The Condor 105:428±438 ᭧ The Cooper Ornithological Society 2003

VARIATION IN MORPHOLOGY AND MATING SYSTEM AMONG ISLAND POPULATIONS OF GALAÂ PAGOS HAWKS

JENNIFER L. BOLLMER1,5,6,TANIA SANCHEZ2,MICHELLE DONAGHY CANNON3,7, DIDIER SANCHEZ2,BRIAN CANNON3,8,JAMES C. BEDNARZ3,TJITTE DE VRIES2, M. SUSANA STRUVE4,9 AND PATRICIA G. PARKER1,6 1Department of Evolution, Ecology, and Organismal Biology, The Ohio State University, 1735 Neil Avenue, Columbus, OH 43210 and Department of Biology, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, MO 63121 2Departamento de BiologõÂa, PontifõÂcia Universidad CatoÂlica del Ecuador, Quito, Ecuador 3Department of Biological Sciences, Arkansas State University, P.O. Box 599, State University, AR 72467 4Charles Darwin Foundation, Inc., 407 N. Washington Street, Suite 105, Falls Church, VA 22046

Abstract. Interspeci®c variation in sexual size dimorphism has commonly been attributed to variation in social mating system, with dimorphism increasing as intrasexual competition for mates increases. In , overall body size has also been found to correlate positively with size dimorphism. In this study, we describe variation in morphology and mating system across six populations of the endemic GalaÂpagos Hawk ( galapagoensis). GalaÂpagos Hawks exhibit cooperative polyandry, a mating system in which long-term social groups contain a single female and multiple males. Comparisons among islands revealed signi®cant differences in overall body size for both adults and immatures. Populations ranged from completely monogamous to completely polyandrous, with varying mean group sizes. Data did not support our prediction that sexual size dimorphism would increase with the degree of polyandry (number of males per group) or with body size; there was no correlation between mating system and sexual dimorphism. We did ®nd a signi®cant negative relation- ship between degree of polyandry and body size among islands, opposite of the pattern predicted. Key words: body size, Buteo galapagoensis, cooperative polyandry, GalaÂpagos Hawk, principal components analysis, sexual size dimorphism.

VariacioÂn en MorfologõÂa y Sistema de Apareamiento entre Poblaciones de Buteo galapagoensis Resumen. VariacioÂn interespecõ®ca en dimor®smo sexual ha sido atribuõÂda comuÂnmente a variaciones del sistema social de apareamiento, de tal manera que el dimor®smo aumenta conforme aumenta la competencia intrasexual por parejas reproductivas. TambieÂn se ha encontrado que el tamanÄo corporal se correlaciona positivamente con el dimor®smo. En este estudio describimos la variacioÂn morfoloÂgica y el grado de poliandrõÂa de seis poblaciones de Buteo galapagoensis, una especie que exhibe un sistema de apareamiento denominado poliandrõÂa cooperativa. En este sistema los grupos de individuos reproductivos incluyen una sola hembra y muÂltiples machos. Se comprobo que existen diferencias signi®cativas en el tamanÄo del cuerpo de adultos y juveniles entre islas. Las poblaciones muestreadas variaron entre monoÂgamas y completamente poliaÂndricas, y el tamanÄo promedio de los grupos fue variable. Los datos no apoyaron las predicciones establecidas inicialmente pues el grado de dimor®smo sexual no aumento con el nivel de poliandrõÂa (nuÂmero de machos por grupo) ni con el tamanÄo corporal, ni hubo una correlacioÂn entre el sistema de apareamiento y el dimor®smo sexual. La relacioÂn entre el tamanÄo corporal y el sistema de apareamiento fue contraria a la que se predijo: hubo una correlacioÂn negativa signi®cativa entre el grado de poliandrõÂa y el tamanÄo corporal entre islas.

