ANIMAL BEHAVIOUR, 2006, 71, 799–807 doi:10.1016/j.anbehav.2005.05.025 Montezuma oropendolas modify a component of song constrained by body size during vocal contests J. JORDAN PRICE*, SARAH M. EARNSHAW* &MICHAELS.WEBSTER† *Department of Biology, St Mary’s College of Maryland, St Mary’s City ySchool of Biological Sciences, Washington State University, Pullman (Received 21 November 2004; initial acceptance 12 January 2005; final acceptance 20 May 2005; published online 24 February 2006; MS. number: A10048R) Physical constraints on animal signals can influence how challenging they are to produce, which in turn can provide potentially important cues about the condition or quality of a signaller. Some components of a signal are often more challenging to produce than others, however, and we expect animals to modify those elements that are especially costly so as to give high-intensity and demanding displays only when necessary. We found that Montezuma oropendolas, Psarocolius montezuma, modify a particular component of their songs during vocal contests, the lowest peak frequency (LPF). Songs that overlapped other males’ songs during interactions at nesting colonies had slightly but significantly lower LPFs than did the same song types produced when singing alone. Other components of song, in contrast, did not change. We also provide evidence that the production of loud, low-frequency notes in this species is constrained by body size, based on a negative bounded relationship between mean measurements of LPF and tarsus length across 102 species and subspecies of New World blackbirds (Icteridae), the songbird family to which the Montezuma oropendola belongs. Measures of LPF in the songs of Montezuma oropendolas fall near the lower limit of what members of the family produce. Taken together, our results suggest that low-frequency notes in these birds might constitute an honest signal of competitive ability during male–male interactions. Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Songs of male songbirds are known to function in both Cynx & Gell 2004). During highly competitive interac- mate attraction and competition with other males over tions, for example, such as close-range, male–male con- territories and mates (Catchpole & Slater 1995). Individual tests over mating opportunities, individuals should variation in songs can have important consequences invest more in their displays and thus should produce for reproductive success, so signal design is likely to be more extravagant signals. In situations with less potential strongly influenced by sexual selection (Andersson immediate benefit, however, such as while a male is sing- 1994). Based on current theory, features of song used in ing alone, we expect less demanding versions of these inter- and intrasexual advertisement should be subject to sounds. some constraint to constitute stable, honest indicators of The Montezuma oropendola, Psarocolius montezuma, quality (Kodric-Brown & Brown 1984; Grafen 1990; Gil provides an extreme case of sexual selection in a songbird. & Gahr 2002). For features constrained by a performance This Neotropical member of the New World blackbird limitation, such as sounds that are limited in some way family (Icteridae) has a harem-defence polygynous mating by morphology (Podos 1997), performing near this biome- system, in which males compete intensely for access to chanical limit should be especially challenging for a singer, sexually receptive females at nesting colonies (Webster and individuals should modify these elements of song 1994). It is also one of the most sexually size dimorphic conditionally based on perceived levels of competition of all birds, with adult males having more than twice and the potential payoffs of an interaction (e.g. Maynard the body mass of females (Webster 1997). Larger males Smith & Parker 1976; Wagner 1992; Kreutzer et al. 1999; tend to have higher ranks in dominance hierarchies than do smaller males, and male rank is closely associated Correspondence: J. Price, Department of Biology, St Mary’s College of with mating success (Webster 1995). Thus, large body size, Maryland, 18952 E. Fisher Road, St Mary’s City, MD 20686, U.S.A. as well as other male morphological features presumably (email: [email protected]). M. S. Webster is at the School of Biological important in courtship and competition for mates, ap- Sciences, Washington State University, Pullman, WA 99164-4236, pears to be a product of strong sexual selection (Webster U.S.A. 1992, 1997). 799 0003–3472/06/$30.00/0 Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. 