Ode to The Farallons

Oh you beautiful, rocky islands, covered in mice and smelling like shit,
I made it within reach and you taunted me once before.
But this time, I'll be staying.  See you soon.

That three-line masterpiece (^^^) was inspired by the fact that I am GOING TO THE FREAKING FARALLONS!  Holy crap, I'm getting so psyched about this.  I know it's been a slow summer (Cory and I officially suck), but get ready, because extremely epic and cool things are going to happen (currently knocking on wood), and I intend to share them with you folks.

Countless amazing vagrants have shown up on these Islands, including Brown Shrike and Common Rosefinch among others, and the annual eastern warblers and other landbirds.  Seabirds migrate by the islands throughout fall, as do cetaceans.  Pinnipeds call the islands home, and thus attract (great) white sharks to the nearby waters.  I cannot wait to be part of this wonderful place.

So I hope you haven't forgotten about us.  We're still here, and the blogging will continue shortly.  I leave for California Sep. 5, and arrive on the island Sep. 8.

Until then, here are some photos you may have seen from the day I was supposed to get on the island, but couldn't.

Stay tuned, dear friends.

Luke Musher

Banded Shorebirds on Delaware Bay

By Luke Musher

Since the mid 1990's biologists have been banding shorebirds along the Delaware Bay shore, a body of water that is an incredibly important stopover site for hundreds of thousands of shorebirds including Red Knot, Ruddy Turnstone, Sanderling, Semipalmated Sandpiper, and many other species.  The main reason that the bay is so critical is the coincidence of shorebird migration and the spawning of horseshoe crabs. Many shorebirds double their weight during their time spent at Delaware Bay, building important fat stores for migration by consuming the crab eggs that become exposed on the beaches, river mouths, and even marshes.

Part of the 1%.  A Semipalmated Sandpiper banded along the Delaware Bay shore in 2010  or earlier that  we recaptured last week.

One method that biologists are using, including the team I work on studying Semipalmated Sandpipers (hereafter SESA), is putting colored flags with unique codes on the birds.  These birds can then be re-sighted by birders.  The recapture rate of these banded shorebirds is roughly 1%, but by flagging the birds biologists can get a 10% return.  Thousands of Red Knots, SESA, Sanderling, and Ruddy Turnstone have been flagged both on Delaware Bay, and on their wintering grounds in South America, and each location has its own flag color.  For example, while woosh-netting at Fortescue this week, we re-sighted a Red Knot with an orange flag.  Although we could not get a combination from it, we knew it had been banded in Argentina by the color of the flag.

One of a few hundred Red Knot present at Fortescue last week, and one of only a few that was flagged.  [Photo by Luke Musher]

Biologists have long pointed out that the shorebirds are feeding almost exclusively on the horseshoe crab eggs while they are here, but certainly many thousands of birds feed elsewhere, away from exposed eggs such as on mudflats in the back bays and marshes at low tide.  The work I am doing with NJ Audubon is continuing what has been done along Delaware Bay for the past two and a half decades, but also supplementing those data with information on what these birds, specifically SESA, are eating.

By taking blood from the shorebirds, we can not only use genetics to find out the sex of each individual we capture, but also fairly accurately evaluate the diet of these shorebirds as they stopover along the bay.  For example, horseshoe crab eggs and marine invertebrates that shorebirds feed on in mudflats have different isotopic signals.  I wont go into too many details about how stable isotopes work or what they even are (this will most certainly bore you and me and probably stable isotope chemists as well), but I will tell you some basics.

Essentially, you are what you eat.  Organisms are made up of, molecularly speaking, the organisms they consume.  If a SESA eats strictly horseshoe crab eggs for days, its blood plasma will contain fatty acids that match the proportion of stable isotopes that are found in the eggs.  This is important because biologists want to know what will happen if horseshoe crab populations continue to plummet.  Will shorebirds be able to replace a diet of mainly crab eggs with other food sources?

A Semipalmated Sandpiper on Kimble's Beach, NJ just after I bled it.

