Geography 316:  Biogeography

In progress 11/20/2001

The Biogeography of the Acorn Woodpecker (Melanerpes formicivorus)

by  Sam Pedone,  student in Geography 316, Fall 2001

Kingdom:  Animalia                                                                         Phylum:  Chordata Class:  Aves Order:  Piciformes Family:  Picidae Genus:  Melanerpes Species:  (Melanerpes formicivorus)
	Adult female acorn woodpecker storing an acorn in a granary.  Photograph © Peter LaTourrette.  2001.

Description of Species:
     The acorn woodpecker will measure anywhere between seven to nine inches in length.  It has a pure black back, a red crown, and white eyes that are surrounded by black.  It will have a white patch on the forehead.  There is also a small, yellowish patch on the throat.  Male and female patterns differ slightly.  The males red crown touches the white forehead while the females red crown is separated from the white forehead by black.  One should notice the white rump and the white patch near the tips of the wings of both sexes while in flight (Stokes 1996). 

Feeding Habits:

     Melanerpes formicivorus will feed on a assorted diet of insects, tree sap, and fruit.  It will occasionally eat grass seeds, lizards and bird eggs (University of Michigan 1995).   Acorns are actually a supplemental portion of the diet for acorn woodpeckers.  They are stored extensively in the fall to be eaten when climatic conditions make insects unavailable (Koeing 1995).  Working together in communal groups, thousands of holes will be drilled into the dead limbs of trees, utility poles, and even buildings for the storage of acorns.  The network of holes that are drilled for the storage of the acorns is called a granary.  The group may store at least 50,000 acorns in a season that will be consumed during the rest of the year (University of Michigan 1995).  Usually, acorns are removed singly from trees, but the bird may also break off a twig that is holding up to three acorns as a time saving practice (Koeing 1995).  Members of the group are constantly rotating and repositioning the stored acorns to protect against predation from squirrels or other competitors.  The acorns must be rotated and repositioned because the acorns, and the holes that they have drilled, will expand and contract from temperature and moisture fluctuations.  The hole in which an acorn is placed into has to be the right fit.  If the hole is too big the acorn may fall out or be taken by a competitor.  If the hole is too small, and has to be  forced in, it will crack the shell and cause the acorn to rot (David video).   

     Insects are the preferred food of the acorn woodpecker due to there high nutritional values.  The acorn woodpecker mostly  feeds on flying ants.  It also feeds on beetles or any other insect that it is capable of catching (Koeing 1995).  An acorn woodpecker will often perch at the tops of trees while flycatching in order to catch and insect in mid flight, otherwise it will forage in or near the canopy of the trees.  In flight they will generally catch insects high above the canopy in a precise flight pattern in which an individual insect is targeted and consumed.  Flights are often nearly vertical and last up to 43 seconds if necessary, but it commonly takes closer to five seconds to catch something (Koeing 1995).  An individual will usually fly once every one to two minutes to catch insects (Koeing 1995).  Insects may be stored for extended periods of time in cracks or crevices to be consumed at a latter time.   

    M.  formicivorus will also engage in communal sap sucking as another dietary supplement.  Sap holes are drilled smaller in diameter and shallower than the acorn storage holes.  They are usually spaced apart in a similar patterns (Koeing 1995).  These holes are generally used for several years before they dry up.  This bird species is also known for feeding on seeds at bird feeders that people set out (Stokes 1996).  

Breeding Habits:

     Melanerpes formicivorus live in communal groups consisting of several active breeders and a number of helpers that are typically related to the breeders.  There are up to four breeding males and one to breeding two females.  The helpers do not mate with each other or the mating pairs.  This kind of mating system is known as polygynandry.  All individuals within the group are close relatives, usually brothers and sisters,  except for the  co-breeder males.  They are never related to joint-nesting females. Incest avoidance is maintained because the helpers only inherit and become co-breeders following reproductive vacancies.  A vacancy will occur when the breeders of the opposite sex die and are replaced by unrelated birds from another group (Hastings Natural History Reserve 2001).  This group will usually produce up to 10 offspring in a season.  All the eggs are laid and hatched in the same nest cavity, and they will all be cared for by all the members of the group.  

