similar to ones noted in Semipalmated Sandpipers shortly before they embarked on their 2,400-mile jaunt to South America. Remarkable!

Switching gears, we know that migrating birds often overshoot their intended breeding destinations. Summer Tanagers, Hooded Warblers and Kentucky Warblers occasionally appear in Maine in the spring but presumably withdraw to their more southerly breeding grounds.  These birds likely made a navigation error.

A different explanation may explain the appearance of some out-of-range birds. Recently in southeastern Florida, a number of Caribbean birds appeared to the delights of Florida birders. These birds included Bahama Mockingbirds, a LaSagra’s Flycatcher, a Fork-tailed Flycatcher, a Thick-billed Vireo, two Cuban Vireos and many Bananaquits and Western Spindalis (a type of tanager).

Why this influx of rarities? Some ornithologists believe that these appearances were driven by the devastation wrought by Hurricane Matthew last fall that hit eastern Cuba and the Bahamas with its full fury. Lots of bird habitat was destroyed. The hypothesis is that some Caribbean birds returned to their normal breeding grounds, found it to be destroyed, and kept on trucking to Florida.

[First published on June 11, 2017]

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Migration Physiology I

September 11, 2017 · No Comments

The spring bird migration is coming to an end. The last species to arrive are usually Black-billed Cuckoos, Yellow-bellied Flycatchers, Blackpoll Warblers, Salt Marsh Sparrows and Nelson’s Sparrows.

We all marvel at the ability of birds to fly thousands of miles to nest and then return to their wintering area each year. These herculean feats become even more astounding when we consider the physiology of birds.

Like mammals, birds are endotherms; they maintain a constant body temperature in the face of changing environmental temperature. All endotherms have a minimum idling speed, called the Basal Metabolic Rate (or BMR, for short). You are likely at your BMR as you read this column.

The BMR varies inversely with size. A hummingbird or a mouse has a relatively high BMR while a hippopotamus or ostrich has a lower BMR. But here’s the kicker, a bird has a higher BMR than a mammal of the same size. The fire of life burns brightest in the birds.

Maintaining the BMR requires energy to produce heat. Those costs become more severe at colder temperatures.

Consider what happens when you push your body to its limits, like running a 400-meter dash. You breathe heavily, your heart rate increases and your metabolic rate rises. For humans, maximum performance results in about a doubling of your BMR. Other mammals may be able to triple their BMR.

Birds outdo mammals by a long shot. The demands of powered flight are huge. A migrating bird increases its metabolism to 8 to 10 times its BMR! That is one blazing furnace.

The ideal fuel to keep the metabolism high should pack the most energy per unit weight. Fats are the fuel of choice for birds. Breaking down a gram of fat produces about twice the energy of a gram of carbohydrate or gram or protein. The burning of fats also produces water as a breakdown product, which can be used by a migrating bird. This metabolic water is a necessity for birds migrating over the Gulf of Mexico or other saline bodies of water.

How much fat should a bird carry on a migratory leg? Obviously, a bird needs enough to fuel its flight, particularly if the journey is over water. But, carrying extra weight means that the bird has to work even harder to fly. So, there is an optimal fuel load that ensures completion of the flight but leaves little fuel in the tank.

How well are birds able to find the optimal fat load? Semipalmated Sandpipers provide a nice example. During the fall migration, most Semipalmated Sandpipers depart from the Bay of Fundy, flying over the ocean to reach Suriname in northeastern South America. This flight requires about four days of sustained flight.

Before departing, the Semipalmated Sandpipers gluttonously feed on the abundant crustaceans in the expansive intertidal Fundy mudflats. Over a period of two weeks, a bird doubles its mass from 20 grams to 40 grams or more!

Shorebird biologists in Suriname wait for the arrivals of the sandpipers. Some are captured and their mass is recorded. Most are at or close to their lean weight of 20 grams. The birds are cutting it pretty close.

Ovenbirds provide a nice example of fat dynamics on a shorter time scale. Like most songbirds, Ovenbirds start a migratory leg in the early evening. Some banding stations keep their nets open during the night, capturing migrating songbirds as they land. The data indicate that Ovenbirds captured around midnight, after a short flight, weighed about two grams more than Ovenbirds captured around dawn after a longer flight. With a lean mass of around 18 grams, some Ovenbirds therefore burn 10% of their body weight in a single night’s flight!

We have some good evidence that those lighter birds linger longer than fatter birds before undertaking the next leg of a migration.

[Originally published on May 28, 2017]

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