March 30, 2014

All real biologists love Charles Darwin, or Saint Chuck as he is sometimes known among the fraternity.   Darwin’s genius was multifaceted; he was first and foremost a naturalist, and a damned fine one at that.   In addition to his monumental bestsellers, The Origin of Species and The Descent of Man, Darwin wrote numerous books about more obscure biological topics, such as the sex lives of orchids, the role of earthworms in soil formation, and the emotional responses of humans and animals, among others.  His fascination with the patterns of distribution and morphological diversity of the Galapagos finches (“Darwin’s finches”) is said to be one of the key observations that led him to his revolutionary evolutionary insights.

His greatest contribution to biology, though, was his elucidation of the principle of natural selection, and later, sexual selection.  Darwin was certainly not the first scientist to suggest that organisms evolve and that one species can transmogrify over time into a new and radically different form.  The Greek philosopher Anaximander is often credited as being the first to suggest that organisms evolve, way back in the 7th century BC.   But Darwin (along with Alfred Russell Wallace, who tends to get lost in the mix), was the first to propose a credible and testable mechanism explaining how evolution and adaptation happen.

Darwin’s principle of natural selection is surprisingly simple, and to those enchanted with biotic diversity and natural history, incredibly powerful as a tool for understanding.  The process of natural selection is based on a few widely confirmed premises: a) individual organisms vary in many traits, b) variability among individuals in traits is heritable (i.e., traits have a partial genetic basis), and c) variability among individuals in these heritable traits results in differences in performance in the natural world.   Most importantly, individual variability affects the ability of individuals to survive and ultimately reproduce.  Those individuals who are more successful at reproduction will pass more of their genes to the next generation than individuals that reproduce less or not at all.  Over the course of generations, the genetic composition of the organism will change.  That’s evolution.  The widely used term “survival of the fittest” does a very poor job of capsulizing the process.   Natural selection is not so much about survival as it is about the ability to produce viable offspring carrying one’s genes, though successful reproduction does require that an individual survive until at least the age of reproductive maturity.   Ultimately, though, difference among individuals in reproductive success, or Darwinian fitness, is the defining feature of Saint Chuck’s principle.

Evidence for the basic premises of natural selection is overwhelming, and obvious to anyone who spends any time studying organisms and how they function in their real environment.   Right now in Florida, evidence for one of those premises is obvious to anyone who maintains feeders that attract American goldfinches.   Individual variability among goldfinches is on full display right now as they begin preparation for their northward migration and breeding season.   It is a challenging task to find two birds that look exactly the same right now. 

Though individual variation in many traits exists in all bird species all the time, often it’s not apparent to dull-witted human observers, though I have no doubt it is obvious to the birds themselves.  They know who they are.  I absolutely adore the American crow family that visits my yard frequently, but except for the patriarch with a slight bill deformity (Longbeak, or LB), I can’t tell them apart, aside from the crude distinction between adults and younger birds that still show hints of brown in their plumage.

American goldfinches are undergoing their pre-alternate molt right now, and males are making the transition from their dull, female-like basic (winter) plumage to their alternate (breeding) plumage.   Each male is on his own schedule, slightly or greatly out of sync with his mates.  Some males have scarcely begun their pre-alternate molt, while others have nearly completed it.  This in turn reflects tremendous variability in a panoply of physiological states correlated with the progression of molt, likely including serum testosterone levels in their blood.  A rapid increase in serum testosterone levels in males during late winter and spring is a common phenomenon among birds breeding in temperate habitats; rising testosterone strongly influences a wide range of traits, including both plumage traits and reproductive behaviors like singing, male-male aggression, and in many species, territorial behavior.

But why is individual variation in progression of molt so apparent in goldfinches, but not so much in other species?  As it turns out, American goldfinches are unique in a number of aspects of their molting and reproductive behavior.   They are the only species among their close relatives (cardueline finches) that undergoes a complete replacement of body feathers in the pre-alternate molt.   Consequently, the magnitude of change between basic and alternate plumages of males is unusually large.  The mottled appearance of many males, who have patches of new bright yellow contour feathers interspersed among the remaining duller basic plumage, makes each male slightly (or greatly) different from others.

Asynchrony in the progression of molt among males is accentuated by their extended molting period, which can occur between March and July.  At the time when most temperate passerines have finished breeding or are working on a second or third brood, American goldfinches are just getting ready to nest.  They are one of the latest breeding species in eastern North America, with nesting activity mostly occurring in June through August.  This may be related to their dietary specialization – they are among the most devoted seedeaters of any North American passerine.  Many species that feed heavily on seeds during the winter, such as sparrows, switch to much greater reliance on insects when nesting, as animal prey provides more protein to the growing nestlings than seeds.  American goldfinches, by contrast, feed their offspring primarily seeds, and in particular, they are addicted to the seeds of various species of thistle.  Their relatively late breeding season may be an adaptation to the reproductive phenology of thistles – goldfinches don’t initiate breeding until thistles are blooming and setting seed.  The young birds hatch at a time of maximum thistle seed abundance.  This delayed breeding behavior may allow them to extend their pre-alternate molt over a longer period than most temperate passerines, allowing a much greater amount of asynchrony in molt than is seen in most other birds.

