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Wing Shape Variation in the Medium Ground Finch (Geospiza Fortis): An

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Wing Shape Variation in the Medium Ground Finch (Geospiza Fortis): An Biological Journal of the Linnean Society, 2009, 98, 129–138. With 5 figures Wing shape variation in the medium ground finch (Geospiza fortis): an ecomorphological approach BIEKE VANHOOYDONCK1*, ANTHONY HERREL2, ANA GABELA3 and JEFFREY PODOS3 1Department of Biology, University of Antwerp, B-2610 Belgium 2Departments of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA 3Department of Biology, University of Massachusetts, Amherst, MA 01003, USA Received 19 October 2008; accepted for publication 19 March 2009bij_1269 129..138 Wing design in birds is subject to a suite of interacting selective pressures. As different performance traits are favoured in different ecological settings, a tight link is generally expected between variation in wing morphology and variation in ecological parameters. In the present study, we document aspects of variation in wing morphology in the medium ground finch (Geospiza fortis) on Isla Santa Cruz in the Galápagos. We compare variation in body size, simple morphometric traits (body mass, last primary length, wing length, wing chord, and wing area) and functional traits (wing loading, aspect ratio and wing pointedness) across years, among populations, and between sexes. Functional traits are found to covary across years with differences in climatic conditions, and to covary among populations with differences in habitat structure. In dry years and arid locations, wing aspect ratios are highest and wings are more pointed, consistent with a need for a low cost of transport. In wet years and cluttered habitats, wing loading is lowest and wings are more rounded, suggesting enhanced capabilities for manoeuvrability. Sexes differ in wing loading, with males having lower wing loadings than females. Superior manoeverability might be favoured in males for efficient territory maintenance. Lastly, in contrast to functional traits, we found little consistent inter-annual or inter-site variation in simple morphometric traits. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 129–138. ADDITIONAL KEYWORDS: climate – ecomorphology – habitat use – inter-annual – inter-population – sexual differences. INTRODUCTION characteristic of migratory than sedentary birds (Mönkkönen, 1995; Lockwood, Swaddle & Rayner, Because the ability to move is of crucial importance in 1998; Fernández & Lank, 2007). Manoeuvrability, on many different ecological contexts, the locomotor the other hand, appears to be the determinant per- apparatus of many organisms is subject to suites of formance trait for birds living and foraging in clut- interacting selective pressures. Adaptations to eco- tered habitats, or in birds that perform acrobatic logical contexts have been noted in the morphology aerial mating displays (Rayner, 1988; Hedenström & and function of bird wings, which may be shaped in Møller, 1992). Manoeuvrability is largely determined part by natural or sexual selection. For example, in by a bird’s wing loading (i.e. ratio of body weight to birds that migrate or that forage over great distances, wing area) because the minimum turning radius is low energetic costs of flight are desirable (Rayner, proportional to body mass and wing area (Norberg, 1988). Energy-efficiency and enhanced speed in long- 2002). Also, having short, rounder wings appears to distance flight can be achieved by long and pointed enhance manoeuvrability (Kaboli et al., 2007). In wings (i.e. high aspect ratio), which are more many species, manoevrability and wing loading have been found to correspond with microhabitat use, for- *Corresponding author. aging behaviour, and sexual displays (Hedenström & E-mail: [email protected] Møller, 1992; Gamauf, Preleuthner & Winkler, 1998; © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 129–138 129 130 B. VANHOOYDONCK ET AL. Kaboli et al., 2007). Lastly, speed and acceleration precipitation are distinct vegetation zones. In the capacity are likely important for take-off (e.g. during present study, we compare the wing morphology of predatory attacks). Because high take-off perfor- three different populations of G. fortis that occur at mance is associated with low aspect ratios and with elevations ranging from sea-level to high altitude low wing loadings, birds susceptible to high ground- (600 m a.s.l.), and in microhabitats ranging from predation rates tend to have less pointed and more open and dry, to densely vegetated, humid forest. rounded wings (Burns & Ydenberg, 2002; Swaddle & For example, in the cluttered, humid upland forest, Lockwood, 2003). medium ground finches can be expected to benefit In the present study, we document aspects of