Monthly Archives: May 2014

CEWA feed ritual_05262014-23_Silver Lake CP

Old birds, new behaviors

 

May 31, 2014

I do most of my birding and natural historizing locally, only occasionally traveling more than 30 miles to a birding destination.   Once a year, though, I drive to northern Virginia sometime in May to hang out with my father for a week or so and experience the explosion of breeding bird activity in that part of the country.   One of the rewards of making this pilgrimage is the opportunity to experience new behaviors of species that winter, but don’t breed, in Florida.   There are a lot of those.  Although I mostly grew up in northern Virginia, and discovered my obsession with birds there, I’m still surprised on most visits by finding species or seeing behaviors that I somehow missed while I was living there.  Cedar waxwing breeding behavior is a case in point.

Silver Lake, in Haymarket, VA

Silver Lake, in Haymarket, VA

In the last couple of years, I’ve started visiting a new site while in Virginia – a county park that was only recently opened to the public.  Silver Lake Regional Park, in Haymarket, Virginia, opened in 2009, and has become one of my favorite birding spots when I’m in the area.  At 230 acres, it’s a postage stamp of a park.  Silver Lake is a 23-acre impoundment fed by Little Bull Run, and the surrounding piedmont is a mosaic of mostly disturbed and successional habitats.   Breeding bird communities of so-called old field habitats in the mid-Atlantic region contain a number of charming birds, and the diversity and density are high enough that during May there is nearly always something happening worthy of watching.  At Silver Lake, there is a large parking area that is designated for horse trailer parking, which abuts a lovely tract of perhaps 20-30 acres of prime old field habitat in the shrub-sapling stage of succession.   I rarely see anyone else at this end of the park; most park visitors cluster around lovely Silver Lake to fish.  So I have this beautiful shrubby old field all to myself.

Cemetery Cedar C_05192012_08_Aden VA

When agricultural land is abandoned and allowed to revert to a natural state, it undergoes a predictable sequence of changes in plant composition, vegetative structure, and characteristic breeding bird communities, called secondary succession.  The first few years of succession are characterized by low stature vegetation consisting entirely of herbaceous grasses and forbs; because of the simple, monolayer structure, the breeding bird community is low in diversity, often consisting of only a few species (grasshopper sparrows and eastern meadowlarks, for example) at relatively low densities.  Within 5-10 years, woody plants begin to invade and become a prominent component of the vegetative structure; these invaders include species such as red cedar, wild cherry, and persimmon, among others.  The increase in vegetative complexity, and concomitant increase in the amount and variety of food resources for birds, results in a big jump in breeding bird diversity, as species like indigo buntings, blue grosbeaks, song and field sparrows, brown thrashers, common yellowthroats, and yellow-breasted chats establish breeding populations.  The density of breeding pairs also increases nearly three-fold between the grass/forb stage of succession and the shrub-sapling stage.  All of the various stages of succession from abandoned field to mature deciduous forest have their own characteristic bird communities; diversity and density of breeding birds is greatest in late successional habitats, which in northern Virginia means various incarnations of eastern deciduous forest.

The average number of species breeding in four stages of secondary succession in the eastern U.S. (from May, 1982)

The average number of species breeding in four stages of secondary succession in the eastern U.S. (from May, 1982)

Grasshopper sparrows are characteristic breeders of very early successional habitats

Grasshopper sparrows are characteristic breeders of very early successional habitats

FISP_05262014-27_Catharpin VA

Field sparrows don’t begin breeding in old field habitats until woody plants have begun to invade.

Where you find breeding field sparrows, you usually find prairie warblers as well

Where you find breeding field sparrows, you usually find prairie warblers as well

The density (number of territorial males/40 hectares, or about 100 acres) at four stages of secondary succession.

The density (number of territorial males/40 hectares, or about 100 acres) at four stages of secondary succession.

Density and diversity of breeding birds generally increase in a predictable pattern with successional age of the habitat, with the greatest abundance and diversity occurring in the so called “climax stage”, which remains relatively stable in plant composition unless it is disturbed by either natural events (fire, blowdowns, etc.) or anthropogenic causes (deforestation).   In the mid-Atlantic region, the climax plant community in many parts of the landscape is some form of eastern deciduous forest. (The concept of a climax community that is stable and unchanging over long time periods is eschewed by many ecologists; it’s a pretty simplistic idea.)  So even though it’s not the most diverse habitat type in the successional continuum, the shrubby stage of old-field succession is hard to beat for superb birding.  Not only are many of the birds breeding there interesting and beautiful, the relatively low stature of the vegetation and open architecture of the habitat make observation of bird activity far easier than in the more diverse mature forests.