In Central Florida, American goldfinches are relatively late migrants.  I don’t usually see them until November.  During the 7 years I did weekly bird censuses at Emeralda Marsh, I saw them in small numbers beginning in mid-November, but didn’t see large numbers until late January and February, when flocks of hundreds of birds could be found feeding on the elm fruits that were just maturing.  Goldfinches are typically quite nomadic in the winter, traveling widely in search of their favored seeds.  During most of the winter, I usually have no more than a half-dozen birds at my feeders, but in late March and April their numbers skyrocket.  I have somewhere around 30-40 birds visiting my feeders right now.  Perhaps some of this increase in numbers late in winter is due to depletion of natural seed crops.

So with all these birds around, massive individual variability in appearance, and physiology, of American goldfinches is apparent.  What are the consequences of this variation in molt and appearance to the reproductive success (Darwinian fitness) of individuals?   It seems like a safe assumption that differences in plumage characteristics are heritable, as they are in other birds whose genetics are well known.   But is there a significant impact of differences in the timing of molt on the ability of males to obtain a mate and successfully reproduce?  That’s a good question.   Some evidence suggests that pair-bond formation  may occur in pre-breeding flocks, so differences in plumage among males could be having their initial effects on reproductive success right now.

In American goldfinches, males winter, on average, further north than females, and precede them in their arrival at breeding habitats by about 2 weeks.  Males that molt earlier may also migrate earlier, allowing them to claim the best breeding sites. In some other passerines, like American redstarts, research has shown that higher-quality males are the first to return to their breeding habitats, and they are able to stake out the highest quality territories.  Does it work that way in American goldfinches as well, even with their delayed onset of breeding?  Maybe.  It bothers me a bit every year that I almost never see males that have fully completed their pre-nuptial molt; even the most brightly colored males usually have at least a few small patches of basic plumage remaining.  Perhaps the males that have completed their molt are already winging their way north.

 

March 16, 2014

The arrivals of fall migrants and winter residents are the highlights of the birding year for me.  Spring migration has its own feel and charm, but for numbers and diversity, it’s the southbound birds that give me more raw birding pleasure.   So it’s always a good day when I get an FOS bird, even if it’s one that’s completely predictable in its time of arrival.   But I get an even bigger kick out of the migrants that are not so foreseeable.  Cedar waxwings are a prime example.

You know they are going to show up every year at some point, but exactly when, where, and in what numbers is never a given.   Cedar waxwing abundance in a particular area can vary by an order of magnitude or more between years, in my experience.

Two winters ago was a big abundance year for waxwings in DeLand; in good years, there are sometimes flocks of hundreds to thousands of birds cruising around town for a couple of weeks in late winter.  The appearance of the waxwings in town often coincides with the shift of American robins from their forest phase to their urban phase;  waxwings and robins commonly flock together.  One morning in February 2012 when I was driving to work I saw several flocks of a hundred or so birds around Stetson’s campus.   As is typical of waxwings, they often don’t stay in one spot for very long, but I was ecstatic to find one large flock hanging around the Rinker Environmental Learning Center.  Some of the birds were visiting puddles of water in the parking lot and on sidewalks, and others were flying down to the grassy lawn behind the RELC, apparently feeding on something , along with dozens of robins.   It was a puzzling behavior, since waxwings feed largely on fruit in the winter, and there were no fruiting trees around that might have dropped fruits into the lawn.   As it turns out, the waxwings were there to drink.  The sessile, round leaves of the profuse pennywort in the less-than-well-manicured Stetson lawn were acting as collecting cups for water, and the waxwings were drinking from  them.

 

I happened to have a camera with me that morning, and I was like a pig in shit for the half-hour or so I was able to photograph the waxwings.   But I also had an 11:00 class to prepare for and I was getting antsy about that, so eventually I had to leave the waxwings and head to the office.  I was able to  process a few of the better photos before class and began that day with a mini-diatribe on the miracle of the nomadic waxwings, in the hope that at least one or two students might look for them in the next day or two.  I don’t know if any ever did, but no doubt it was at least slightly more entertaining than the scheduled topic of the day.  Cedar waxwings vs.  Hensen’s nodes and dorsal lips of the blastopore – no contest.

Four years ago I planted a small dahoon holly (Ilex cassine) in one of my feeder gardens, hoping at some point, years down the road, to have a decent fruit crop and perhaps attract a feeding flock of waxwings to come and partake.   Dahoon holly in the flatwoods of Tiger Bay seems to be one of the mainstays of winter frugivores; its berries seem to usually disappear long before some of the other less favored fruits, like winged sumac, or even American beautyberry.     In addition to its bird-attracting properties,  dahoon is just a lovely plant by itself, with a startlingly beautiful and apparent fruit crop in some years.

