Morphological divergence in the House Wren (Troglodytes aedon) species complex: a study of island populations with a focus on the Grenada House Wren (T. a. grenadensis)

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Wetten, Kimberley
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House Wrens (Troglodytes aedon) are common throughout North, Central and South America; however, there is a knowledge gap regarding the morphology of certain populations, particularly those residing on islands, which may have adapted to the unique geographic pressures compared to the conditions House Wrens face on the mainland. I examined three questions to understand the morphology of insular house wrens, by using both museum samples and live-captured male House Wrens and other closely related species within the genus Troglodytes (n = 1,189). I analyzed six morphological characteristics: wing chord, tarsus length, bill length, bill width and bill depth. First, I examined whether island House Wrens were morphologically different from mainland House Wrens as well as different among each island population. Insular birds were larger than mainland birds in all morphological measurements. I found morphological features were unique to each island, suggesting there are different factors on each island influencing morphology. I then used linear regression to examine the influence of island proximity and island size on House Wren morphology of thirteen island populations. House Wrens on smaller islands had shorter tails and tarsi, and smaller bill depth and width. Islands that were further from the mainland had House Wrens with longer tails and smaller bills. Lastly, I examined whether House Wrens on Grenada, the southern-most Caribbean island, are morphologically distinct within the House Wren complex, based on seven morphological characteristics. When the Grenada House Wren was compared to 26 other subspecies and closely related species within the genus Troglodytes, morphology of Grenada House Wrens was different in 81% of morphological comparisons, and of these, was bigger in 88% of comparisons. These findings on insular House Wren morphology allows for a deeper understanding of the debated taxonomy of these House Wrens while also filling a data gap for the understudied island populations.
House Wren, Morphology, Islands, Grenada, Caribbean