Oak-hickory-beech forest is the terminal or "climax" stage of succession in parts of northern Virginia.

Oak-hickory-beech forest is the terminal or “climax” stage of succession in parts of northern Virginia.

So on two of the five mornings I was in Virginia, I found myself at Silver Lake Regional Park, ensconced in my car (a blind of sorts) to just sit and soak in the stunning beauty of spring in Virginia.   The primary object of my attention was the yellow-breasted chats that breed in this patch of habitat; I see chats only rarely in Florida, and then typically very briefly.  There are few bird species skulkier than a yellow-breasted chat.   That doesn’t change all that much when they are breeding, but they are such vocal birds that even if you can’t see them much of the time, you can keep track of their movement and activities by the nearly constant outpouring of croaks, grunts, whistles and other varied mechanical sounds these oversized warblers produce.  They do a killer imitation of a distant crow cawing; on several occasions, they momentarily fooled me with this call even though I knew I was listening to a chat.  They’re that convincing.  On my first visit to Silver Lake, I had a remarkable half-hour or so watching and listening to yellow-breasted chats that on occasion abandoned their skulkitude and FULLY EXPOSED THEMSELVES.  Amazing.

Yellow-breasted chat in mild skulk mode.

Yellow-breasted chat in mild skulk mode.

Yellow-breasted chat IN THE OPEN!!

Yellow-breasted chat IN THE OPEN!!

So it shouldn’t be hard to understand how I can easily pass an hour or two sitting by this patch of old field habitat, watching and listening to the comings and goings of the breeding birds. It was while I was doing just that on my second visit, parked next to a small copse of some fruit-bearing sapling, that I saw a pair of cedar waxwings fly into the dense cover at the back of the grove, nearly hidden from sight.  Cedar waxwings are a species that I saw fairly regularly when I lived in Virginia, but always as nomadic flocks of fruit-scouring pirates during the non-breeding season.   At Silver Lake, though, they seem to be common breeders.  I had discovered a nesting pair of waxwings frequenting a dense clump of vine-tangled cedars on my first visit to Silver Lake, but those birds were in such dense cover that they were nearly impossible to observe when they were on or near the nest.

The male waxwing with his gift.

The male waxwing with his gift.

By contrast, this pair of waxwings in the little grove by the parking lot put on a show for me.  One of the birds, presumably the male, flew out of the back of the clump towards me, and snagged a pair of small, green fruits.  He was soon joined by his mate, and they began a ritualized behavior that was entirely new to me.   The male presented the unripe fruits to the female, which I interpreted as courtship feeding.  Cool to see, but not particularly unusual.   Many passerines and non-passerines perform similar ritualized feeding during courtship and pair-bond maintenance.  I see it every summer between the cardinals that breed in my neighborhood.   But the female didn’t eat the fruits – she moved away from the male a few inches and held it, then moved back to the male and passed it back to him.   He held them for a few seconds, then returned them to the female.   She followed suit.  For the next minute or two, they repeated this behavior at least 5 times.  Eventually one of the waxwings flew away; I don’t know if it was the male or the female that left first, or if he or she even ate the fruits.  It was a trip to see.

CEWA feed ritual_05262014-19a_Silver Lake CP

Waxwings show a number of distinctive aspects of their breeding behavior.   Anyone who has marveled at the antics of big flocks of waxwings wintering in Florida as they decimate the fruit crop on a chosen tree knows they are extremely social birds, and this extends to the breeding season.  They aren’t territorial when breeding, and sometimes nest in loose colonies of 10 or more breeding pairs.   Compared to most other passerines, they are among the latest to begin breeding activity.  Eggs aren’t usually laid until late May or early June, which seems to be an adaptation for synchronizing the appearance of the greedy youngsters to the availability of ripening fruit.  Waxwings are one of the few primarily frugivorous birds in North America; while many species feed on fruit opportunistically, none are as specialized to a fruit-eating diet as waxwings are.   They do incorporate more animal prey into their diet during the breeding season, probably for the protein content, but still fruit makes up a substantial portion of the diet of nestling birds.