Needless to say, it is very stressful for shorebirds to be netted, handled, and bled.  They already have a long, highly energy-demanding journey to complete without being consumed by predators such as Peregrine Falcons or Merlins.  They only have a short period of time to build the fat stores they need to get to the arctic, and even begin breeding (early arriving individuals may precede the availability of invertebrate prey in the arctic).  However, the more we can learn about how shorebirds utilize migrations stopover sites and wintering grounds, the more we can help to manage their populations and maximize conservation efforts.

One of the greatest parts of this project is that you, aka birders, can help too!  Many people see flagged or banded birds and report them to the USGS or biologists working in Delaware bay, and are helping tremendously with shorebird conservation efforts. Still, many people see flagged birds and never report them.  Perhaps they don't know how to do so, or don't think it matters.  If you see a flagged bird along the east coast (or anywhere for that matter), try to write down the code and flag color.  It is now incredibly easy to report flagged birds.  Delaware Bay shorebird biologists created a website, bandedbirds.org.  Go on and check it out.  There's lots of great information on the project, the birds, and how to report the flags you see.  So if you're going out this month, or ever, looking at shorebirds, don't forget to look at the legs of the birds as well as plumage and structure, and write down your sightings.  Good birding1

Fox Sparrows and the Species Concept: Part 1

Here at BoomCha we've begun, and hope to continue, talking about some big questions in ornithology (see Gavin Leighton's recent interesting posts on the evolution of flight).  Another major question, though not at all limited to avian biology, is how to define the idea of the species.  In my experience species definitions (and similarly definitions of higher taxa - genera, family, etc) often seem arbitrary.  The constant lumping and splitting baffles the birding bystander, yet remains unquestioned.  In many cases the same taxa are lumped and then subsequently split or vise versa!

A Fox Sparrow of the thick-billed race, probably representing the subspecies brevicauda.  I would argue (and am not alone) that the four races of Fox Sparrow represent evolutionarily independent units and should be considered distinct species.

Bird watchers and birders in general may not care - though certainly many do - all that much about why species are classified the way they are or who is making those decisions.  For the purposes of birding and bird watching alone, the American Ornithologists Union (AOU) is fairly helpful in keeping a standardized checklist of birds that everyone uses in North America.  Still, every once in a while (i.e. every time a supplement to the AOU Checklist comes out), you have to ask, why ornithologists at the AOU make the decisions they make.  Why split one taxon with multiple independent populations but not others?  Indeed the duty of sorting through the current literature, and choosing what species definitions to use is a difficult one, but it is often done in a way that seems almost arbitrary.

A short hike in February with Steve Howell (author of Gulls of the Americas, A Field Guide to the Birds of  Mexico and Northern Central America, and most recently Petrels, Albatrosses & Storm-Petrels of North America among many others) on Pine Mountain in Marin county, CA to look at three of the four Fox Sparrow races (sooty, thick-billed, and slate-colored) got me thinking about the species concept.  This trip was followed by some interesting discussions with Steve on how to define species, and some major problems with the AOU.  In part one of this series I will discuss very basically about the idea of the species, and some of the ways species (at least for birds) come about.  In later posts I will discuss some of the ways that biologists delimit species, and how these ideas pertain to Fox Sparrow races (the four probably represent separate species but have not been split).  Further, I will attempt to discuss possible ways to deal with the problem of delimiting species.  I will not, however, write an in depth criticism of how the AOU Committee on Classification and Nomenclature (CCN) should do their job (I would not be authoritative on this matter, but if you are interested Steve Howell wrote a very informative commentary on the forty-first supplement to the AOU checklist that covers some examples of inconsistent decisions made by the CCN in the past and the need to standardize a system for taxonomy).

Though there is much debate in biology on how to best delimit species, most biologists agree that a species is the smallest evolutionarily independent unit.  Speciation is the evolutionary process leading to the rise of new species - a consequence of gene flow (essentially gene flow is just interbreeding between two populations; gene flow between populations means speciation won't occur), mutation, natural selection, and genetic drift (changes in gene frequencies due to random chance alone) acting on separated populations.  The most common form, known as allopatric speciation, involves geographic isolation between two populations that then diverge as the aforementioned processes act differently on each population.  Geography, ecology, and genetics all play a major role in this process.  Despite such wide agreement on how species arise and what they are in the most basic sense, there are a lot, in fact more than 25, of species concepts out there, each with a different way of classifying species.