Manuscript received 25 April 2002; accepted 20 March 2003. 5 E-mail: [email protected] 6 Present address: Department of Biology, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, MO 63121. 7 Present address: Department of Forest Science, Oregon State University, 321 Richardson Hall, Corvallis, OR 97331-5752. 8 Present address: Oregon Department of Fish and Wildlife, Corvallis Research Lab, 28655 Highway 34, Corvallis, OR 97333. 9 Present address: CH2M Hill, 13921 Park Center Road, Herndon, VA 20171. [428] MORPHOLOGY AND MATING SYSTEM IN GALAÂ PAGOS HAWKS 429

INTRODUCTION Morphological variation among or pop- ulations is driven by a number of factors, one of which is sexual selection. Interspeci®c variation in sexual size dimorphism has commonly been attributed to variation in social mating system and differences in parental care (Darwin 1871, Andersson 1994). The high levels of intrasexual competition for mates characteristic of polyga- mous systems may result in large body size in the competitive sex. Increased sexual size di- morphism was found to be associated with in- creased polygyny in the New World blackbirds (Icterinae; Webster 1992) and increased polyg- FIGURE 1. Map of the GalaÂpagos archipelago show- yny and polyandry (reverse size dimorphism) ing islands where we acquired morphological mea- within the Charadrii (SzeÂkely et al. 2000). surements and group sizes for GalaÂpagos Hawks: Vol- Among 73 species, Owens and Hartley (1998) can Alcedo on Isabela, Santiago, Pinta, Marchena, found a signi®cant positive relationship between Santa Fe, and EspanÄola. The entire range of the Ga- laÂpagos Hawk is limited to nine islands: the six men- sexual size dimorphism in body mass and degree tioned above, Santa Cruz, PinzoÂn, and Fernandina. of polygamy, while in a larger analysis (n ϭ 1031 species), Dunn et al. (2001) found a similar relationship, with polygynous and lek species size dimorphism disappeared after controlling being more dimorphic than monogamous spe- for mating system, which Webster interpreted to cies. mean that body size affects dimorphism indi- Across taxa, species with greater size dimor- rectly through its effects on mating system. In phism also tend to be larger in overall body size. their study, Dunn et al. (2001) also found a pos- Webster (1992) reviewed 11 hypotheses that itive relationship between sexual size dimor- have been proposed to explain this relationship, phism and body size. all of which are based on males competing for In this study, we look at the relationship be- females in polygynous systems. Four of these tween morphology and mating system in the Ga- ideas are based on intrasexual competition as the laÂpagos Hawk (Buteo galapagoensis), which is cause of sexual size dimorphism. They predict endemic to the GalaÂpagos archipelago (Fig. 1). that the association with body size results from In addition to forming pairs, the GalaÂpagos polygyny being more likely among large-bodied Hawk exhibits cooperative polyandry, in which species, larger females being favored as better one female mates with up to eight males (usually competitors, genetic correlations increasing fe- two or three), and all aid in the care of a single male body size as male size increases, or the brood (Faaborg and Patterson 1981, DeLay et al. change in male body size being greater in larger 1996). Group members are unrelated (Faaborg species. Other hypotheses predict that while sex- et al. 1995) and highly territorial, defending ual selection drives dimorphism, larger species year-round, all-purpose territories (de Vries may have fewer ecological or energetic con- 1975). Observations of marked birds indicate straints on body size, or only the female is con- that group membership is stable across years, strained to a smaller body size. Still other hy- especially for males (Faaborg et al. 1980, Do- potheses argue that larger species may be more naghy Cannon 2001). dimorphic because of increased niche partition- Immatures are not territorial, and live in the ing, male predator defense, differences in ge- ¯oater population until they are at least three netic variation among the sexes, or allometric years old and able to secure a position in a ter- growth, with sexual selection having no effect. ritorial group. In addition to immatures, non- Among the New World blackbirds, Webster breeding adult-plumaged hawks also occur in (1992) found that sexual size dimorphism cor- the ¯oater population. On Santiago, adult male related with body size but less strongly so than ¯oaters are rarely seen; however, adult females with polygyny. The effect of body size on sexual account for a sizeable proportion of the ¯oaters 430 JENNIFER L. BOLLMER ET AL.

