The Similar Affinities of the Brown-headed and Pygmy Nuthatches

The brown-headed nuthatch (Sitta pusilla) and the pygmy nuthatch (Sitta pygmaea) both descend from a common ancestor that used to range all across the North American continent.  Now, the former inhabits the southeast, while the latter lives in disjunct populations scattered across the southwest.  They are similar in size, appearance, and habitat preference.  The pygmy nuthatch has a darker brown cap than the brown-headed nuthatch, and its tail has white spots not found on its cousin, but otherwise they are physically identical.  Both favor mature pine forests prone to frequent fire.

Brown-headed nuthatch in its usual position, perched upside down on  a tree trunk.  They live in mature southern pine forests and originally were abundant in long leaf pine savannahs.

Range map of a brown-headed nuthatch.  I suspect the average abundance figures on this map are misleading.  They are probably more abundant on the coastal plain than the piedmont, but more people live in the piedmont and report seeing them there.

Pygmy nuthatches live in Ponderosa pine forests out west.

Range map of the pygmy nuthatch.

Fossil evidence of a small forest-dwelling bird, such as a nuthatch, is understandably scarce, yet does exist.  The oldest fossil specimen from the Sitta genus dates to early in the Miocene (~23 million years ago) and was excavated in France.  In North America 2 specimens of nuthatches dating to the middle Pleistocene (~600,000 BP) were found in Porcupine Cave, Colorado.  One was positively identified as the still extant white-breasted nuthatch (Sitta carolinensis).  Brown-headed nuthatches and pygmy nuthatches became geographically isolated from each other during the latter stages of the Pleistocene.  A desert scrub and grassland corridor existed during the early and middle Pleistocene that linked southeastern North America with the west.  During some undetermined time in the mid to late Pleistocene ecological changes closed this pathway for some species.  Like burrowing owls and Florida scrub jays, brown-headed nuthatches became isolated from their western populations.  In the cases of the jays and the nuthatches, speciation occurred between the eastern and western populations.  As far as I know, no attempt has ever been made to see if these 2 species of nuthatches would interbreed in captivity.  I bet they could.

Ponderosa pine forests were reduced to 2 refuges durng the Last Glacial Maximum when they were displaced by spruce forests.  Studies of pygmy nuthatch genetics suggests they only survived in 1 of the refuges.

Nuthatches are smart birds that use chips of bark to probe for insects burrowing in wood.  Seeds are also an important part of their diet.  Family groups usually consist of a male, female, and a male offspring who helps take care of the nestlings.  The annual survival rate is about 70%.  The brown-headed nuthatch has an outlying population in the Bahamas where it lives in Carribbean pine forests.  Prospects for survival there are doubtful because developers are going to eliminate all of the pine forests, none of which are protected by nature preserves.

I’ve never seen a brown-headed nuthatch, though supposedly they range into Augusta, Georgia.  However, I did see a white-breasted nuthatch at the Silverbluff Audubon Center in Silverbluff, South Carolina.  See https://markgelbart.wordpress.com/2011/11/28/the-silver-bluff-audubon-center/

Florida Sand Scrub Habitat Hosts Pliocene-Age Relicts

About 2 million years ago, an exceptionally dry climate phase occurred over most of North America.  Grasslands and scrub habitat stretched in a continuous belt from southern California to Florida.  Most of the large vertebrates that thrived in such an environment such as llamas, camels, flat-headed peccaries, pronghorns, horses, and donkeys have been rendered extinct or extirpated from the southeast, but the sandhill habitats of north and central Florida still host scores of relict invertebrates in addition to the Florida scrub jay.  Specimens found at the Inglis fossil site in Citrus County, Florida provide a glimpse of the fauna formerly inhabiting the once extensive arid grassland and scrub habitat that existed across the southeast during the late Pliocene.  In addition to the above mentioned species, the antelope jack rabbit (Lepus alleni), now confined to the American southwest, was a common component of the ecosystem when the climate was drier.

Antelope jack rabbits and an extinct species of jack rabbit lived in southeastern North America during the late Pliocene when the climate was much drier than it is today.