Typical polygynandrious composition of a group of acorn woodpeckers.  (Koeing 1987)

    Joint-females will often engage in reproductive competition when the group is newly formed or in times of low food production and storage.  During this breeding competition, females regularly destroy eggs laid by their co-breeders in order to assure that their genetic composition is passed along.  After females have established a normal laying sequence within the group or high food yields allow for the competition to be relaxed, egg destruction stops.  Reproduction competition between males comes in the form of attempts by one male attempting to disrupt copulation between another pair (University of Michigan 1995).           The breeding season for acorn woodpeckers extends from late April to mid-August.  The average clutch size is five white, elliptical eggs per one breeding female.  The incubation period is 11 days and both male and female breeders will engage in the incubation of the egg.  Once hatched, the nestlings will leave the nest after 30-32 days (University of Michigan 1995).  All members of the group are active in excavating nest sites and with the feeding and brooding of the young.  Nest cavities are drilled into large dead or living limbs in trees or snags.  The cavity has to be big enough to house the entire group at once.  The inside of the nest cavity is lined with fresh wood chips that act as insulation.  The same nest holes may be used for several seasons by the same communal group (Koenig 1987). 

Habitat:

    M.  formicivorus are closely associated with Oaks and Pine-oak woodlands.  They will also be found along riparian corridors, and in Douglas-fir, redwood, and tropical hardwood forests as long as oaks are near by (Koeing1995).   They are most abundant in the mountains up to the distributional limit of oak trees.  They are also common in urban parks and suburban areas wherever oak trees or other natural or human-made structures are available for acorn storage (Koeing 1995).  

Distribution of Oak Woodlands in California (  )	Photograph of a Oak Woodland.  Santa Rosa Plateau, Riverside County.  Photo © Dr.  Sharon Johnson (1985).

Distribution
     The Acorn woodpecker is found from western Oregon, California, and the arid American Southwest (including West Texas), through Mexico and the Central American highlands to Colombia (Husak 2001).  The Acorn Woodpecker is a year-round resident in its habitat (Husak 2001).  In California the range of M.  formicivorus extends from the north throughout  the Kalamath Mountain region and Cascade Mountains east to southwest Modoc County, and south along the western slopes of the Sierra Nevada Mountain range to the mountains encircling the south end of the San Joaquin Valley, and south almost to the Mexican boarder (Koeing 1995).

Map of Distribution:

Range of the Acorn Woodpecker.  Birding.com.  2001

Evolution:
     Birds are descended from reptilian ancestors. They arose nearly 200 million years ago, during the Mesozoic, the age of dinosaurs. In fact, birds are probably the closest living relatives of the dinosaurs. Many still question today how modern day birds evolved.  Did bird evolution originate from tree-dwelling reptiles, or was it from ground-dwelling reptiles?  How and why did feathers evolve into what they are today?  Were they simply a mechanism that regulated these early reptiles body temperature?  Or did feathers evolve specifically for flight?  Did these early reptilian species evolve feathers and wings from jumping from tree to tree in search of food, or was it a response to escaping from predators more efficiently?  These are only some of the questions that litter how and why modern bird evolution.  Many thought that the discovery of a small bird-like fossil in Germany would bridge together all of these different questions into one coherent answer to how modern birds evolved.  The fossil is called Archaeopteryx.

     The is the first defined species of bird fossil found. It was hailed as the "missing link"  between reptiles and birds (Gamlin 1986).  It was discovered and excavated in1861 at Bavaria, Germany.  About the size of a crow, Archaeopteryx was similar to birds in that it had asymmetrical feathers, three toes with claws, and long straight legs, and was capable of flight (Attenbourogh 1999).  Archaeopteryx possessed clawed digits on its' wings. This suggests to some a semi-arboreal existence in which Archaeopteryx climbed high up into the trees, using both wings and feet, in order to glide down in pursuit of insect prey (Gamlin 1986). The leg structure indicates that Archaeopteryx was capable of perching like a bird.  It was similar to reptiles in that it had a reptilian head containing teeth, a long bony tail, and clawed fingers (web page on evolution ). 

