Sexing Bay-capped Wren-Spinetails
By Gregory Gough
In some kinds of birds the males and females look pretty much the same. One example is the tropical ovenbirds, a large family consisting of many species, featuring such hobbit-like genera as the: earthcreepers, barbtails, firewood-gatherers, brushrunners, and plushcrowns.
One member of this family is the bay-capped wren-spinetail. It lives in coastal marshes in Argentina. Although the males and females resemble each other greatly, scientists wondered if there were subtle differences.
The first step was to actually identify which were males and females among museum collections. For this, scientists used molecular techniques to accurately sex 65 wren-spinetails.
Next, they took 10 measurements. They found that the best markers that differentiate the males were a stouter bill and longer rufous cap and wings.
However, the measurements would not be able to identify all the birds correctly to sex. For males, 71 percent could be correctly identified, and for females, 89 percent.
So yes, male and female bay-capped wren-spinetail do look different, subtly, and maybe other members of the ovenbird family do as well.
This article summarizes the information in this publication:
Cardoni, D. A., Maldonado, Jesús E., Isacch, J. P. and Greenberg, Russell S. 2009. Subtle Sexual Dimorphism in the Bay-Capped Wren-Spinetail (Furnariidae) Uncovered through Molecular Sex Determination. Ornitologia Neotropical, 20: 347-355.
The family Furnariidae comprises putatively sexually monomorphic species, such as the Bay-capped Wren-Spinetail (Spartonoica maluroides). The goal of this study was to test for possible dimorphism in this species. We sexed 65 individual Bay-capped Wren-spinetails using molecular techniques. We performed subsequently a stepwise discriminant function analysis (DFA) on ten morphological measurements to select the best subset of variables capable of differentiating the sexes. We found that males had a longer rufous cap and wings than females. Furthermore, the bill length-depth ratio values were higher in females than males, indicating that males had more robust bills than females. In an a posteriori analysis, the discriminant function correctly determined the sex of 80% of the overall samples, (71 and 89% of males and females, respectively). We suggest that, in the light of these results indicating sexual dimorphism in a species previously thought to have none, other furnariid species be better examined to determine whether sexual dimorphism is actually common, albeit subtle, in this family. The functional significance of these differences needs further exploration.
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