(ϳ17%; Donaghy Cannon 2001). Our observa- Hawks were caught on six islands (Fig. 1): Santa tions between 1998 and 2002 suggest that com- Fe (n ϭ 23), EspanÄola (19), Santiago (223), Vol- petition among females for breeding spots may can Alcedo on Isabela (91), Pinta (12), and Mar- be high: (1) there is higher turnover among ter- chena (25). The hawks were caught using two ritorial females than males (Donaghy Cannon methods: a bal-chatri trap (Berger and Mueller 2001); (2) four new territories carved out of area 1959), baited with live prey such as a rat (Rattus defended by existing groups were established by spp.), and a rope noose on a stick (Faaborg et ¯oater females and 1±3 males, never by males al. 1980). Each hawk was banded with either an alone (Donaghy Cannon 2001); and (3) we have aluminum or anodized color band bearing an al- observed ¯oater females ®ghting each other phanumeric code, or with both types of bands. (JLB, pers. obs.). The following morphological measurements Island populations tend to be small and iso- were taken: wing chord (un¯attened, to the near- lated, which fosters genetic and phenotypic dif- est mm), tail length (posterior base of uropygial ferentiation among them (Grant 1998, Whittaker gland to tip of central rectrices, to nearest mm), 1998). Similar to most , GalaÂpagos Hawks cranium (posterior of cranium to tip of mandi- are reluctant to cross wide expanses of water ble, to nearest 0.1 mm), culmen (anterior edge where thermals are not reliably present (Kerlin- of cere to tip of mandible, to nearest 0.1 mm), ger 1985); thus, their populations are likely to bill depth (vertical distance from dorsal to ven- be genetically isolated, with only a rare ex- tral surfaces of mandibles at anterior edge of change of individuals between islands. Consis- cere, to nearest 0.1 mm), hallux claw (chord tent with this suggestion, some authors have de- from proximal to distal extent of claw, to nearest scribed morphological and behavioral differenc- 0.1 mm), foreclaw (as measured for hallux), and es across islands: larger hawks on EspanÄola mass to nearest 10 g. (Swarth 1931, de Vries 1973) and variation in Given the large degree of reverse sexual size the frequency of polyandry (de Vries 1975, Faa- dimorphism present in the GalaÂpagos Hawk, we borg et al. 1980). were able to identify sex in the ®eld. We used a Here, we quantify variation in morphology PCR-based genetic sexing technique (Fridolfs- and mating system across six populations of the son and Ellegren 1999) to con®rm sex assign- GalaÂpagos Hawk. First, we use principal com- ments for 328 of 330 birds (two Santiago im- ponents analysis to assess morphological differ- matures identi®ed as males were found to be fe- ences among adult and immature hawks from males). We were also able to place the birds into different islands. Second, we characterize the age classes, because their plumage gets progres- mating system on each of the islands based on sively dark until they achieve adult plumage at the mean group size for each population. Finally, ®ve years of age (de Vries 1973). we look at whether there are relationships be- Mating system was characterized by counting tween morphology and mating system across the number of adults present in each territory. populations. We test the prediction that the de- We counted individuals during observations of gree of sexual size dimorphism should be posi- chick provisioning at nests and when the nest or tively related to the degree of cooperative poly- territory was being defended during our banding andry. If a greater frequency of polyandry is as- visits. All members of a group aggressively pro- sociated with more competition among females tected the young when we approached nests. for access to males, then we would expect an Most territories were visited multiple times, so increase in female body size, which would result we feel con®dent that recorded group sizes were in increased sexual dimorphism. We also test the accurate. predictions that body size will be positively re- lated to sexual size dimorphism and to the de- STATISTICAL ANALYSES gree of polyandry, as has been shown among Principal components analysis (PCA) was used polygynous species (Webster 1992). to describe morphological differences among populations. Components with eigenvalues METHODS greater than one were retained, and eigenvectors FIELD METHODS were rotated using varimax rotation. Analyses Fieldwork was conducted in the GalaÂpagos Is- were performed on log-transformed data. Fe- lands between May and August of 1998±2001. males are on average 31% larger in mass than MORPHOLOGY AND MATING SYSTEM IN GALAÂ PAGOS HAWKS 431 males across populations (Table 1), and so the ues reported in the Results section are means Ϯ sexes were analyzed separately to prevent the SD. variance due to sexual dimorphism from mask- ing variation among populations. All analyses RESULTS were done using SPSS 10.0.5 for Windows ADULTS (SPSS Inc. 1999). We conducted four PCAs: separate analyses In general, birds from EspanÄola were the largest, on adult females, adult males, immature females, then those from Isabela, Santa Fe, Santiago, Pin- and immature males. The last two analyses were ta, and Marchena; however, there was some var- restricted to individuals from three populations iation in this order among the different mea- surements (Table 1). The PCAs yielded two because only single immature individuals of the components for both the females and males, ex- same sex were captured on the other islands. plaining 63% of the total variance in both cases. Kruskal-Wallis tests were performed