800 ANIMAL BEHAVIOUR, 71,4 Several factors suggest that the songs of male Monte- male–male contests: the lowest peak frequency (LPF). zuma oropendolas are subject to strong sexual selection as Songs produced during vocal overlapping situations at well. These vocalizations consist of loud, elaborate cas- colonies have lower peak frequencies than do the same cades of notes often accompanied by a dramatic visual song types produced by males singing alone. We also display in which the singer bows forward on a branch, provide evidence that the production of loud, low- often until hanging nearly upside down. Songs are pro- frequency notes in song is constrained by body size, based duced frequently during the breeding season, especially by on our finding of a negative bounded relationship be- dominant males defending harems, and they appear to be tween these variables across the entire family Icteridae. important in male competitive interactions (Jaramillo & Measures of LPF in the songs of Montezuma oropendolas Burke 1999). At large colonies, singers often overlap the fall near the lower limit of what members of the family songs of other males with their own by beginning produce. This song component therefore might constitute a song before their opponent’s song is complete (personal a physically limited, and thus honest, signal of male size observation); in many cases these songs occur almost si- and competitive ability during male–male interactions. multaneously. Overlapping an opponent’s song is thought to be a more aggressive signal than producing songs that do not overlap those of other males, based on studies of METHODS a variety of other songbird species (black-capped chicka- dees, Poecile atricapillus: Mennill & Ratcliffe 2004; great Study Species and Song Analysis tits, Parus major: Langemann et al. 2000; European robins, Erithacus rubecula: Dabelsteen et al. 1997; nightingales, The Montezuma oropendola is a large, colonial-nesting Luscinia megarhynchos: Naguib 1999). Vocal overlapping songbird found in lowland rainforest habitats from south- could have importance for male competitors in disrupting ern Mexico to the Canal Zone of Panama (Howell & Webb the courtship signals of other males (Webster & Robinson 2000). It is one of the largest and most polygynous mem- 1999; Todt & Naguib 2000), and might have additional bers of the oropendolas and caciques (genera Psarocolius, importance in conveying information about contestants Ocyalus and Cacicus; Webster 1992; Jaramillo & Burke both to each other and to nearby receptive females (Men- 1999), a monophyletic subgroup of the New World black- nill et al. 2002). bird family (Icteridae). Other major clades in this family Evidence of past selection on Montezuma oropendola are the orioles, grackles and allies, meadowlarks and allies, song comes from phylogenetic comparative studies of the and the monotypic cup-nesting species Amblycercus holo- entire oropendola group (genera Psarocolius and Ocyalus), cericeus (Lanyon & Omland 1999). Male Montezuma oro- in which the evolution of various vocal features was re- pendolas have more than twice the body mass of constructed using robust molecular phylogenies (Price & females (Webster 1997). The sexes do not differ in plum- Lanyon 2002, 2004). Reconstructed changes in song indi- age coloration, but adult males have brightly coloured, cate that vocalizations have changed recently and rela- swollen facial wattles and elongated outer primary tively dramatically in the Montezuma oropendola. Far feathers not found in females or juvenile males. more evolutionary changes have occurred in the recent The mating system of Montezuma oropendolas is ancestors of this species than in any other oropendola lin- harem-defence polygyny, a system common in mammals eage (Price & Lanyon 2002), and many changes have re- but rare in birds (Webster 1994). Females nest colonially sulted in song features that are probably costly for during the dry season in trees separated from the sur- singers to develop or perform, such as broad frequency rounding forest canopy (Skutch 1954). Males compete in- ranges and long continuous sounds (Price & Lanyon tensely for breeding positions at these nesting colonies 2004). and form linear dominance hierarchies in which high- Sounds that are challenging to produce, because of ranking males are able to physically exclude others from mechanical limitations of the vocal tract or other physical nest-building females. Subordinate males excluded from constraints, could provide useful information to listeners a colony tree frequently perch in nearby trees and display about a singer’s quality or motivation and thus could have (i.e. sing) as females fly to and from the colony. Top- important consequences for reproductive success (Gil & ranked males obtain virtually all observed copulations Gahr 2002). Recent studies