            The dinosaurs ruled the planet during the Mesozoic.  During this time period of over 100 million years, the Earth saw the come and go of a tremendous assortment of reptiles.  From within this bewildering amount of extinct dinosaurs, there are only a few potential avian ancestors that can be singled out (Wallace 1955).  Among these were the pterosaurs or pterodactyls, buoyant, light boned flying forms capable of avian flight.  The Saurischia was usually a heavy-bodied quadrupedal giant dinosaur, but it also had small light-bodied, fleet-footed bird-like relative.  The Ornithischia were bipedal runners with heavy armor for defense (Wallace 1955).  Various members in each of these groups possessed some avian features and could have been the beginning of the modern day bird.  Still, no one can be sure how the jump was made into this avian lineage.  Even these groups of dinosaurs mentioned above cannot prove any type of theory because there is no one defining piece of fossil evidence that shows any of these species are related to modern birds.  There are many (Wallace 1955) that believe that all birds have evolved from a single ancestor, perhaps close to Archaeopteryx, and it remains central to all discussions on bird origins because it is unique in being the oldest known complete fossil bird (Feduccia1996). 

Natural History:
    M. formicivorus is in the order Piciformes, family Picidae (Feduccia 1996).  Also in this order is the tree-creeper, honey-guide, toucan, wrynecks, jacamar, puffbirds and other perching birds that poses the zygodactyl foot, or a foot that has its' second and third toes on the anterior and the fourth toe on the posterior (Feduccia 1996).  It has modified in order for the bird to forage along tree trunks and in hitching up trees.  To do so, piciform birds spread the toes to the side and use their stiff-ended tail feathers as a brace on the trunk (Felucca 1996). 

     The Piciformes is an ancient order of bird, dating back to the Eocene (Feduccia 1996).  The piciforms used to be widely distributed during the Cenozoic time.  They are found today through out the tropical regions of the Old and New Worlds.  They were not represented in  Madagascar and Australia.  Due to climate deterioration during the late Cenozoic, combined with competition from the more advanced perching birds, the woodpeckers are the only piciforms found in the temperate zone (Feduccia 1996). 

Postulated cladistic relations of some major avian higher taxa (Feduccia 1996).

 
 Bibliography 

Bent, Arthur C.  1964.  Life Histories of North American Woodpeckers.  New York, NY.  Dover Publications.
 
Birding.com.  2001.  “Acorn Woodpecker:  Melanerpes formicivorus.”  [On-line]    
    http://www.birding.com  [October30, 2001].

Feduccia, Alan.  1996.   The Origin and Evolution of Birds:  Second Edition.  New Haven and London.  Yale University Press.

Husak, Michael S.  2001.  “Acorn Woodpecker:  Melanerpes formicivorus.”  [On-line] 
      http://tbba.cbi.tamucc.edu  [October 1, 2001].

Johnson, Sharon.  1985.  "Engelmann Oak Woodland #2."  [On-line]
    http://geoimages.berkeley.edu/Geoimages/Johnson/Biomes/BiomesSub/EngelmannOaks2.html

Koenig, Walter D.  1987.  Population Ecology of the Cooperatively Breeding Acorn Woodpecker.  Princeton, NJ.  Princeton University Press.

Koeing, Walter D.  & F.  Haydock.  2001.  “Social Behavior of the Cooperatively
    Breeding Acorn Woodpecker.”  [On-line] http://www.hastingsreserve.org/AcrnPkrs/AcrnPkrs.html  [September 28, 2001]. 

LaTourrette, Peter.  2001.  “Acorn Woodpecker (Melanerpes formicivorus).  [On-line] http://www.stanford.edu  [October 28, 2001].

Marie S. Harris.  1997.  “Melanerpes formicivorus: Acorn Woodpecker.”  [On-line]  http://animaldiversity.ummz.umich.edu [October 3, 2001].

Sibley, Charles  g and Burt monroe.  1991.  Distribution and Taxony of birds of the world.  New haven and London. Yale University Press.  

Skutch, Alexander F.  1985.  Life of the Woodpecker.  Santa Monica, CA.  Ibis Publishing Company.

State of California.  Division of Forestry.  Map

Stokes, Donald and Lillian.  1996.  Field Guide to Birds:  Western Region.  New York,   NY.  Little, Brown and Company.

The Life of Birds : To Fly of not to Fly.  Writ.  David Attenbourgh, Produce  David Silbely.  1999.  Twentieth Century Fox.

Wallace, George J.  1955.  An Introduction to Ornithology  New York, NY.  The Macmillan Company.

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