on the PC The ®rst principal component (PC1) for both scores to test for signi®cant differences among sexes described overall body size, correlating populations within sex and age classes. positively and highly with all variables except We captured birds over a period of four years the claw measurements (Table 2). Hallux and fo- (1998±2001), so it is possible that year effects reclaw loaded heavily onto the second compo- could have in¯uenced our results. To test this, nent (PC2) in both sexes (Table 2). we performed Kruskal-Wallis tests on PC scores Kruskal-Wallis tests performed on PC1 from adult females and males captured on San- showed that the populations are signi®cantly dif- tiago in different years, as well as Mann-Whit- 2 ferent in size for both females (␹ 5 ϭ 54.7, P Ͻ ney U-tests on PC scores from second-year fe- 2 0.001; Fig. 2a) and males (␹ 5 ϭ 69.3, P Ͻ males from two different years on Santiago. 0.001; Fig. 2b). PC2 was also different among Sample sizes from other islands and other age 2 females (␹ 5 ϭ 29.5, P Ͻ 0.001; Fig. 2a) and classes were not large enough from multiple 2 males (␹ 5 ϭ 26.1, P Ͻ 0.001; Fig. 2b). These years to permit analysis. results indicate that there is a signi®cant amount We calculated a sexual size dimorphism index of morphological divergence among populations (Lovich and Gibbons 1992) as follows: sexual of GalaÂpagos Hawks. size dimorphism ϭ (female measurement/male measurement) Ϫ 1. We evaluated whether there IMMATURES were signi®cant differences in degree of sexual For the immature females, individuals from Is- size dimorphism in overall body size across pop- abela, Santiago, and Marchena were used, and ulations by testing for differences in degree of for immature males, individuals from Santa Fe, dimorphism in wing length and mass, the two Isabela, and Santiago were used. Three compo- variables that most strongly predicted overall nents were kept in each analysis, explaining adult body size according to the PCAs. We ran- 74% of the total variance in females and 73% in domly paired the data from males and females males. The variables loaded similarly in both within each population using all the adults, and sexes (Table 2): hallux and foreclaw correlated calculated a size dimorphism index for each highly with PC1, the head measurements (cra- pairing. We conducted 100 randomizations for nium, culmen, and bill depth) all loaded on PC2, each population using the PopTools (v. 2.5.3; and wing and tail length correlated most closely Hood 2002) add-in for Microsoft Excel. We as- with PC3. sumed that our sampled individuals were rep- For the females, Kruskal-Wallis tests on PC1 resentative of their populations, although our 2 2 (␹ 2 ϭ 9.9, P ϭ 0.007), PC2 (␹ 2 ϭ 26.9, P Ͻ sample sizes were limited. Because the distri- 2 0.001), and PC3 (␹ 2 ϭ 11.5, P ϭ 0.003) showed butions did not have equal variances, a Kruskal- signi®cant differences among islands (Fig. 2c). Wallis test was used to test for signi®cant dif- For the males, PC1 did not differ signi®cantly 2 2 ferences among populations in degree of wing (␹ 2 ϭ 5.1, P ϭ 0.08), while PC2 (␹ 2 ϭ 46.6, P 2 and mass dimorphism. Additionally, overall size Ͻ 0.001) and PC3 (␹ 2 ϭ 7.8, P ϭ 0.02) did dimorphism indices were calculated for all eight show signi®cant differences among populations variables for each population using the mean (Fig. 2d). These results indicate that, as in the measurements for males and females. The val- adults, there are signi®cant morphological dif- 432 JENNIFER L. BOLLMER ET AL. 83 (10) 37 (9) 96 (9) 59 (6) 64 (8) 63 (9) 91 (53) 54 (74) 107 (4) 75 (17) 118 (3) 33 (8) Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ mass Body 873 903 844 891 1137 1013 1578 1494 1354 1295 1223 1235 0.8 (8) 1.0 (7) 0.6 (10) 0.5 (8) 1.0 (6) 0.4 (7) 1.0 (54) 1.0 (75) 1.0 (4) 0.9 (17) 0.4 (2) 1.1 (9) Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ chord Foreclaw 28.0 27.3 27.4 27.7 27.0 26.1 31.0 30.0 30.2 31.5 29.3 29.7 1.0 (10) 0.6 (9) 1.0 (11) 0.4 (7) 0.9 (9) 0.5 (9) 1.3 (54) 1.1 (74) 0.8 (4) 0.8 (17) 2.2 (3) 0.4 (6) Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ chord Hallux 31.0 31.4 31.4 32.0 30.3 31.4 27.5 27.4 27.7 27.8 27.1 26.0 0.3 (9) 0.5 (8) 0.7 (10) 0.4 (8) 0.5 (9) 0.4 (9) 0.6 (54) 0.4 (71) 0.4 (4) 0.3 (17) 0.3 (3) 0.3 (8) Bill Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ depth Âpagos Hawks from six islands. All measurements are in mm except for 21.1 20.9 21.5 20.7 20.0 20.0 19.5 18.6 18.5 18.0 17.6 18.2 0.6 (10) 0.8 (8) 0.7 (11) 0.5 (7) 0.7 (8) 0.5 (8) 0.9 (52) 0.8 (74) 0.5 (4) 0.5 (17) 0.2 (3) 0.8 (9) Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ length Culmen 32.6 32.6 31.5 32.0 30.6 29.2 28.8 28.1 26.8 27.8 26.3 26.1 2.1 (10) 1.5 (9) 1.0 (9) 0.8 (7) 3.7 (8) 0.9 (9) 2.0 (53) 1.5 (73) 0.8 (4) 1.0 (17) 2.1 (3) 1.2 (8) Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ length Cranium 91.6 91.1 90.9 91.2 87.6 88.1 85.0 84.3 83.5 83.4 80.5 81.8 6 (10) 3 (8) 3 (10) 8 (8) 8 (9) 8 (8) 6 (54) 6 (72) 7 (4) 6 (17) 1 (2) 5 (8) Tail Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ length 233 228 228 224 216 214 257 255 253 245 239 239 ) for eight measurements taken on adult female and male Gala n SD ( Ϯ 7 (10) 3 (8) 8 (11) 8 (8) 11 (8) 7 (9) 8 (51) 10 (71) 5 (4) 5 (16) 9 (3) 11 (9) Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Ϯ Wing chord 415 400 399 392 381 381 458 439 440 429 424 417 Äola Sex Female Male Female Male Female Male Female Male Female Male Female Male TABLE 1. Means Island Espan mass, which is in g. Isabela Santa Fe Santiago Marchena Pinta MORPHOLOGY AND MATING SYSTEM IN GALAÂ PAGOS HAWKS 433

