Ornis Fennica 91:178186. 2014 The further the flight, the longer the wing: relationship between wing length and migratory distance in Old World reed and bush Warblers (Acrocephalidae and Locustellidae) Jaros³aw K. Nowakowski, Justyna Szulc* & Magdalena Remisiewicz J.K. Nowakowski, Bird Migration Research Station, University of Gdañsk, ul. Wita Stwosza 59, 80-308 Gdañsk, Poland J. Szulc, Bird Migration Research Station, University of Gdañsk, ul. Wita Stwosza 59, 80- 308 Gdañsk, Poland. * Corresponding authors e-mail: [email protected] M. Remisiewicz, Bird Migration Research Station, University of Gdañsk, ul. Wita Stwosza 59, 80-308 Gdañsk, Poland Received 10 June 2013, accepted 4 June 2014 We analysed how body mass, migration distance, taxonomic family and breeding on small islands vs. continents affect wing length in 72 species of closely related Old World reed and bush warblers (Acrocephalidae and Locustellidae), based on literature data. The species we analysed share similar morphology, habitat, food preferences, feeding habits and breeding systems, but their migratory behaviour varies from sedentariness to long- distance migration. The mean wing length of these species was strongly correlated with their body mass, migration distance and taxonomic family (R2 = 0.824; p =3.226). The wing was on average 23.4% longer for each doubling of body mass and 2.7% longer for each 1,000 km of migration distance. Breeding on small islands was not significantly re- lated to wing length. Species of Acrocephalidae had on average 11.7% longer wings than Locustellidae with the same body mass and migration distance. The relationship between migration distance and standardised wing length was identical in both families (differ- ence between slope coefficients b: t-test: t = 0.19, p = 0.85). After we partly controlled for the effects of different habitats and behaviours, our results showed that at inter-specific level migration affects wing length in proportion to migration distance. 1. Introduction Tris & Tellería 2001, Leisler & Winkler 2003, Fiedler 2005, Milá et al. 2008, Baldwin et al. Wing morphology affects the energetic efficiency 2010). Comparisons at the intra-specific level, be- of flight (Pennycuick 1989, Bowlin & Wikelski tween populations or subspecies, have shown that 2008). Many authors have shown the relationship the migrants tend to have longer and more pointed between birds wing length and pointedness and wings, which helps in fast and efficient flight, than their migration distance, at the inter- and intra-sp- the sedentary birds, which tend to have shorter and ecific level (Calmaestra & Moreno 2001, Pérez- more rounded wings for improved manoeuvra- Nowakowski et al.: The further the flight, the longer the wing 179 bility (Pérez-Tris & Tellería 2001, Milá et al. species of these families show little sexual dimor- 2008). However, comparisons of these wing fea- phism, live in dense bush or reed habitats, feed on tures between species have not provided a clear invertebrates and do not perform spectacular dis- pattern in these relationships. Some authors have play flights (Kennerley & Pearson 2010). But the shown that migrant species have longer wings than species in these families do present the whole sedentary species (e.g. Marchetti et al. 1995, Milá spectrum of migratory behaviour, from complete et al. 2008), but others could not confirm this rela- sedentariness, through altitudinal or short-dis- tionship or have shown only a weak correlation be- tance movements, to long-distance seasonal mi- tween wing length and migration distance (e.g. gration between continents. This makes them a Mönkkönen 1995, Calmaestra & Moreno 2001, suitable model to study the relationships between Baldwin et al. 2010). migration distance and wing morphology. An- This has led some authors (Marchetti et al. other important factor, which differs among these 1995) to conclude that wing length depends more species, is that some breed on small islands in the on a species ecology such as habitat, food type, Pacific or Indian Oceans, and most of those spe- size of prey and feeding habits (Winkler & Leisler cies are sedentary, but others breed and overwinter 1992, Swaddle & Lockwood 1998, Lockwood et over vast land areas, such as large islands or conti- al. 1998, Tornberg et al. 1999, Dawson 2005) nents (Kennerley & Pearson 2010). Living on than on migration distance. The selective pres- small islands limits the likelihood of long move- sures of the different elements of birds habitat or ments during the breeding and wintering seasons, behaviour can have opposing effects on the wing which considerably affects wing morphology (Se- length (James 1982, Alatalo et al. 1984) that lead nar et al. 1994, Desrochers 2010) and might even to an equilibrium in which none of these factors lead to flightlessness (Grant 1965, Olson 1973, effects prevail over the others. Many authors have Bicudo et al. 2010). also shown