Joaquin Aldabe Virtual Poster

INTRODUCTION

The Woodcreepers (Dendrocolaptidae) are a family of birds within the Order Passeriformes that contains 52 different species and are found in the Neotropical region from Mexico to central Argentina (Fig. 1). These birds chiefly inhabit forests although a few species occur in semi-open habitats. They are characterized by their habit of climbing up tree trunks and branches to forage on insects, spiders and other arthropods (Fig. 2) and use their tail feathers as a body support when climbing. As a result of this foraging habits, woodcreepers have stiff tail feathers, curved claws, and toe modifications that aid in creeping on trees. However, there is one species, the Scimitar-billed Woodcreeper (Drymornis bridgesii) of south central South America that forages mainly on the ground where it is even capable of running with little difficulty (Fig. 2) and normally only climbs trees for refuge and nesting.

The lifestyle of Drymornis is shared with the ovenbirds, the sister group of the woodcreepers, and their anatomical similarities are explained by convergent evolution, as Drymornis has a derived phylogenetic position in the woodcreeper tree.

As Drymornis is facing different selection pressure from the typical woocreepers, it is expected that anatomical differences exist between this species and the rest of the woodcreepers.

This study attempts to identify morphological adaptations associated with the niche shift of Drymornis to a dominantly terrestrial habit, by examination of skeletons and study skins of museum specimens.

METHODS

Osteological measurements including the skull, mandible, sternum, pelvic girdle, pygostyle, and the 3 principal limb segments were made using a Mitutoyo digitamatic caliper. As the taxa studied differed in body size, the humerus length was used as a standard of comparison in order to make the osteological measurements comparable within this group of birds. Analysis of the variance (ANOVA) and Scheffe pairwise comparison were performed using Systat® software. Genera with small sample sizes were excluded from the statistical analysis.

Because the tail feathers are known to provide support when the birds are climbing, the feather rachis width at the tip of the central tail feather (remix) was measured. This measurement was taken dorsoventrally and at a distance from the tip equivalent to 5% of the length of the central tail feather on 71 specimens belonging to 19 species. The weight of each species was recorded and plotted against the tail measurements.

The middle claw of Drymornis was also examined. A distance measurement was taken between the claw tip and the point where the inner curve meets the flesh (see Figure 3, T1 and L2 for measurement specifics). The ratio of these two measurements was calculated by dividing the absolute inner length (L2) by the total curve length (T1). Measurements were compared to a perfectly flat claw value of 1.0.

RESULTS

Osteological examinations show that Drymornis differs in the leg proportion of the hind limbs. The tibiotarsus (Tib) is longer in Drymornis than all of the other genera studied except Xiphocolaptes and the metatarsus (Tar) was significantly different in all genera studied (Table 1). These two leg bones are also larger in relation to the femur than in other woodcreepers, the tarsometatarsus being clearly longer than the femur in Drymornis (Figure 4).

Another osteological feature that differs between Drymornis and the other genera studied is the height and shape of the sternal keel. Drymornis has a higher and more curved keel than the rest of the woodcreepers, which are characterized by a small, straight keel (Figure 5).

The tip width of the central tail feather in Drymornis is markedly thinner than that of the strictly arboreal species in this family (Figure 6).

No differences in claw curvature were found, and Drymornis has a typical woodcreeper claw (Figure 7).

DISCUSSION

The elongation of distal bones of the leg is a characteristic of ground birds as it increases the speed and maximizes the energy per step. Drymornis has this feature as a consequence of its different habits. However, Xiphocolaptes major also has a long tibiotarsus. This species may also be seen foraging on the ground but not as frequently as Drymornis, and it does not walk but hops. Xiphocolaptes is probably in an early stage of adaptating to ground foraging habits. A possible explanation for the particular behavior of both taxa may be that they inhabit the Chaco region, an area of deciduous scrub habitat type, where during the driest part of the year arthropods may be difficult to find in the trees and these birds are forced to forage on the ground.

Typical woodcreepers need to be as close as possible to the trunk or branch of the tree when they climb so that they can reduce the effect of gravity. This problem has been solved by evolving a shallow keel of the sternum so that the breast is closer to the climbing surface. Drymornis has a markedly higher and curved sternal keel as a result of its different ecological niche.

In climbing birds, tail feathers are usually attached at both extremes, the pygostyle of the bird and the trunk of the tree. To avoid rupture by compressive forces, the rachis of the tail feathers is extremely thick and the width is proportional to body mass. In Drymornis the tip width of the central remix rachis is markedly smaller than in the strictly arboreal forms; presumably because Drymornis does not need a strong tail feather to support its weight.

Claw curvature in terrestrial birds is usually low. As shown by claw morphology Drymornis is not completeley morphologically adapted to terrestrial life and has retained some of its original tree-climbing adaptations possibly to facilitate hiding and nesting in trees.

ACKNOWLEDGEMENTS

I am very grateful to my advisors Carla Dove and Storrs Olson for guidance throughout this project. I wish to thank Marcy Heacker and Helen James for assistance in statistical analysis and poster preparation. Rebecca Snyder provided logistical and software support for claw analysis. Gabriel Rocha and Agustin Carriquiri (Aves Uruguay ONG, Uruguay) and Jorge Cravino (Ministerio de Ganaderia,Agricultura y Pesca, Uruguay) provided photos. I would like to thank all of the people in the Division of Birds, NMNH who created an excellent atmosphere for me to work. I am grateful to LSU Museum of Zoology and the American Museum Natural History for allowing me access to specimens in their care. Thanks to Chris Milensky and Scott Whittiker (NMNH) who supported this research in various ways. I would like to thank the Research Training Program for providing me the opportunity to study at the Smithsonian Institution.

This research was supported by a grant to the NMNH Research Training Program from the Smithsonian Latino Initiatives Fund.

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The information presented here represents preliminary research as the result of ten-weeks of investigation in-residence at the National Museum of Natural History. This is not an official publication of the information.

As preliminary information, results and/or findings should not be cited as part of conclusive work. Please contact the authors first if you wish to utilize the information presented here.


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Research Training Program VIRTUAL POSTER SESSION 2004

Research Training Program
VIRTUAL POSTER SESSION
2004