ferences among immature hawks from different islands. re similar 0.57 0.85 0.38 0.15 0.02 0.04 0.15 16

Ϫ Ϫ Ϫ Ϫ YEAR EFFECTS is species. In Kruskal-Wallis tests on the PC scores for San- tiago adult females (n ϭ 53) captured in four 0.07 0.03 0.62 0.79 0.87 0.04 0.10 24 different years showed no signi®cant differences

Ϫ 2 among years (both ␹ 3 Յ 2.4, P Ͼ 0.5). The same was true for the adult males (n ϭ 72) from Immature males 2 Santiago (both ␹ 3 Յ 3.7, P Ͼ 0.8). Second-year females on Santiago, however, did show a sig- 0.21 0.24 0.29 0.06 0.92 0.88 0.52 33 PC1 PC2 PC3

Ϫ Ϫ ni®cant difference between two years. Individ- uals caught in 1999 (n ϭ 9) were signi®cantly larger at PC1 (U ϭ 14, P ϭ 0.01) and smaller at PC2 (U ϭ 17, P ϭ 0.02) than those caught in

15 1998 (n ϭ 10). 0.07 0.65 0.87 0.34 0.08 0.04 0.28 Ϫ VARIATION IN MATING SYSTEM AND SEXUAL SIZE DIMORPHISM The degree of polyandry varied greatly across 18 0.04 0.33 0.68 0.81 0.78 0.15 0.13 islands, from 0% of territories sampled on Es- Ϫ panÄola to 100% of territories sampled on Pinta,

Immature females Marchena, and Isabela (Table 3). The mean number of males per group varied signi®cantly

40 2 PC1 PC2 PC3 0.18 0.22 0.28 0.93 0.88 0.29 0.08 across islands (Kruskal-Wallis test: ␹ 5 ϭ 32.5, Ϫ P Ͻ 0.001). Islands with larger mean group sizes