Speciation is the evolutionary process that results in new species.  It has happened millions if not billions of times in earth's history resulting in species of all kinds such as this Mosque Swallow in west Africa (above) and this leopard, Pantera pardus, from South Africa (below).  [Photos by Lukas Musher]

The problem lies simply in the fact that delimiting species is intrinsically complex.  That is to say, since species arise from other species and microevolution is often occurring in natural populations, the point at which speciation happens is often ambiguous.  Species concepts attempt to draw the line that defines the species, and the good ones do it consistently, systematically, and in a way that is broadly applicable, corresponding to discrete entities that exist in nature.  However, no single concept is agreed upon by all biologists, and each has it's own set of problems.

Still, to most, if not all, people the basic idea of the species is not difficult to grasp.  I would argue that for the most part what a species is, is intuitive.  We know that Northern Cardinal is not the same as a Greater Roadrunner.  They look nothing alike, they make different vocalizations, they have different habits, live in different habitats, and so on.  A little more subtly, most people could tell that a Yellow Warbler is different than a Wilson's Warbler if compared next to each other.  However, more difficult, but arguably as intuitive is deciding when cryptic species are different.  We know that Willow and Alder flycatchers are not the same.  They may look the same (almost) and have awfully similar habits, but they make different vocalizations, have different DNA, and so on.

One use of bird song is to help distinguish between other species. Prothonotary Warbler, Magee Marsh, OH [Photo by Lukas Musher]

So it isn't just an evolutionary and molecular understanding that is required to identify species, it is generally something that is easy to do.  Perhaps classification is a major part of being human.  Pre-Darwinian natural historians as well as non-western native cultures have recognized most valid species that we recognize today (most, if not all, exceptions include cryptic species).  Even species can tell species apart.  Song Sparrows, among others, have been shown to be capable of distinguishing between their own and other species.  Needless to say all birds do it on a daily basis during the breeding season (sometimes unsuccessfully in the case of hybridization) with song.

In the words of the great ecologist, Ernst Mayr, species are "natural kinds."  Ultimately, though, a species in it's most basic definition is just an entity that evolved independently of other such entities.  In order to be independent, and thus for speciation to occur, they must not breed with other such entities during the course of their evolution.  In the next part of this series I will discuss some of the major ways of delimiting species in biology.

By Luke Musher

The Why of Fly - The Origin and Evolution of Flight in Birds: Part 2

To read The Why of Fly - The Origin and Evolution of Flight in Birds: Part 1 by Gavin Leighton, Click here.

How flight evolved is one of the oldest and yet most uncertain questions in avian evolutionary morphology.  Below, Gavin explains some of the prominent hypotheses that may explain the evolution of flight in the context of ecology.  White Hawk, Chan Chich, Belize [Photo by Lukas Musher]

In the first post I explored some of the requisite physiological/morphological changes necessary for flight in birds.  Importantly, these physiological changes did not arise in a vacuum, and there remain the interesting questions of what selective forces could change the body plan of birds so that flight was possible.  Since flight evolved in birds millions of years ago, one can not definitively define the specific selective pressures that contributed to bird evolution.  Despite this difficulty, we still have a set of several competing hypotheses for the selective factors promoting flight.  These factors are described below in conjunction with the evidence for each hypothesis. 

The first explanation is the oldest explanation, having been proposed in 1879 by Samuel Williston.  This first explanation for flight is the cursorial hypothesis (Figure 1).  The cursorial hypothesis posits that the bipedal ancestors of modern birds would run to catch their prey.  To facilitate catching prey that was flying away (think insects), the ancestors would leap into the air to obtain the prey.  While the cursorial hypothesis seems technically possible, the theory is not parsimonious.  First, to gain sufficient ground speed for considerable ascent the ancestors of birds would have to have been faster runners than the birds today.  Second, after liftoff, the increased drag would after liftoff would have limited the ascent.  Finally, and perhaps convincingly, we don’t see this behavior in any extant birds today, suggesting that either this explanation is erroneous, or that feathers evolved according to the cursorial hypothesis and then the behavior was subsequently lost by any and all ancestral birds. 

