Introduction

One important objective of systematic biology is to classify variation in the natural world into a meaningful evolutionary and taxonomic framework. In the field of ornithology, this has most often been done using qualitative analysis of plumage characters. These assessments, however, may not reflect natural evolutionary units (species or subspecies). Now, with new morphological and genetic techniques available, it is possible to quantitatively assess variation among and within species and determine whether current taxonomic designations do, in fact, reflect evolutionary history.

This project evaluated morphological and genetic variation in three Amazonian antwrens in the genus Myrmotherula: M. longipennis (Long-winged Antwren), M. menetriesii (Gray Antwren), and the species complex M. hauxwelli/guttata (Plain-throated and Rufous-bellied Antwrens) [Figure 1]. This is the first quantitative study of plumage variation and is among the first comprehensive studies of genetic variation in pan-Amazonian birds. It is part of a larger study of genetic, morphological, and vocal variation in these antwrens.

Methods

• Plumage color reflectance measurements were obtained for different body regions on a total of 491 birds using an OceanOptics S2000 spectrophotometer with OOIBase32 operating software. This reflectance data was converted to measures of hue, chroma, and brightness [Figure 2] using the program Spectre, v.1.07. Morphological measurements such as wing, tail, and bill length were also recorded. Statistical analyses were performed using the programs SYSTAT 11 and PAST.

• The mitochondrial gene ND3 was amplified using standard PCR protocol and sequenced using an ABI 3100 Genetic Analyzer. Sequences were aligned with the program Sequencher 4.1, and trees were constructed using PAUP*.

Results

Major results:

All sequenced subspecies formed genetically distinct units [Figure 3].

Female plumage was found to be distinct between nearly all genetic units [Table 1, Figure 4].

Male plumage was less informative.
In addition:

The subspecies M. hauxwelli clarior appears to contain two distinct genetic units, probably delimited by the Rio Tapajós.

Although no comparative genetic information was available, two distinct plumage groups were identified within M. menetriesii berlepschi.
M. h. hauxwelli, while genetically distinct, was difficult to distinguish using morphological characters.

Discussion

• Our study, like other emerging studies of tropical birds, found plumage-based subspecies to be genetically distinct units, whereas studies of temperate zone birds have often found plumage differences to be more pronounced than genetic differences.

• While previous genetic studies have relied on existing subspecies designations, the independent quantitative examination of plumage in this study revealed new insights into comparative patterns of genetic and morphological variation.

• Consistent with earlier studies of typical antbirds, female plumage exhibited much more informative geographic variation than male plumage, a phenomenon known as heterogynism.

• While most subspecies boundaries appear to be river-delimited, M. longipennis and M. menetriesii both have two distinct plumage variants in the interfluvium between the Madeira and the Tapajós, one in the upper reaches and another in the lower, possibly reflecting the existence of a historical barrier.

• Future research will investigate plumage variation at a finer geographic scale, improve genetic sampling, and analyze vocal variation among populations.

Acknowledgements

This research was supported by a grant to the Smithsonian’s Research Training Program from the NMNH Office of the Director. I would like to thank the following institutions for use of their specimens and tissues: the National Museum of Natural History, the American Museum of Natural History, the Field Museum of Natural History, the Carnegie Museum of Natural History, the Academy of Natural Sciences, the Kansas University Museum of Natural History, and the Louisiana State University Museum of Natural Science.

I would also like to thank Jennifer Reed for completing much of the molecular work for this project, Rob Fleischer of the NMNH Genetics Laboratory, Gary Graves for the use of his spectrophotometer, Jessica Armenta for assistance with color analysis, and Lee-Ann Hayek for invaluable help with statistics.

I would also particularly like to thank Terry Chesser and Mort Isler for their patient guidance with this project, and Mary Sangrey for giving me the opportunity to do this research through the Research Training Program.