Interspecific Competition Influences Fitness Benefits of Assortative Mating for Territorial Aggression in Eastern Bluebirds (Sialia sialis)

Abstract

Territorial aggression influences fitness and, in monogamous pairs, the behavior of both individuals could impact reproductive success. Moreover, territorial aggression is particularly important in the context of interspecific competition. Tree swallows and eastern bluebirds are highly aggressive, secondary cavity-nesting birds that compete for limited nesting sites. We studied eastern bluebirds at a field site in the southern Appalachian Mountains that has been recently colonized (<40 yr) by tree swallows undergoing a natural range expansion. The field site is composed of distinct areas where bluebirds compete regularly with tree swallows and areas where there is little interaction between the two species. Once birds had settled, we measured how interspecific competition affects the relationship between assortative mating (paired individuals that behave similarly) and reproductive success in eastern bluebirds. We found a strong tendency toward assortative mating throughout the field site. In areas of high interspecific competition, pairs that behaved the most similarly and displayed either extremely aggressive or extremely non-aggressive phenotypes experienced higher reproductive success. Our data suggest that interspecific competition with tree swallows may select for bluebirds that express similar behavior to that of their mate. Furthermore, animal personality may be an important factor influencing the outcome of interactions between native and aggressive, invasive species.

Physical habitat quality and interspecific competition interact to influence territory settlement and reproductive success in a cavity nesting bird

“…However competition environments. Eastern bluebirds appear to be behaviorally subordinate to tree swallows in our population, as tree swallows have a large effect on bluebird reproductive success (Harris and Siefferman, 2014;authors, pers. obs.).…”

“…Following the methods of Harris and Siefferman (2014), we defined zones as areas where nest boxes are <0.50 km apart (mean distance between boxes in a zone = 0.15 km) and zones are separated by >1 km (mean distance between zones = 1.32 km). High and low competition sites were assigned based on historical occupancy rates of bluebirds and tree swallows, as they are the most abundant and aggressive competitors with bluebirds in our field site (authors, pers.…”

“…However, settlement dates may be poor proxies of individual quality in year-round residents because individuals are often present without actively defending a territory, and thus, are harder to detect. Further, it is important to consider the consequences of settlement in areas where aggressive competition for nesting locations may lower the realized habitat quality (Johnson, 2007) of a territory or reduce fitness (e.g., Harris and Siefferman, 2014).…”

“…Tree swallows are aerial insectivores (Robertson et al, 1992), and thus, competition for food resources between swallows and bluebirds is limited. However, tree swallows and bluebirds compete aggressively for nesting cavities and this competition has a greater effect on bluebird breeding success than conspecific competition (Harris and Siefferman, 2014). Moreover, bluebirds may have difficulty in assessing potential competition levels, as tree swallows begin breeding approximately 1 month after bluebirds and swallows arrive on the breeding grounds a few weeks prior to bluebird egg laying (Robertson et al, 1992;Gowaty and Plissner, 1998;authors, pers.…”

“…obs.). Although intraspecific competition likely plays an important role on territorial settlement of eastern bluebirds (Siefferman and Hill, 2005a), post-settlement aggressive behaviors between conspecifics are uncommon (Harris and Siefferman, 2014).…”