Figure 1: The cursorial hypothesis.  Therapod dinosaurs that could achieve short bursts of lift may have been better able to catch flighted prey (i.e. insects; dinosaurs like Archeopteryx were not much larger than an American Robin), thus giving them an advantage in survival and reproduction.  Unfortunately this hypothesis is not well-supported.

One of the major hypotheses for why flight evolved capitalizes upon observations of contemporary birds.  Since many birds spend significant time in the trees, the arboreal hypothesis of flight argues that wings evolved to help birds navigate from tree to tree.  The progression of evolution begins with individuals living primarily arboreal lifestyles (i.e. foraging in trees and spending most of the time in tree canopies).  Such a lifestyle would put selection pressure on individuals to move from tree to tree without having to return to the ground first.  These observations led to the arboreal theory.

The arboreal theory is the most strongly supported theory and also provides a plausible progression of feathers.  Specifically, the first arboreal individuals would have utilized the feathers to glide from branch to branch, instead of flapping their wings.  Indeed, research indicates that many of the early feathers would not have been able to withstand the force of a downstroke during flight (Nudds and Dyke, 2010).  Since individuals could not flap their wings to take off, one would expect that the first flight, or proto-flight, took place when birds would jump from branches to reach another branch. 

The strength of the arboreal hypothesis derives from multiple sources.  The first is that the requisite physiology necessary for flight was not present in many feathered theropod dinosaurs, and thus, self-powered flight was not possible.  Therefore, climbing a tree to achieve flight would explain how flight could be achieved without all of the pieces being in place.  Second, contemporary birds are arboreal, and inhabitat almost every vertical niche one can think of.  Third, there are many other arboreal inhabitants that have evolved the ability to glide due to their arboreal lifestyle.  For example, flying squirrels and lizards with skin flaps jump from trees and use various adaptations to glide to another branch.  And finally, the arboreal hypothesis provides an argument for the extensive feathering we see on the bodies of Microraptor and Archaeopteryx (Figure 2). 

Figure 2: Many therapod dinosuars in the avian lineage, such as this Microraptor, are known to have been covered with  feathers,  including long feathers extending from both forelimbs and hindlimbs, as well as from the tail, suggesting that early birds were gliders rather than capable of powered flight.

An explanation on the periphery is that wings were primarily helpful for young birds that would climb trees to return to nests they had fallen from.  This idea, known as assisted-incline running is argued most forcefully by Ken Dial (Dial, 2003).  Dr. Dial has studied chukars (Alectoris chukar) in the lab and notice that they will pump their wings to scale inclines in the lab (Figure 3).  The fact that there is a modern bird that uses wing-inclined running makes it more attractive than the cursorial theory that is not supported among modern birds.  In contrast, the theory suffers from fossils that are incongruent with wing-assisted incline running.  Specifically, fossils such as microraptor have feathers on both the hindlimbs and tail; and the feathers in these areas would be unnecessary if used for wing-assisted incline running.   

Figure 3: One potential explanation for how flight evolved involves using wings to help scale inclines.  Although plausable and supported empirically, it probably isn't as good of an explanation as the tree-down, or arboreal hypothesis. 

Most recently, a group from Montana State University has proposed that the evolution of feathers in theropod dinosaurs was used primarily to help stabilize the predator while it was pinning it’s prey with feet (Fowler et al., 2011).  The argument is that theropod dinosaurs, like birds of prey today, would pin their prey down using both feet.  Pinning the prey was enhanced by strong legs and large talons that are used to hold prey that are large enough that they may escape.  Importantly, once the prey has been pinned, the prey may still struggle, thus causing the theropod to lose balance – since it’s legs are being used to grasp the prey.  To help stabilize the predator, the authors argue that feathers would have evolved and wing beats could be used to stabilize the predator while it consumed the prey.  Similar to the other hypotheses, this idea is plausible; however, it still does not explain the extent of the feathers on the entire bodies of many of the earliest bird ancestors. 

