Short communication: Absence of protandry in a population of Gray Catbirds Dumetella carolinensis

Abstract

Sex-based differential arrival on the breeding grounds at the completion of migration has been poorly studied in monochromatic and monomorphic species because individuals are difficult to sex without recapture or molecular tests. We examined sex-based arrival timing to the breeding grounds of the Gray Catbird Dumetella carolinensis, a monochromatic species. We also examined differences in size and arrival by age as these are well documented to affect arrival timing. Older birds were larger than younger birds, and older males were larger than older females. Older birds arrived back on the breeding grounds earlier than younger birds, as documented in many species, but males did not precede females. Our results provide evidence of sexual size dimorphism in Catbirds and add to a growing body of information on arrival timing in monochromatic species. Differences in intensity of sex-based competition for breeding resources are hypothesized as the primary selective force for one sex to precede the other to the breeding grounds (Kokko 1999, Kokko et al. 2006). Typically, males arrive at the breeding grounds in advance of females (Morbey & Ydenberg 2001), a phenomenon known as protandry (Rubolini et al. 2004). This is because males are thought to experience more intensive selective pressure for acquiring territories or other resources for attracting a mate than females (but see Kokko et al. 2006 for a discussion of why females may also be under selection pressure for earlier arrival to compete for resources or mates). Furthermore, comparative studies suggest that the extent of sexual selection experienced by males, as evidenced by the degree of sexual dichromatism (Moller & Birkhead 1994, Owens & Hartley 1998), sexual size dimorphism (Owens & Hartley 1998, Dunn et al. 2001), polygyny or rate of extra-pair paternity (Coppack et al. 2006), affects the degree of protandry. While protandry is well documented in dimorphic species (Rubolini et al. 2004), little is known about arrival dates by sex in monomorphic or monochromatic species (Rubolini et al. 2004, Edwards & Forbes 2007, Newton 2008). The primary reason for this is the difficulty in sexing individuals upon arrival at the migratory destination. Moreover, the limited studies to date have produced equivocal results. For example, male Spotted Flycatchers Muscicapa striata preceded females during passage on the island of Ventotene, Italy, (Mabey 2002), and male White-throated Sparrows Zonotrichia albicollis and Least Flycatchers Empidonax minimus preceded females during spring migration at Long Point Observatory, Ontario, Canada (Mills 2005). In contrast, Edwards and Forbes (2007) presented evidence that Song Sparrows Melospiza melodia did not exhibit protandry upon arrival at breeding grounds in Ontario. Our research on arrival and breeding in the Gray Catbird afforded us the opportunity to assess arrival timing by sex and age in a songbird migrant described as monochromatic and monomorphic (Cimprich & Moore 1995). One advantage of our study is that we captured birds both during the arrival period and throughout the breeding season, permitting us to determine an individual’s sex and relate it back to when it arrived at breeding grounds in northeastern Pennsylvania. Although there are several hypotheses regarding differences (or lack of differences) in arrival timing between the sexes (Morbey & Ydenberg 2001) here we focus on assessing whether catbirds are indeed monomorphic and whether they exhibit protandry.