Âpagos Hawks: adult and immature males and females. Numbers are variable loadings tended to have a higher proportion of polyan- drous groups as well, although our sample sizes from some of the islands were low (Table 3). 0.04 0.89 0.88 0.07 0.32 0.31 0.10 0.47 17

Ϫ There was some variation in the degree of sexual size dimorphism among islands (Table 4). The degree of wing and mass dimorphism of

Adult males males and females randomly assigned to pairs 46 PC1 PC2 0.75 0.67 0.65 0.58 0.81 0.08 0.15 0.79 showed signi®cant differences among popula- 2 tions (Kruskal-Wallis: ␹ 5 ϭ 26.6, P Ͻ 0.001 and 2 ␹ 5 ϭ 119.3, P Ͻ 0.001, respectively). However, there was no relationship for any measurement 0.05 0.01 0.45 0.41 0.21 0.88 0.91 0.12 24 between degree of dimorphism (as calculated

Ϫ Ϫ Ϫ from population means for each sex) and degree of polyandry across islands (all r Յ 0.65, P Ͼ 0.16), nor was there a relationship between di-

Adult females morphism and overall body size as measured by 0.83 0.78 0.47 0.60 0.67 0.07 0.03 0.81 40 PC1 PC2

Ϫ mean male (all r Յ 0.54, P Ͼ 0.27) and female (all r Յ 0.51, P Ͼ 0.30) PC1 scores. There was, however, a strong relationship be- tween overall body size and mating system. Body size (PC1) of both adult females (r ϭ Ϫ0.72, P Ͻ 0.001; Fig. 3a) and males (r ϭ Ϫ0.69, P Ͻ 0.001; Fig. 3b) was signi®cantly

Variable negatively correlated with degree of polyandry. PC2 was not signi®cantly related to mating sys-

TABLE 2.on Results each of signi®cant four axis separateboth principal and immature components analyses, percentage (PC) we ofwithin analyses omitted total of age body Gala variance groups, mass explained. but as We quite a analyzed different variable sexes between because separately age we because lacked groups. of this the measurement considerable for sexual many dimorphism individuals in from th Isabela. Variable loadings we Wing chord Tail length Cranium length Culmen length Bill depth Hallux chord Foreclaw chord Body mass % Variance explained tem (r ϭϪ0.09, P ϭ 0.41) in females, but it 434 JENNIFER L. BOLLMER ET AL.

FIGURE 2. Principal components (PC) analysis of GalaÂpagos Hawk morphology. Results are from four sep- arate analyses. Variable loadings were similar between sexes but different between ages (see axis descriptions). Analyses were conducted on (a) 91 adult females, (b) 127 adult males, (c) 75 immature females, and (d) 95 immature males from six (adults) or three (immatures) different islands. was in males (r ϭϪ0.20, P ϭ 0.03), despite a body size and claw size. The degree of polyan- large degree of scatter. dry also varied widely across populations. We had predicted that the degree of sexual size di- DISCUSSION morphism would increase with increasing poly- We sampled GalaÂpagos Hawks on six of the nine andry and body size because of potential intra- islands they inhabit, and found that they varied sexual competition (Webster 1992). Although greatly in morphology and mating system. The the degree of sexual size dimorphism differed principal components analyses described signif- somewhat across islands, there was no relation- icant differences among populations in overall ship between size dimorphism and polyandry or

TABLE 3. Island area and degree of polyandry found in GalaÂpagos Hawks breeding on six islands.

No. of territories Mean Ϯ SD no. of Island Area (ha)a sampled Percent polyandrous males per territory EspanÄola 6048 11 0 1.0 Ϯ 0.0 Santa Fe 2413 6 50 1.5 Ϯ 0.6 Santiago 58 465 32 89 2.3 Ϯ 0.8 Isabela 458 812 3 100 2.3 Ϯ 0.6 Marchena 12 996 6 100 2.8 Ϯ 1.0 Pinta 5940 6 100 3.5 Ϯ 1.6 a Black 1973. MORPHOLOGY AND MATING SYSTEM IN GALAÂ PAGOS HAWKS 435

TABLE 4. Sexual size dimorphism indices for eight measurements taken from six GalaÂpagos Hawk popula- tions. The indices were calculated as the female population mean divided by the male population mean minus one. All variables are measured in mm except for body mass (g).