Similar to the diversity of birds we see today, there is a diversity of hypotheses that have been offered to explain the evolution of flight in birds.  The four hypotheses: the cursorial, arboreal, wing-assisted inclined running, and predator stabilization, all provide potential explanations for flight.  Some of these hypotheses are even reinforcing.  For example, an arboreal lifestyle would have likely favored making nests in trees, which would have then favored individuals that fell out of nests to re-ascend into the tree.  Therefore, the non-mutually exclusive arboreal and wing-assisted incline running hypotheses could complement each other.  In total, however, the main hypothesis that is still considered the most likely is the arboreal hypothesis.  The arboreal hypothesis can explain many of the phenomena we see in extant birds, and much of the physiology in ancestral birds.  Therefore, birds arguably evolved flight to glide first, and over time gained the adaptations necessary to perform powered flight.  Thus resulting in the avifauna we see today. 

Barrow's Goldeneye, Rodeo Lagoon, Marin Headlands, CA [Photo by Lukas Musher]

By Gavin Leighton


Gavin is a PhD candidate at the University of Miami studying cooperative behavior in Sociable Weavers.  To learn more about Gavin, see our Guest Writers page.

Citations:

Dial, K. (2003). Wing-Assisted Running and the Evolution of Flight.  Science. 17: 402-404

Fowler et al. (2011) The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds. PLoS ONE 6(12).

Nudds, RL., Dyke, GJ. (2010). Narrow Primary Feather Rachises in Confuciusornis and Archaeopteryx Suggest Poor Flight Ability.  Science. 14: 887-889. 

How Climate Change might affect California Birds

As birders we are ingrained with an appreciation for the beauties of the natural world, and a fear of the impending loss of them.  Biodiversity is by no means taken for granted by birders (generally speaking of course).  This is why thousands of American birders congregate in southern Texas or Florida to see hundreds of species of birds in just a few days.  Anthropogenic climate change is a looming threat to avian diversity, and has been shown by many authors to pose risks as great as extinction for many of the world’s taxa.  Still, systems for ranking threats to species have often overlooked their vulnerability to climate change despite its widely appreciated consequences.  In an effort to quantify the vulnerability of California's birds to climate change, a recent publication by PRBO Conservation Science researchers here at the Palomarin field station (Tom Gardali, Nat Seavy, and Ryan DiGaudio) in collaboration with California Fish and Game develops a new method to evaluate the effects of climate change on birds, and pin-points species and subspecies of highest concern in California.

Coastal species such as these Wandering Tattlers or Common Murres (below) are quite vulnerable to climate change likely because rising ocean levels will alter rocky shorelines, beaches, or estuaries used by many species for foraging habitat (as with the tattlers) or nesting habitat (as with the murres).

According to the publication, California birds may be sensitive to climate change in any of the following ways: 1) habitat specialization—species with narrow habitat preferences may be more sensitive to climate change than habitat generalists; 2) physiological tolerances—species with broader physiological tolerances may be less likely to be affected by climate change because they are more resilient to extreme temperatures; 3) Migratory status—migratory species may be more sensitive to climate change because the timing of their movements critically depend on climatic conditions for survival and successful reproduction; and 4) Dispersability—species with poor dispersal ability may be more sensitive to climate change because they lack the mechanisms to rapidly habitat track.

Further, climate change poses risks to species by exposing them to any of the following conditions: 1) Changes in habitat suitability—exposure to changes in habitat structure in any of a variety of ways may pose risk if habitat suitability decreases for a given species; 2) changes in food availability—exposure to changes in the availability or abundance of food sources undoubtedly affects survival and reproductive success; and 3) changes in extreme weather—extreme weather has been shown numerous times to lead to low fecundity or even nest failure in many species.

The authors scored species on all of these seven criteria and ranked vulnerability of the top 25% of scores from most vulnerable to least vulnerable to climate change.  In doing so, they added five taxa not originally listed in the California Bird Species of Special Concern monograph (BSCC; 2008), and raised the priority of ten more.  Further, it was found that 21 of California’s 29 state or federally threatened or endangered species were susceptible to the consequences of climate change.