CEWA feed ritual_05262014-18_Silver Lake CP

CEWA feed ritual_05262014-10_Silver Lake CP

CEWA feed ritual_05262014-19a_Silver Lake CP

CEWA feed ritual_05262014-13_Silver Lake CP

So this pair of birds engaging in repeated acts of fruit passing were likely still in the courtship/pair bonding stage of the nesting cycle.   The entry for cedar waxwings at Cornell’s Birds of North America Online site gives this account of the behavior I observed:

Typical courtship display in which mates alternately approach one another on a perch with hopping movements, sometimes touching bills. Usually initiated by male; successful when female reciprocates (Putnam 1949). This display is termed the Courtship Dance or Courtship-Hopping (Silloway 1904Crouch 1936Lea 1942). Courtship-Hopping begins in migrant flocks, and has been noted as early as Apr in California (Feltes 1936) and in Ohio (Putnam 1949). Courtship-Hopping often includes passing a small item (usually food item such as a fruit, insect, or flower petal, but sometimes inedible items, and occasionally object-passing may be merely simulated, with no object actually passed; Fig. 3) between male and female, interspersed with short hops away from and back toward mate. Display usually initiated by male, who obtains a food item and joins female at a perch (Putnam 1949). Male approaches female by hopping sideways and passes item to female with turn of head (usually both birds face same direction). Female typically hops away from male, then hops back and returns item to male. Male then responds by hopping away, often performing bowing movements between hops, before hopping back and repeating the sequence. The display may be repeated a dozen times or more (Tyler 1950) and is usually terminated when the female eats the food item (Putnam 1949). Bouts of courtship-feeding may be interspersed with fast circular flights around nest area. Crouch (1936) observed an apparent extension of passing behavior in which female would pass last food item back to male after he had delivered food to her, either at or away from nest. Then the mates would allopreen and bill. Copulation is usually preceded by Courtship-Hopping (Putnam 1949).”

CEWA feed ritual_05262014-16_Silver Lake CP

Though I saw no allopreening, circular flights, or copulation, I was ecstatic about observing this fascinating behavior.  One of the great joys of natural history study is knowing that even after observing a species, sometimes extensively, for years or even decades, there is always the potential for learning something new about them.

 References:

May, P.G.  1982.  Secondary succession and breeding bird community structure: Patterns of resource utilization.  Oecologia 55(2): 208-216.
Witmer, M. C., D. J. Mountjoy and L. Elliot. 1997. Cedar Waxwing (Bombycilla cedrorum), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:  http://bna.birds.cornell.edu/bna/species/309

 

Crotalus ad DOR_05172014-01_Ocala NF FR05

Shameful shit

 May 18, 2014

I have no words to fully express my shock and profound sadness on finding this magnificent animal, head and neck crushed by a cretinous driver, in Ocala National Forest yesterday. I’ve been fantasizing about photo ops of an EDB crossing a forest road for several years now; this was not the picture I had envisioned. We watched with disgust and disbelief as he slowly writhed and tried to gape while the last spark faded from his defiant eyes.

Crotalus ad DOR_05172014-08_Ocala NF FR05

A 4′, heavy-bodied healthy eastern diamondback crossing a pale sand road in Ocala National Forest. This animal was run over intentionally.

What kind of deeply depraved mindset does it take for someone to do this?

Some of my Facebook friends captured some of the thoughts that occurred to me, and some that didn’t.

“Damn, Peter, that ruined my day. People suck. Some people suck.” – John Jett

Ours too, JJ.

“Only someone unhappy would do this.”  – Mary Ohlman Shaperow

Unhappy and morally retarded, Mary.

Crotalus ad DOR_05172014-03_Ocala NF FR05

“The real issue, is that it’s really hard to change people’s minds on this, it is really, really entrenched in so many…just ignorance multiplied and taught to others.”  – Chris Kincaid

Education is one answer, Chris.  But it’s futile when dealing with closed minds.

“Oh man, I hate hate hate hate hate to see this. I’m preaching to the choir here, but I truly can’t understand how/why so many people aren’t able or willing to respect this amazing (and very important) species.” – Janson Jones

Keep preaching, Janson.  You make a difference.

Crotalus ad DOR_05172014-20_Ocala NF FR05

“That is FUCKED UP. Period.”  – Patrick McGowan

Right on, Patrick.  Right on.

 

EATO_032411_03_Tiger Bay RR

Why no sparrows?