Wing Tail Cranium Culmen Bill Hallux Foreclaw Body chord length length length depth chord chord mass EspanÄola 0.10 0.10 0.08 0.14 0.09 0.13 0.11 0.39 Santa Fe 0.10 0.11 0.09 0.18 0.16 0.13 0.10 0.55 Isabela 0.10 0.12 0.08 0.15 0.12 0.14 0.11 0.47 Santiago 0.10 0.09 0.09 0.16 0.14 0.15 0.14 0.47 Marchena 0.11 0.11 0.09 0.16 0.14 0.12 0.09 0.45 Pinta 0.09 0.12 0.08 0.12 0.10 0.20 0.14 0.39 body size. There was, however, a strong nega- were taken over a span of four ®eld seasons. tive correlation between body size and degree of Immature females caught in different years on polyandry. Santiago differed signi®cantly in size, but there There were some potential sources of error in were no differences among adults. We doubt that the morphological analyses. The majority of year effects greatly affected the analyses of the birds were measured once, so we lack data on immatures, because the majority of immature measurement repeatability. Also, measurements males from Santiago (20 of 24), females from Isabela (33 of 36), and males from Isabela (29 of 35) were captured during a single year, 1998. A third source of error was variation in sample size; certain age and sex categories from some of the islands had limited sample sizes.