Some results:

Studies such as this provide a salient understanding of how anthropogenic climate change will affect natural populations, and are critical to effective conservation since threats such as climate change pose risk for extinction.  For instance, alpine species with restricted temperature tolerances would be unlikely to survive long-term global warming because habitat tracking ends at the top of the mountain.  Delimiting which taxa are most vulnerable will allow conservationists and managers to prioritize species of high concern and further work to protect them in addition to their habitat requirements.  


By Luke Musher


To read the open access article visit: http://www.plosone.org/article/info:doi/10.1371/journal.pone.0029507

Gardali T, Seavy NE, DiGaudio RT, Comrack LA (2012) A Climate Change Vulnerability Assessment of California's At-Risk Birds. PLoS ONE 7(3): e29507. doi:10.1371/journal.pone.0029507

Mouse on the Menu

Mousing is a fairly well-known behavior in some of the larger members of the heron family, Ardeidae.  While we generally think of herons and egrets as being waterbirds strictly eating animals living in water - fish, frogs, crustaceans, etc - herons such as Great Blue Heron and Great Egret are largely opportunistic yet formidable predators, eating just about whatever small animal crosses their path.  It is not uncommon to see either of these two species in farm fields or meadows, far from water, hunting rodents among other small terrestrial vertebrates. Herons, in general, are stalking predators, walking slowly and stabbing their prey in one quick motion with their dagger-like bills - an effective method to say the least.

On Monday, 1/23/12, at Las Gallinas Valley Sanitation District in San Rafael while waiting for the wintering Short-eared Owls to appear after sunset we watched a Great Egret catch at least two rodents in the marsh.  Though dark, I was able to obtain some photos of this interesting behavior.

We first noticed this Great Egret catching rodents just past sunset, so though dark, there was still enough light to get some photos.

A vole perhaps?

It manipulated the rodent in its bill for a short period of time before swallowing.

Scrumptious

Check out the lump in its throat.

Got another one!

By Luke Musher

Red-shouldered Hawk Indeed: Gawk at this Hawk

Three days ago we had a post about one of the two Red-shouldered Hawks we caught this past week.  The second hawk, to us banding amateurs with little hand-hawk experience, looked odd especially compared to the first bird we caught (read first RSHA post).  It had features that reminded us easterners of Broad-winged Hawk.  Look at the photos here and see what we mean.  Turns out, after running it by the hawk banders at Golden Gate Raptor Observatory, the bird is definitely within the normal range of variation seen in the California Race of Red-shouldered Hawk, Buteo lineatus elegans.  Here are some photos taken by Dan Lipp of the overzealous Frenchie holding this beautiful SY RSHA.  

This photo shows the pale grayish brown eye that is typical of a young Red-shouldered Hawk.

An adult California Red-shouldered Hawk would show a nearly solid orange breast with an orange belly with white barring. The brownish barring on this SY RSHA is very reminiscent of a Broad-winged Hawk, and it has a much smaller, streaky bib than our previous SY RSHA. However, this RSHA's plumage is within the normal variation one could expect to see within the California subspecies.

Here you can see the tail with fewer, thicker bands than our first Red-shouldered Hawk, but the California subspecies immature can show a more adult-like tail than the eastern subspecies. 

By Luke Musher and Cory Ritter

The Why of Fly - The Origin and Evolution of Flight in Birds: Part 1

By Gavin Leighton

The most salient trait of birds is their ability to fly. Flight allows peregrine falcons to dive at just over 200 miles/hour, flight allows for migration, and flight allows birds like albatrosses to make vast expanses of ocean their home. Indeed, the evolution of flight was a critical event (and maybe the critical event) in the evolution of birds. Given the importance of flight for the class Aves, evolutionary biologists have long studied the origins of flight, and researchers are beginning to delineate how and why birds evolved flight.

Peregrine Falcon, Point Reyes, CA.  Dec. 2011 [Photo by Lukas Musher]

How birds evolved wings with flight feathers is an interesting story, especially since flight has evolved independently at least four times (insects, bats, pterosaurs, and birds). To begin I will very briefly outline the major physiological steps that produced wings that could produce self-powered flight. In the second part of the series, I will try to describe the selective mechanisms that favored the following physiological changes.