varia- from enhanced manoeuvrability (Rayner, 1988; tion in wing morphology in the medium ground finch Norberg, 2002). We thus hypothesize that birds with (Geospiza fortis) on Isla Santa Cruz, Galápagos, high manoeuvrability and low wing loadings will be Ecuador, at three levels. First, we compare the wing favoured in upland forest. As demonstrated by the morphology in a single population across three suc- negative correlation between wing loading and eleva- cessive years. Second, we test whether wing morphol- tion among 43 species of hummingbirds (Altshuler & ogy varies among different populations. Finally, we Dudley, 2002), wing loading at the highland site can question whether sexes differ with respect to wing also be expected to be lower than at the coastal sites. morphology. By contrast, dry, open sites with sparse vegetation The environmental conditions on the Galápagos and food, should favour birds with a low cost of Islands can be considered unpredictable and harsh in transport, having high aspect ratio and highly many ways. One of the most striking features of the pointed wings. Galápagos environment is the extraordinary annual Our analyses also address the question of whether variation in rainfall. Some years are characterized wing morphology and function differs between males by extensive, heavy rains, whereas, in other years, and females, due to sexual differences in behaviour hardly any rain falls. During dry years, food supplies and ecology (Burns & Ydenberg, 2002; Fernández & are scarce and mortality in Galapagos finches is very Lank, 2007) or sexual selection on wing shape. As high (Grant & Grant, 1989; Grant, 1999). In the demonstrated in other bird species, males performing present study, we compare wing morphology of one acrobatic aerial displays need to be agile and manoeu- population of G. fortis across three consecutive years vrable, and tend to express comparatively small sizes (2005, 2006, and 2007). The amount of rainfall on and low wing loadings (Hedenström & Møller, 1992; Santa Cruz varied greatly during these years, with Székely, Reynolds & Figuerola, 2000; Fernández & hardly any rain in 2005, more typical rainfall in 2006, Lank, 2007). Whereas male birds constantly patrol and heavy rains in 2007 (Grant & Grant, 2006; Huber their territory and actively chase other (male and et al., 2007; Hendry et al., 2009). Food supplies were female) birds, females do not defend territories and limited in 2005, whereas, in 2007, seeds and cater- fly mostly in foraging related contexts. Comparison of pillars were very abundant. Food abundance in 2006 male versus female wing aspect ratios and wing load- was intermediate compared to 2005 and 2007 (A.H. & ings may provide insight into the balance between B.V., pers. observ.). We hypothesize that, during dry manoeuvrability or energetic efficiency in territorial years, such as 2005, birds with energetically efficient defense. flight should be favoured. Because efficient flight is associated with high aspect ratios and wing pointed- ness, birds with more pointed wings should be MATERIAL AND METHODS favoured during dry periods. By contrast, the lush vegetation and the presence of insects during wet FIELDWORK years should favour enhanced manoeuvrability and We conducted fieldwork on the island of Santa Cruz, thus favour lower wing loadings and more rounded Galápagos, during the month of February of three wings. Notably, interannual variation in wing aspect consecutive years (2005–07). In 2005, we mist-netted ratio, wing pointedness, and wing loading may be ten individuals (Nǩ = 6, NǨ = 4) at Academy Bay, near influenced not only by selection, but also by habitat the Iguana pens at the Charles Darwin Research choice (see below) and by variation in individual Station (CDRS), and four individuals (Nǩ = 3, NǨ = 1) birds’condition, especially with regard to body mass, in the highlands at Los Gemelos. In 2006, we used feather growth, and moult. mist nets to capture 44 birds (Nǩ = 24, NǨ = 20) at the Ecological variation occurs not only over time, but CDRS. In 2007, we captured 57 birds at the CDRS also across localities. On Santa Cruz Island, there is (Nǩ = 26, NǨ = 31) and 59 birds at El Garrapatero an approximately three-fold difference in rainfall (Nǩ = 32, NǨ = 26) using mist nets. All birds were between coastal and inland higher elevation sites banded upon capture. In general, recapturing rates (Grant, 1999). Associated with the local differences in were low, but, if the same individual was captured © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 129–138 WING SHAPE VARIATION IN G. FORTIS 131 multiple times, only one set of measurements was the tip of the respective primary. We used the factor used in the analyses. The Los Gemelos site consists of scores on the first two axes (PC1
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