Sunrise in the scrub.  Forest Road 46, Ocala National Forest

Sunrise in the scrub. Forest Road 46, Ocala National Forest

May 10, 2014

I find being in big expanses of native habitat around sunrise has the effect of producing brief moments of clear-headed thinking.  For me, this is especially true of the more open habitats, like early stage scrub, where you can easily track the incremental effects of the ascending sun as the surging morning light progressively highlights newly visible features of the environment.  I was sitting in just such a dense patch of oaky scrub in the Juniper Prairie Wilderness area of Ocala National Forest on Wednesday when one of those rare moments of lucidity raced through my normally muddled brain.   I kid you not that I was actually sipping my tea when I heard the clear whistle and trill of drink-your-tea  coming from the dense scrub.  At that moment, I realized that I had made a foolish and easily refutable claim in my last post, in which I pondered the absence of breeding sparrows in most Florida habitats.  I claimed that for the most part Bachman’s sparrow was the only breeding species in most inland or upland habitats of the Florida peninsula.   

As it turns out, Bachman's sparrow isn't the only widespread breeding sparrow in the Florida peninsula.

As it turns out, Bachman’s sparrow isn’t the only widespread breeding sparrow in the Florida peninsula.

Astute birders and natural historians no doubt immediately recognized the fallacy of that statement – there’s one species of sparrow that breeds in a variety of habitats in peninsular Florida, and can be incredibly abundant in some, including scrub.  Towhees are sparrows, really.  We just don’t call them sparrows.  A bit larger and more conspicuous in plumage than the typical cryptically-hued sparrow-type sparrows, but members of the same family (Emberizidae) nonetheless, and classified within that family as belonging to the same clade as the New World sparrows.   In most respects, their ecology is similar to that of the smaller sparrows – they are omnivores, feeding more on seeds and plant-derived foods during the fall and winter, and switching to more of an animal-based diet during the breeding season when their voracious offspring need more protein than is available in most plant foods.  They are fond of open or early successional habitats, though in Florida they nest in open-canopy woodlands like flatwoods or scrub as well.  Eastern towhees were by far the most common breeding birds I heard in most areas of the Juniper Prairie Wilderness scrub on this beautiful May morning.

Drink your tea, he said.  I was way ahead of him.

Drink your tea, he said. I was way ahead of him.

The juvenal plumage of eastern towhees clearly shows their affinity to sparrows.

The juvenal plumage of eastern towhees clearly shows their affinity to sparrows.

So we do have a common breeding sparrow in many peninsular Florida habitats.  But that doesn’t really resolve the conundrum – in some ways it magnifies it.  If this one species of emberizid can successfully maintain viable populations here, why not the other sparrows with which it frequently co-occurs in breeding bird communities further north?  It’s not hard to find towhees nesting along with other species typical of the shrub-sapling stage of old field succession, such as song and field sparrows.  The enigma is further confounded by the fact that field, song, grasshopper, chipping, swamp, and several other rarer sparrows (Henslow’s, LeConte’s, Lincoln’s) can be found wintering in these habitats in Florida.  But none stay to breed.  Why not?

Field sparrows winter in peninsular Florida, but don't stay to breed here.

Field sparrows winter in peninsular Florida, but don’t stay to breed here.

The sparrow problem is just one component of the riddle of peninsular Florida’s low breeding bird diversity.   One of the most well-documented trends in landscape ecology is the profound latitudinal gradient in species diversity among a wide range of taxa – as you move from the temperate zones towards the tropics, the number of species of many, many groups of organisms increases dramatically.  Though this pattern is widely known, it hasn’t been clearly explained in terms of an underlying cause or mechanism.   More than a dozen hypotheses have been suggested to explain the higher diversity in the tropics, but none is universally accepted, and in fact most of the hypotheses are not even mutually exclusive.  Like many complex ecological phenomena, the origin of these diversity gradients is probably multifactorial, arising from a number of interacting factors and causes.  Higher productivity, greater climatic stability, a longer evolutionary history, greater importance of biotic interactions such as competition, predation, and parasitism – all of these and many more may contribute to the higher tropical diversity.  But despite the fact that Florida is at a lower latitude than most of North America, and might therefore be expected to have higher bird diversity than areas further  north, at least for breeding species of land birds that isn’t true.   The picture with respect to breeding bird diversity in eastern North America is more complex and perplexing. 

 

Number of breeding land bird species in North America, from a paper by Cook (1969).

Number of breeding land bird species in North America, from a paper by Cook (1969).