Birds’ closest ancestors were theropod dinosaurs (1). We all know what theropods dinosaurs are thanks to Jurassic Park (and as far as I’m concerned, only one Jurassic Park movie was ever made and it came out in 1993). As depicted in Jurassic park dinosaurs such as Velociraptor (see below), which is actually much smaller than depicted in the movie , and Tyrannosaurus, which may have been more of a scavenger than a predator, are both examples of theropod dinosaurs. In general, theropods were bipedal dinosaurs with strong hindlimbs and a long tail that was likely used for balance. From theropods we get birds, here is a woefully brief description of how.

An artist's rendition of velociraptor [Wikipedia]

To get from velociraptor to a contemporary bird, the first thing we need is feathers. Feathers were traditionally thought to only occur in birds, but a cascade of recent evidence demonstrates that many theropod dinosaurs had feathers as well (2) – I know what you are thinking, and no, Tyrannosaurus Rex could not fly. The theropod feathers were not used for flight, so other explanations must be invoked. The two common explanations are that the dinosaur “protofeathers” were used for signaling during sexual selection (due to the types of melanin researchers have found in fossils we know the dinosaur feathers were black, white, or red-brown type colors [3]); or, for facilitated thermoregulation. These two ideas are not mutually exclusive and hopefully future work will indicate the selective pressures that favored the evolution of the first feathers.

An artist's rendition of anchiornis [National Geographic]

At this point we have feathers on a roughly bi-pedal animal, but we still aren’t close to powered flight. To get powered flight we must move from the proto-feathers (see below) here is a very simple model of how feathers evolved) that resemble the downy feathers of chicks, to asymmetrical flight feathers. To get to contemporary flight feathers, feathers passed through intermediate stages, and an excellent review of this process was “Which came first, the feather or the bird?” by Prum and Brush (2003 – see citation 2). One of the main intermediate stages is the symmetrical flight feather (3). The symmetrical flight feather, compared to the asymmetrical flight feather, is actually worse for flying due to physics that will not be described here. Despite problems with a symmetric feather, components of the symmetrical flight feather were useful. For example, there are tiny hooks called barbules that help smooth the feather to maintain a feather’s airfoil shape. The evolution of barbules almost certainly occurred with or after the evolution of the symmetrical flight feather, thus making the symmetrical flight feather a useful stage to focus on. So, let’s skip ahead a bit and assume we have some sort of flight feathers on the arms, what comes next is the development of adequate wings.

Proto-feather evolution in chronological order. [Wikipedia]

The change from forelimbs with separated digits to the common bird wing involved numerous skeletal changes that I won’t describe.

Wings, such as the long wings allowing Black-footed Albatross to wander countless miles over vast expanses of ocean
evolved from the forelimbs of therapod dinosaurs. Pelagic out of Fort Bragg, CA. Aug. 2011. [Photo by Lukas Musher]

What I will mention as an aside is the downstream evolutionary consequences after wings evolved. After proto-birds lost any sort of forelimb dexterity, the duty of manipulating objects was transferred to the beak. This transfer likely explains the extraordinary diversity of bill forms, compared to other groups. So lets assume that the wing morphology is coming into place, even with wings, other physiological structures and adaptations were necessary to yield contemporary birds. Among the adaptations are hollow bone structures that largely reduce the weight of a bird (1), and save the bird significant amounts of energy during flight. And to actually get into flight, birds rely on their oversized breast muscles, which became so powerful that they need to be anchored to a keeled sternum.

Archeopteryx lithographica, Wyoming Dinosaur Center.  Taxonomically the first bird.  One of 8 specimens worldwide.
Note the details of feathers and other traits that link dinosaurs to birds including the furcula. [Photo by Lukas Musher]

All of these physiological adaptations compose the part of the story that describes “how” bird physiology changed over time to allow for powered flight. In addition to how birds anatomy changed, a complementary question is “why” birds are the way they are, and not otherwise. To answer why birds can fly, several ideas have been posited with varying degrees of support. In part 2 of this series, I will limn the most well established ideas for why birds evolved flight and some of the newer ideas as well. 