Pleistocene glacial cycles also fostered drier climates and an increase in scrubland and grassland habitat, but these environments never again formed an unbroken corridor from west Texas to Florida.  I hypothesize that pine and oak forest species evolved a greater drought tolerance and were able to grow in some areas with favorable conditions, thus forming interdicting fingers of habitat that prevented some scrubland species, such as jack rabbits, from recolonizing the southeast.

The shaded black areas indicate sand scrub habitat–an environment that once stretched from southern California to Florida.  A continous sand scrub belt hasn’t existed for at least 1 million years.  Many small species have become isolated in these relict habitats.

Most of Florida is so low lying in elevation that numerous high sea stands have inundated much of the state.  High sea stands have occurred on many occasions dating as far back as the Miocene and as recently as the Sangamonian Interglacial of the late Pleistocene.  High hilltops, however, remained above sea level as islands surrounded by sandy beaches.  The sand scrub areas of today are simply remnants of these sandy beaches.

The Florida scrub jay (Aphelocoma coerulescens) used to be considered the same species as the Western scrub jay (A. californicus).  About 20 years ago scientists declared they were a separate species because they have a shorter broader bill and are less able to disperse following an acorn crop failure.  Fossil evidence shows that Florida scrub jays were a distinct species as long as 2 million years ago.

The Florida scrub jay is an example of a scrubland species now isolated from populations of its ancestral species–the Western scrub jay.  The fossil evidence from Inglis shows that by 2 million years ago, the Florida scrub jay was already a distinct species from the Western scrub jay.  Florida scrub jays are habitat specialists that will not even travel through unsuitable habitat.  They are endangered today because much of their habitat has been transformed into subdivisions and citrus orchards.  Families of scrub jays living on 1 patch of remnant scrubland will not fly through an orange orchard to reach another patch of scrubland.  Studies show that for the bird to survive, they will need corridors of protected scrubland habitat to prevent extinction through inbreeding.

Scrub jays are tame birds, known for taking food from people’s hands.  Offspring help care for young, a habit that makes them semi-communal birds.  Scrubland habitat in Florida provides a reliable crop of acorns which along with seeds, insects, and small lizards makes up the bulk of their diet.  Despite growing stunted, sand live oaks (Quercus germinata). myrtle oaks (Q. myrtlefolia), and scrub oaks (Q. iopina) provide plenty of mast, unlike Rocky Mountain oaks which may fail to produce acorns in the harsher climate there.  This accounts for the behavioral difference between Florida scrub jays and Western scrub jays. The former never evolved the habit of dispersing when the acorn crop fails because in Florida’s climate that seldom happens, but the latter did of necessity and is therefore more widespread and not endangered.

Over 70 species of invertebrates are also unique to the Florida scrublands.  Because it’s such a harsh environment, most plants growing there are high in toxins and have evolved thorny structures to discourage herbivores.  But these defenses don’t deter many of the insects that have co-evolved with them.  Scrub rosemary is a toxic plant that unwillingly hosts species of a grasshopper, moths, and beetles. 

Florida sand scrub wolf spider killing an insect.  Note the spider is the same color as the sand.

Florida sand hair ant (Componotus floridanus).  The hair enables them to travel through sand without sinking.

Other unusual invertebrates are specially adapted to living in sand.  They have waxy armor that protects them from being shredded by sharp grains of sand, and they have stiff hairs that help them locomote through sand without slipping backward.  One species of harvester ant has hair under its mandible in the shape of a basket to carry sand when they excavate their 3 foot deep nests.  Hunting wasps are common, and they actively defend their paralyzed prey because in the thinly vegetated habitat, it’s more likely to be discovered by other carnivores such as tiny yellow predatory ants, wireworms, and robber fly larvae which abound under the sand. 

 Each isolated sand scrub community has its own species of short-horned scrub grasshopper…an ideal case study for biologists interested in evolution.  The wealth of unique arthropod species found in the Florida scrub attracts entomologists and evolutionary biologists who consder the scrublands a mecca of potential new discoveries.  And just think–these invertebrate species once shared a wider range with now extinct megafauna.

Reference:

Deyrup, Mark and Thomas Eisner

“Last Stand in the Sand”

Natural History Magazine (102) 12 December 1993