that wing length can vary greatly be- We aimed to identify the relationship between cause of weather and feeding conditions while the wing length and migration distance among passer- primaries are growing, and also because of age- ines, while controlling for the effects of different and sex-dependent differences in the length of habitat and feeding behaviour by choosing taxa of feathers (Björklund 1996, Dawson et al. 2000, similar habits. We also aimed to identify the Hall & Fransson 2000, Fiedler 2005, Milá et al. strength of this relationship and whether it is simi- 2008, Baldwin et al. 2010, Polakowski & Janko- lar in bird groups of different phylogenesis, and to wiak 2012). These numerous and insufficiently determine if breeding on small islands might affect explored factors affecting wing morphology re- this relationship. quire multifactorial analyses to identify the rela- tionships between wing length and the elements of behaviour typical of the species (including migra- 2. Material and methods toriness or sedentariness) and of its habitat, or con- trol of the remaining variables that could affect a This paper is based on the overview of biometric conclusion (Baldwin et al. 2010). The need to con- data published in Kennerley & Pearson (2010) for sider the history of the species, or of its popula- reed and bush warblers of the Old World. We ana- tions, is also emphasised, including the time dur- lysed all 72 species of the families Acrocephalidae ing which different types of selective pressures and Locustellidae for which that study provided have acted, such as the time during which a species the mean wing length and the mean body mass, and its ancestors had been sedentary (Baldwin et and for which it was possible to determine an ap- al. 2010). This history is reflected by relationships proximate migration distance (e.g. we did not in- between different taxa. clude three species of partial migrants; see Appen- Bearing in mind the effects of habitat, behav- dix 1). iour and a species history, for this analysis we se- All the mean values of parameters such as mi- lected Acrocephalidae and Locustellidae, two gration distance, wing length and body mass, were closely related families of passerines that share the means for a species or a subspecies. When similar morphology, habitats and behaviour. Most Kennerley & Pearson provided these values for a 180 ORNIS FENNICA Vol. 91, 2014 few subspecies, we used in our calculations the nis palliseri), the Luzon Islands in the Philippines data for the subspecies that had the best-defined (Long-tailed Bush Warbler Bradypterus cauda- winter quarters or the most complete dataset (the tus), or larger areas (for the importance of such a largest sample size for which mean values were division, see Newton 2003). Appendix 1 presents provided). If separate measurements were pro- the data we used in our analyses. We applied the vided for males and females, we calculated the logarithmic (ln) transformation to the biometic mean values for both sexes combined to standard- data before further calculations. ise the data for comparisons with species in which We used the body mass and the migration dis- the sexes were not distinguished. We used the tance as continuous variables, and the taxonomic body mass and the wing length for the population identity (the family) and the island factor as class for which we could estimate the migration dis- variables in our analyses of the factors that might tance. For sedentary species, mainly tropical and affect the wing length (response variable) of the subtropical, that do not show large seasonal studied species. We compared all 16 possible changes in body mass, we used the mean body models with different combinations of the factors mass given by Kennerley & Pearson (2010), or the using Bayesian Information Criteria (BIC) to cal- mean of minimum and maximum body masses culate the relative weight of each model m as pe- where only those values were provided. For mi- nalised likelihood: L(m) = exp{0.5 BIC(m)} grants, which show large differences in the body and to estimate posterior probabilities P(m)= mass between the migration period and the rest of L(m)/S L(m) (Congdon 2006, Lunn et al. 2012). m the year, we calculated the mean value using the We chose the model that had the highest posterior mean body mass during migration and the mean probability in further analyses by linear multiple body mass outside the migration period, allocating regression. equal weights to both means. In this way we con- We checked if the regression lines for both sidered the effect of the body mass on the wing families were parallel by testing the difference be- length in both periods of the migrants life, but tween the slope coefficients b using Students t- without assuming which of these periods was test. more crucial for this effect. We estimated the migration distance as the dis- tance between the middle of the breeding range 3.