The figure above, originally published in a 1969 Systematic Biology paper by R.E. Cook, contains a wealth of head-scratching trends in diversity.  I first saw this figure in Eric Pianka’s classic little book Evolutionary Ecology over 30 years ago, I think, and it has taunted me ever since. This map shows the number of breeding land bird species in 300-square mile blocks, and even at this crude level of resolution, the contradictions and puzzles are enough to make me swoon.   If you focus on the numbers of species breeding in the middle of the continent, starting in the prairie provinces of Canada and working towards Mexico and Central America, the temperate-tropical diversity gradient is apparent.   But to the east, something funky is going on.  Diversity actually is greatest at higher latitudes.  In particular, notice the column of blocks that includes Florida – there are 141 breeding species in the region of the Great Lakes, but in the southeast block that includes Georgia and North Florida, there are only 93 breeding species.  Though there is no number for the block that contains peninsular Florida, by my reckoning that number is about 72.  I’ll repeat my previous claim – breeding bird diversity of land birds in the Florida peninsula is abysmal relative to the rest of eastern North America.

The reduced numbers of species of some taxa in Florida has been explained at times by the so-called peninsula effect.  For a variety of types of organisms, peninsulas often show reversed diversity gradients from base to tip, though the mechanism for this trend is difficult to pin down.  One component for some organisms may have to do with the colonization and extinction dynamics of populations in the peninsula (I’m referring here to extinction of individual populations in an area, not an entire species).   Because peninsulas have less direct connectivity with nearby land areas that may serve as a source of colonizing organisms, they may lack populations of species with poor vagility that are unable or unlikely to reach the more distant parts of the peninsula.   Further, smaller extents of appropriate habitats in peninsulas may support smaller populations of the organisms that do manage to colonize, producing higher extinction rates for these populations.   Finally, the range of habitat types may be reduced in peninsulas, preventing some species from colonizing in the first place.  With respect to birds, the colonization argument just doesn’t work.  Many of the bird species whose absence as breeding birds puzzles me migrate through or winter in the peninsula in large numbers, so getting here isn’t the problem.  The population size and habitat availability arguments may contribute to Florida’s low breeding bird diversity, but they aren’t the whole story.   

Northern parulas are abundant foliage-gleaners of hammock habitats, but other warblers and foliage-gleaners typical of forest habitats further north are absent.

Northern parulas are abundant foliage-gleaners of hammock habitats, but other warblers and foliage-gleaners typical of forest habitats further north are absent.

For both islands and peninsulas, greater land area is related to greater species diversity, called the species-area effect.   That may explain some of Florida’s lower breeding diversity, but not all of it. Look at the number of breeding bird species in the block that includes that little sliver of land called the Isthmus of Panama – it has around 600 breeding species! Land area isn’t everything. 

One of the problems with the peninsular effect as an explanation for Florida’s low diversity of breeding birds is that this reversed diversity gradient of breeding birds isn’t restricted to the Florida peninsula – it is general to the southeastern United States.  But even so, many bird species common as breeders in the southeast don’t make it into the peninsula.  There’s something else going on here.  One of the proposed explanations for reduced densities and diversity of breeding land birds in the southeast is related to the dynamics of primary productivity in temperate habitats.  Simply put, temperate habitats further north experience a much more concentrated burst of plant growth in spring as all of the dormant vegetation begins leafing out around the same time, providing huge amounts of tender nutritious leaf material for herbivorous invertebrates.  This burst of productivity works its way up the food web, resulting in a glut of food for the breeding birds.   Spring certainly brings a burst of new growth in Florida, but probably not as dramatic and concentrated in time as in more northerly habitats.   One factor contributing to this reversed diversity gradient among forest bird communities of eastern North America, which benefit hugely from this spring burst of productivity,  is that more northerly bird communities show show both decreased extinction rates of individual species in the community, and lower turnover rates in community composition (number of species that disappear or appear between years).  Stated another way, more southerly populations of these forest-breeding birds are more likely to disappear over time, and more likely to be replaced by other species.  

Red-eyed vireos are far more abundant in migration than as breeders in peninsular Florida

Red-eyed vireos are far more abundant in migration than as breeders in peninsular Florida

The breeding bird communities of peninsular Florida’s broad-leaved forest habitats (hammocks) have always struck me as being particularly depauperate.  Northern parulas are usually abundant, but other species of foliage-gleaning warblers are hard to find.   There are no ground-foraging forest warblers breeding here at all, though ovenbirds are common in migration in these habitats.    Red-eyed vireos, one of the most abundant breeding species of eastern deciduous forest, are present as breeders in many Florida hammocks, but at much lower densities than further north.   It seems to me that both the productivity burst effect and area effects may be at work here.  Many of the characteristic tree species of hammocks are evergreen; even though these species do put out new foliage in the spring in a leaf flush (live oaks, for example), the boom-bust nature of the resource experienced by birds breeding in forest habitats  further north is not as dramatic here.   In addition, hammocks themselves tend to be more patchily distributed and limited in area than do deciduous forest tracts in eastern North America.  Smaller extents of habitat support smaller populations, which are more likely to go locally extinct, and exclude wide-ranging species that need large expanses of appropriate habitat in order to maintain viable populations.  Hammock habitats in the peninsula are actually probably far more extensive now than they were historically; fire suppression in some formerly extensive habitats has resulted in expansion of fire-intolerant hammock habitats in many areas that once supported vast tracts of fire-dependent plant communities like sandhills and scrub.