Gavin Leighton is a PhD candidate at the University of Miami, studying cooperative behavior in Socialble Weavers.  See more on Gavin at Guest Writers.

Citations:
1 Kaiser, G. The inner bird: anatomy and evolution. 386 (2007).
2 Prum, R. & Brush, A. Which came first, the feather or the bird? Sci Am 288, 84-93 (2003).
3 Vinther, J., Briggs, D. E. G., Clarke, J., Mayr, G. & Prum, R. O. Structural coloration in a fossil feather. Biol Letters 6, 128-131, doi:10.1098/rsbl.2009.0524 (2010).

To read the Why of Fly - the Origin and Evolution of Flight in Birds: Part 2, click here.

Find the Molt Limit

A quick and dirty molt lesson that will focus on a few aspects of molt that are necessary to know if you want to understand how and why we age birds the way we do:

All birds (for our intents and purposes) undergo one main molt, or growing-in of new feathers, each year. This molt is called the prebasic molt in AHY (after hatch year) birds, and the preformative molt in HY (hatch year) birds. Molts can be complete, incomplete, or partial. During a complete molt, birds grow in all new feathers--this includes all body feathers, and all flight (wing and tail) feathers. Birds that undergo an incomplete molt will grow in new body feathers and some, but not all, flight feathers. Finally, during a partial molt, birds grow in new body feathers and a varying number of wing coverts, but generally no flight feathers.

Now, why is this important? It is important because almost every adult (AHY) bird will undergo a complete prebasic molt, but the HY birds that have a prefomative molt will usually have a partial or incomplete molt. And it is specifically this partial or incomplete molt that will allow us to see a molt limit!

Molt limits are the "boundaries between replaced and retained feathers, resulting from partial or incomplete molts" (Pyle 1997). These boundaries are visible because the newer, replaced, feathers are generally less worn and less faded than older, retained, feathers. So, if all (minus an exception or two) adult birds have complete prebasic molts, then a bird showing a molt limit must be a........................hatch year bird!

Bird Topography.

We had a typical but interesting day at Palo today, with two Varied Thrush and a brand new hatching year female Spotted Towhee.  Check out some photos and descriptions of how we age and sex these species.

HY male Varied Thrush.  Females would generally not have such dark auriculars (cheek band) and breast band. [Photo by Cory Ritter]

HY male Varied Thrush.  Most of the rectrices (tail feathers) look fairly tapered rather than truncate, which is a good sign that this bird is a HY.  HY birds of most species retain many of their juvenal flight feathers.  These feathers are grown in the nest and quickly, so the quality is much lower than those of adults. [Photo by Cory Ritter]

HY Varied Thrush.  Note the overall ratty appearance and brown wash to the head and back (brown on the back is hard to see in the photo).  The wear and overall messiness is not a definitive indicator of age, but it certainly is supportive of a first year bird.  Again, this is because juvenal flight feathers are grown rapidly, and so are of poor quality, and become worn quickly in the nest. [Photo by Cory Ritter]

Male Varied Thrush. Deeper orange and blacker black than the previous HY above. [Photo by Luke Musher]

Male Varied Thrush.  Note neater, better quality flight feathers, broad and rather truncate primary coverts, indicative of an after hatching-year bird. [Photo by Luke Musher]

These rectrices appear more tapered than those of the previous Varied Thrush.  The inner edges of the outer rectrices form a gradual curve.  After-hatch-year birds will show rectrices whose inner edges form abrupt angles.  This ambiguity kept us from aging the bird. [Photo by Luke Musher]

HY female Spotted Towhee.  Its brownish rather than blackish head, wings and back indicate that it's a female.

HY female Spotted Towhee.  This Is textbook HY rectrix shape, narrow and tapered. [Photo by Dan Lipp]

Can you find the molt limit in this photo? [Photo by Dan Lipp]

Okay okay, not the best photo, but enlarging the photo, and putting in a nice red arrow, should help you spot the molt limit. The replaced alula covert on top is darker (less faded) than the retained alula feather.  This is another clue that the towhee is a HY. [Photo by Dan Lipp]

By Cory Ritter, Luke Musher, and Dan Lipp