As it turned out, the cultivar of dahoon that I bought at my favorite nursery was not exactly like the wild dahoons  I’m used to seeing in the flatwoods; the berries are smaller, the fruit color is more orangeish than scarlet, and the presentation of the fruit is less clumped and spectacular.  Nonetheless, in the second year I had the dahoon, its meager fruit crop attracted waxwings to my yard on a morning I happened to be home, and I was able to get a few shots of the birds feeding.   This year, the little sapling has had a tremendous growth spurt and is now 10-15 feet tall, with an impressive fruit crop.  I was guardedly optimistic about photographing waxwings this winter.

It’s been a low abundance waxwing winter here, though.  I first saw or heard waxwings back in November, but as in most years, I didn’t start seeing them with regularity until February.   But always small groups this year – I think the biggest flock I’ve seen has been 50 or so birds.  Around my neighborhood, I’ve seen no more than a dozen at a time.

In the past week or so, on several occasions, I had seen groups of 2 to 5 waxwings hanging around the yard, and flying into the dahoon to feed.   Their feeding forays on all of those occasions were aborted though, due to the aggressive behavior  of my resident pair of northern mockingbirds, who have claimed the dahoon as part of their territory.  They are pretty damned vigilant sentries; every time I’ve seen a waxwing in the area of the dahoon, a mockingbird has swooped in within a minute and driven the potential competitors away. It was starting to piss me off.

The mockingbirds harvest their fruit crop methodically; I’ve never seen one eat more than 3 or 4 fruits at a time.  I suppose I should admire their planning and foresight, particularly compared to the gluttonous behavior of waxwings, who will sometimes completely strip a fruiting tree or shrub within hours of their initial attack.    But I wanted photos of the waxwings, so the mockingbirds were not at the top of my list of favorite birds.

Yesterday, I heard several waxwings trilling and seeting from one of the wild cherries in my yard, and saw a couple more abandoned feeding sorties into the dahoon.  I decided to exercise my very limited capacity for patience and set up on the dahoon and wait the waxwings out.  I was also thinking that perhaps if I was somewhat apparent, it might have an inhibitory effect on the aggressiveness of the mockingbirds.   Fat chance.

I’ve rediscovered recently, probably for the 4^th or 5^th time since I’ve been doing photography, that if you want to get the most out of your optics, you have to use a tripod.  I hate tripods.  I had pretty much stopped using them entirely when I first started using lenses with internal vibration reduction (VR in Nikon lenses, OS in Sigma), but at some point a few months ago I tried some feeder shots with a tripod just for shits and giggles.   The increased resolution and detail in the resulting photos left no doubt that as good as the vibration reduction systems are, handholding nearly always results in some image degradation under less than optimal conditions.  So I set up the tripod, pointed the lens at the dahoon, and waited.

I got lucky.  On several occasions in the next hour or two, groups of 3-4 waxwings came down to feed.  I was particularly struck by how different their behavior was from what I’m used to.  Flocks of feeding waxwings can be riotous with activity and noise, with birds constantly in motion, popping in to eat a few fruits and then flying off, to be replaced by other members of the flock.    The waxwings yesterday were as silent as church mice, and fairly restrained and deliberate in their movements.   They would stay in one spot and eat all the fruit within reach before a short hop or climb to a new fruiting spray.   They were totally silent.  Correlation doesn’t prove causation and all that rot, but I’m pretty sure the subdued behavior was due entirely to the terrorizing mockingbirds.

After the fifth or sixth time one of the mockers chased off a group of waxwings within seconds of them landing in the dahoon,  my blood was boiling.  How dare they?   I tried a little playback of a variety of northern mockingbird vocalizations thinking that might distract them somewhat from the holly and allow the waxwings to sneak in and feed.   No dice.   They paid no attention to any form of playback.  Based on that one piece of empirical evidence, I conclude that playback has no adverse effects on breeding passerines whatsoever.   Insert sarcasm emoticon here.

But eventually I was able to photograph a couple of feeding forays, and the waxwings were still around when I had to leave around 4:30; in the end it was all good.  My animosity for the mockers disappeared entirely.  Well almost.  They are pretty bad-ass little bullies sometimes, particularly towards birds smaller than them.  That’s a behavior I have trouble getting behind.  I think Atticus Finch was slightly underinformed when he told Scout that mockingbirds don’t do anything but make pretty music for us to listen to all day.  They kick serious ass at times.

This morning brought a new perspective on the mockingbirds.   Although there were several big clusters of fruit still on the dahoon yesterday when I left the waxwings,  by 9:00 this morning it had been picked clean.   The efforts of the mockingbirds to thwart the waxwings didn’t amount to much.  The persistence and patience of the waxwings won the day.   So my new and enlightened view of the mockingbirds is this:  had they not kept the waxwings at bay for a couple of weeks, the dahoon would have been stripped bare long ago, probably at a time when I wasn’t around to see or shoot it.  The mockingbirds were actually ackling for me in a way –  they prolonged the presence of the waxwings long enough that I was ultimately able sneak a few shots.  Thanks,  NOMOs.  I take back all that nasty shit I said about you.