Hammocks are historically patchily distributed habitats in peninsular Florida, but have expanded greatly due to fire suppression.

Hammocks are historically patchily distributed habitats in peninsular Florida, but have expanded greatly due to fire suppression.

Other forest breeding guilds (a guild is a group of species that use similar resources in a comparable way) besides the foliage-gleaning warblers and vireos are equally perplexing.   Tyrannid flycatchers, for example.   Eastern deciduous forests further north typically support several species of forest-breeding flycatchers, including great crested flycatchers, eastern wood pewees, and Acadian flycatchers.  We have lots of great cresteds, but Acadian flycatchers and pewees become harder and harder to find the further south you go in the peninsula.   It’s tempting to suggest competition with insects as a possible link to the lowered diversity of breeding flycatchers in Florida – the superabundant and diverse dragonfly community must to some degree reduce the resource base, flying insects, on which tyrannids depend. 

Do dragonflies, which presumably compete with flycatchers for aerial prey, reduce diversity and density of tyrannids in Florida?

Do dragonflies, which presumably compete with flycatchers for aerial prey, reduce diversity and density of tyrannids in Florida?

Great crested flycatchers are common breeding birds in Florida, despite any competition from odonates.

Great crested flycatchers are common breeding birds in Florida, despite any competition from odonates.

Other forest tyrannids, like this Acadian flycatcher, are scarce as breeders.

Other forest tyrannids, like this Acadian flycatcher, are scarce as breeders.

But none of those explanations seem to fit the sparrows.  Most sparrow species in eastern North America are characteristic of disturbed or successional habitats – song, chipping, field, grasshopper, and so on.   Successional or disturbed habitats by their nature are patchy in distribution, often limited in areal extent, and prone to disappearance over time as they are replaced during the process of secondary succession.  Superficially, an old-field habitat in Florida is remarkably similar to one in Virginia, except that breeding bird diversity and density is dramatically lower.  Examine the breeding density maps prepared by the U.S. Fish and Wildlife Service (http://www.mbr-pwrc.usgs.gov/bbs/bbs.html) for any of the missing Florida breeders.  I’ve pasted these breeding density maps below for four species: song sparrow, field sparrow, chipping sparrow, and eastern towhee.  The low breeding abundance or complete absence in peninsular Florida of the three “typical” sparrows is apparent, and in marked contrast to that of the towhee, which actually shows increased breeding abundance in the central-southern portion of the Florida peninsula.  

Song sparrow, a common winter resident that doesn't breed in Florida

Song sparrow, a common winter resident that doesn’t breed in Florida

Song sparrow breeding abundance

Song sparrow breeding abundance

Field sparrow

Field sparrow

Field sparrow breeding density

Field sparrow breeding density

Chipping sparrow

Chipping sparrow

Chipping sparrow breeding density

Chipping sparrow breeding density

Does the smaller burst of insect productivity in spring prevent species like this field sparrow from successfully raising young here?

Does the smaller burst of insect productivity in spring prevent species like this field sparrow from successfully raising young here?

Florida race of the eastern towhee, Pipilo erythropthalmus alleni.

Florida race of the eastern towhee, Pipilo erythropthalmus alleni.

Breeding density of eastern towhees.  Why are they so much more successful in the southeast than other sparrows?

Breeding density of eastern towhees. Why are they so much more successful in the southeast than other sparrows?

So towhees love Florida, even though the productivity burst model may affect Florida towhees to some degree as well.   Eastern towhees in Florida show the same shift in diet between winter and spring as do more northerly populations; they switch from a greater reliance on plant-based foods in the winter to more animal prey in the spring and summer.  However, the magnitude of the shift is of a lesser magnitude in Florida towhees, who rely more on plant-based foods during the breeding season than do northern populations, perhaps hinting at a lower availability of insects in the Florida habitats used by towhees as well. 

But what is it about towhees that makes them so successful in Florida, while all the other sparrows of similar habitats hightail it north in the spring?  I’m still awaiting enlightenment.