VARIATION IN MATING SYSTEM The mating system of the hawks ranged from monogamous pairs to large polyandrous groups. Our sample of groups from Santa Fe was small, but the level of polyandry we observed (50% of territories) is similar to previous estimates. In 1969 and 1979, 47% of territories (n ϭ 15; 1.5 Ϯ 0.6 [SD] males per territory) and 50% of ter- ritories (n ϭ 16; 1.7 Ϯ 0.8 males per territory; Faaborg et al. 1980) held polyandrous groups, respectively. The current absence of polyandry on EspanÄola is similar to what de Vries (1975) found in 1970, when he recorded only one trio in 10 territories. Similarly, on Santiago Faaborg et al. (1980) found that 85% of territories in 1979 were polyandrous compared to our ®nding of 89% in 2000. There are no previous mating- system data for Marchena, Pinta, or Isabela. We found 100% polyandry on each of these islands. While our sample sizes from Santa Fe, Marche- na, and Pinta are much smaller than our sample FIGURE 3. The relationship between body size and from Santiago, their populations are also much degree of polyandry in GalaÂpagos Hawks on six is- smaller, so the proportion of territories sampled lands. Body size was measured as the ®rst principal may actually be larger. Our characterization of component in individual analyses of (a) adult females and (b) adult males. Degree of polyandry was mea- mating system on Isabela, however, is clearly sured as the mean number of adult males per territorial preliminary. Given the large size of Isabela, group for each of six islands. mating system and morphology may vary across 436 JENNIFER L. BOLLMER ET AL. the island, and our characterization may be just lective forces may be acting on the two sexes for hawks on Volcan Alcedo and not Isabela as (Price 1984). a whole. On average, female GalaÂpagos Hawks are This variation in degree of polyandry across 31% heavier than males, making them one of islands could be related to variation in sex ratio, the most dimorphic Buteo species (of 22 species survivorship, and ecology. Polyandry may arise compared; Paton et al. 1994). Aside from mating from male-biased sex ratios; however, the pres- system, other factors may play a role in produc- ence of large numbers of adult female ¯oaters ing this large degree of dimorphism. Diurnal on Santiago (Donaghy Cannon 2001) suggests raptor species that pursue aerial or high-speed that mates are not limiting for male GalaÂpagos prey are generally more dimorphic (Reynolds Hawks on this island. Sex ratio has not yet been 1972, Wheeler and Greenwood 1983), although studied on the other islands. Across species, co- GalaÂpagos Hawks mostly capture their prey on operative breeding is associated with high an- the ground (de Vries 1976). Alternatively, size nual adult survivorship, which leads to low turn- dimorphism may minimize intrasexual compe- over of breeders on territories (Arnold and tition for food (Storer 1966). Island species may Owens 1998). Faaborg et al. (1980) hypothe- evolve greater sexual dimorphism due to de- sized that the variation in degree of polyandry creased interspeci®c competition or increased among GalaÂpagos Hawk populations may result intraspeci®c competition from high population from interisland variation in survival of both densities (Stamps et al. 1997). Among closely adult male ¯oaters and territorial males. Annual related buteos (Riesing et al. 2003), the island- survivorship of nonterritorial hawks on Santiago dwelling (Buteo solitarius)is (50±58%) is greater than on Santa Fe (0±33%), similarly dimorphic (Paton et al. 1994, Clarkson where there is less polyandry (Faaborg 1986, and Laniawe 2000), while the mainland Swain- Donaghy Cannon 2001), which suggests that son's Hawk (Buteo swainsoni), is less so (Fried- more ``excess'' nonbreeders accumulate in the mann 1950, England et al. 1997). ¯oater population of Santiago. As another alter- BODY SIZE AND POLYANDRY native, polyandry may have arisen in the GalaÂ- pagos Hawk due to a limitation in suitable There was a strong relationship of decreasing breeding territories (Faaborg and Bednarz 1990). body size with increasing degree of polyandry The number of suitable territories, and hence the across populations. Our prediction, made from degree of such limitation, probably varies as a the female point of view, was the reverse: as result of ecological factors peculiar to each is- intrasexual competition for mates increases, land (Arnold and Owens 1999, Hatchwell and body size should increase. Taking the male point Komdeur 2000). of view, though, we might have predicted this result. Among hawks, smaller males are more SEXUAL DIMORPHISM AND POLYANDRY agile (Andersson and Norberg 1981) and may be favored in aerial ®ghts. We could speculate, The degree of sexual size dimorphism was not then, that as mean group size increases, the level related to variation in polyandry, a ®nding for of competition for limited space also increases. which there are a number of possible explana- Territory defense should become more dif®cult tions. Sexual selection may be weaker when fe- (especially for smaller groups neighboring larger males (as opposed to males) compete for mates, ones), thus selecting for smaller body size, per- because females generally experience lower var- haps in both males and females. iance in reproductive success (Payne 1984). Al- Alternatively, the correlation between poly- though studies have shown that classically poly- andry and body size is not causal, but is the androus species are more dimorphic than mo- result of a third, unknown factor. For example, nogamous species (SzeÂkely et al. 2000, Dunn et a number of studies have attributed interisland al. 2001), intraspeci®c variance in female repro- morphological differentiation to variation in ductive success among GalaÂpagos Hawk popu- available resources, presence of competitors, or lations may be too weak to drive changes in size predation pressure across islands (Schwaner dimorphism. In addition, factors other than sex- 1985, Freeman-Gallant 1996, Yasushi et al. ual selection can drive sexual size dimorphism 1999). Schoener (1969) and Schwaner and Sarre (Hedrick and Temeles 1989), and different se- (1990) predicted that optimal body size should MORPHOLOGY AND MATING SYSTEM IN GALAÂ PAGOS HAWKS 437 be related to prey size or availability. At this DONAGHY CANNON, M. 2001. Breeding ecology of co- time we do not know enough about the different operatively polyandrous GalaÂpagos Hawks (Buteo galapagoensis) on Santiago Island, GalaÂpagos. island populations to evaluate the in¯uence of M.Sc. thesis, Arkansas State University, Jones- these factors on body size. boro, AR. DUNN, P. O., L. A. WHITTINGHAM, AND T. E. PITCHER. ACKNOWLEDGMENTS 2001. Mating systems, sperm competition, and the evolution of sexual dimorphism in birds. Evolu- We would like to thank the GalaÂpagos National Park tion 55:161±175. and the Research Station for permit- ENGLAND, A. S., M. J. BECHARD, AND C. S. HOUSTON. ting us to do this project and for their logistical sup- 1997. Swainson's Hawk (Buteo swainsoni). In A. port. J. Faaborg aided in the initial design of the pro- Poole and F. Gill [EDS.], The birds of North Amer- ject and in securing funds. N. Whiteman, K. Huyvaert, ica, No. 265. The Birds of North America Inc., A. Lombeida, K. Levenstein, P. Jimenez, A. Lara, G. Philadelphia, PA. Jimenez, and P. Castillo all aided in data collection. K. FAABORG, J. 1986. Reproductive success and survivor- Halbert performed the molecular sexing of the birds. ship of the Galapagos Hawk Buteo galapagoensis: K. Huyvaert, N. Whiteman, C. Roy, and J. Eimes pro- potential costs and bene®ts of cooperative poly- vided helpful comments on previous versions of this andry. Ibis 128:337±347. manuscript. Funding for this project was received from FAABORG, J., AND J. C. BEDNARZ. 1990. 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