EATO_030711_08_Ocala NF Hopkins

 

Reference:  Cook, R.E. 1969. Variation in Species Density of North American Birds.  Systematic Biology 18 (1): 63-84.
BOBO 050109_4 Emeralda MCA

The saddest season

April 27, 2014

Thursday last week, I saw my first bobolinks of the season,  in the marshes of the Tomoka River near Tomoka State Park.  Several flocks of 30-40 birds were winging across the marsh, with occasional dink notes.  It was the calls that first made me aware of them.  I saw more bobolinks on Saturday – a flock of perhaps 100 birds or so flying over the marshes and impoundments of Emeralda Marsh Conservation Area.

Bobolink males at Lake Woodruff NWR

Bobolink males at Lake Woodruff NWR

Bobolinks are probably my most anticipated spring migrant, at least among the species that I see reliably every year.   When I read the listserve reports of amazing warbler lists being tallied at Ft. DeSoto and some of the Brevard County sites, I suffer from severe attacks of envy, and in those moments have dozens of new most-anticipated species.   But I have yet to see a Swainson’s warbler, after decades of birding, so snagging that one doesn’t appear to be imminent.   But I do see bobolinks somewhere every year, and that almost makes up for the more glamorous stuff I can’t make myself drive hundreds of miles to see.

Seeds are a major food item while bobolinks are migrating.

Seeds are a major food item while bobolinks are migrating.

But bobolinks count for a lot.   It’s a pity we get to see them for such a short time each year; they are very interesting little blackbirds.  Bobolinks are devoted seed-eaters; the epithet in their latin name Dolichonyx oryzivorus explicitly references their fondness for grain crops (oryzivorus = rice eater).  The color pattern of males in breeding plumage is splendid and rare, one of the relatively few species of birds to exhibit reverse countershading, in which the upper surface is more brightly colored than the underside.   The more typical countershading places the darker hues on top and lighter below, which is hypothesized to make them less apparent in a top-lit environment.  Reverse countershading  does the opposite – it makes the bird stand out against its background. Not surprisingly, bobolinks show a high degree of sexual dichromatism – the females are predominantly a lovely straw-colored yellow-brown, and so are the males during the non-breeding  season.   The highly apparent colors of the male are coming in while bobolinks are in Florida – some males still show signs of molt going on.

Male bobolink showing a trace of basic (winter) plumage in the belly feathers.

Male bobolink showing a trace of basic (winter) plumage in the belly feathers.

Female bobolinks are more subtle than males, but equally beautiful.

Female bobolinks are more subtle than males, but equally beautiful.

The reverse countershading of a male bobolink in alternate plumage makes them very conspicuous in open environments, like the grasslands where they breed.

Their extreme sexual dichromatism may be related to their breeding system, another unusual  component of their life history. The majority of passerines are monogamous, but bobolinks are one of a number of blackbird species that practice polygyny, at least occasionally.  Some males have multiple mates, usually 2 but ranging up to 4.   The quality of a male’s territory is probably a big factor determining which males get multiple mates while others are monogamous, but you have to figure the visual displays of the male play some part.  Quite likely, the two traits are correlated – males with brighter plumage probably also have better territories and other qualities of interest to a discriminating hen bobolink.

Bobolink males arrive on their breeding grounds before females to stake out the best territories, and maybe get a second mate.

Bobolink males arrive on their breeding grounds before females to stake out the best territories, and maybe get a second mate.

What I love most about bobolinks though is their exuberance for life.  They seem to almost always be on the move, with definite places to go to in mind.  Most of my sightings of bobolinks are of flocks in transit, and my views of them are cruelly brief.   But every now and then I get to see  a group working methodically through an overgrown field or marsh, birds in the rear of the group constantly leap-frogging those near the front.   And if I’m really lucky, I get to hear a big flock in which the males are singing.  The song of the bobolink is one of the most rollicking, joy-filled vocalizations of any bird I’ve actually experienced.   How’s that for egregious anthropomorphism?

BOBO 050109_5 Emeralda MCA_3

As much as I love seeing bobolinks when they finally appear in late April, they bring mixed emotions.  They are the tail of the progression of migrants that has snaked through the peninsula in the previous several months.   And as the last stragglers of those transient migrants finally leave Florida by the second week of May or so, an ugly and inexplicable fact hits me in the face again every year – Florida’s summer bird diversity sucks.  Okay, so that’s a massive overstatement – even at its worst, Florida’ avifauna is always amazing, but it reaches its nadir of diversity in mid-summer.  And preceding that low point, all through the latter half of spring, diversity and abundance of birds generally follows a protracted and depressing downward slide.

What makes this vexing to me is that it is exactly the opposite of the seasonal patterns of bird diversity for birders in most of North America.   At a time when most birders are enjoying a dramatic increase in abundance and diversity, Florida birders have to wait until fall migration begins in mid-summer for avian diversity to begin to climb.  Bird species richness reaches a peak in Florida in late-winter to early spring, and then crashes in late spring, in the last week of April and first week of May.   The graph below shows the severity of the decline in diversity – these data were collected over the course of 7 years of surveys at Emeralda Marsh Conservation Area in Lake County, and though they don’t represent the entire range of inland habitats, they are a pretty accurate representation of changes in diversity in peninsular Florida.

Seasonal changes in the number of species seen per census at Emeralda Marsh Conservation Area.

Seasonal changes in the number of species seen per census at Emeralda Marsh Conservation Area.

This decline in diversity isn’t limited to any one specific group of birds – it’s a general phenomenon among many taxonomic groups.   The graphs below show the seasonal changes in diversity for 2 taxonomic divisions – passerines (Order Passeriformes, the perching birds) and non-passerines (all other orders), and the difference in the overall pattern between these two groups is minimal.  Breeding diversity for most taxa of birds that occur in Florida is at a minimum in the summer.

The decline in diversity in spring is a bit more drastic for passerines, but in general most bird taxa show the same patterns.

The decline in diversity in spring is a bit more drastic for passerines, but in general most bird taxa show the same patterns.

Non-passerine

Every spring after the semester ends, I drive to northern Virginia to visit my dad for a week, and spend as much time as I can in the glorious Virginia countryside when bird breeding activity is kicking into high gear.  The difference between Virginia and Florida in both diversity and abundance of breeding birds is obvious and inescapable.

Why don't chipping sparrows breed in Florida?

Why don’t chipping sparrows breed in Florida?

Part of the reason for such divergent phenological trends in diversity between Florida and the rest of eastern North America  derives from the incredibly high diversity of wintering birds here, of which I’m very appreciative.  But it makes the dearth of birds during the breeding season so much more painful and sad.  When they leave to repopulate the north, they aren’t replaced by tropical migrants coming in to take their place.  What’s really puzzling to me is why so few of the species that winter here don’t establish breeding populations.  There are so many species that are abundant breeders in the mid-Atlantic for which there appears to be suitable habitat for breeding, but for whatever reason, they don’t breed here.  Take the ubiquitous winter visitor, the chipping sparrow. They are common breeders in Virginia in a variety of human-modified environments, many of which seem to be replicated in Florida, yet they don’t breed here.  It’s something of a conundrum to me why so few sparrows breed in Florida – the specialized Bachman’s sparrow is the only breeding sparrow found in inland habitats in most of peninsular Florida (the critically endangered Florida grasshopper sparrow has a very limited range and is an extreme habitat specialist in the dry prairies of the southern half of the peninsula). In Virginia, it’s easy to find chippers, song sparrows, grasshopper sparrows, and field sparrows in a variety of successional or disturbed habitats during the breeding season.  Why are there so few breeding species of sparrows in Florida?

Indigo buntings are common as dirt in agricultural or successional habitats in northern Virginia, but far less common as breeders in Florida.

Indigo buntings are common as dirt in agricultural or successional habitats in northern Virginia, but far less common as breeders in Florida.

Even for species with wide breeding ranges that do breed in Florida, like blue grosbeaks or indigo buntings, the difference in abundance between Florida and Virginia is striking.  In mixed agricultural habitats, indigo buntings are as regular as telephone poles along some rural roads; it’s hard to find a spot in appropriate habitat where you can’t hear one singing somewhere.   In Florida, indigo buntings aren’t terribly difficult to find as breeders, but in nothing like the densities seen further north.

This abundance map of indigo buntings during the breeding season was prepared from Breeding Bird Survey data by the U.S. Fish and Wildlife Service.  The difference in abundance between most of the eastern half of the continent and Florida is huge.

This abundance map of indigo buntings during the breeding season was prepared from Breeding Bird Survey data by the U.S. Fish and Wildlife Service. The difference in abundance between most of the eastern half of the continent and Florida is huge.

For now, all I can do to deal with the loss of birds that’s underway is to learn to appreciate even more the hard-core birds that actually stay here to breed.   And look forward to the first southward movement of migrating passerines, which will begin some time in July.  That’s not that far away.