Behavioral Differences among Eastern Bluebird Populations Could Be a Consequence of Tree Swallow Presence: A Pilot Study

Abstract

Aggressive interference competition for limited resources is frequently observed among animals. However, these behavioral interactions within (intraspecific) and between (interspecific) species are costly as they can be energetically expensive and cause injury or death. To avoid these agonistic interactions, numerous species alter their behaviors and resource requirements. Spatial variation in nest site competition allows for investigation of concurrent variation in territorial defense behavior. Further, among species with bi-parental nest defense, behavioral similarity in territorial defense may benefit pairs. Here, we studied territorial aggression between two eastern bluebird (Sialia sialis) populations (North Carolina and Alabama, USA) that differ in avian community structure; those in North Carolina have recently begun to experience strong interspecific competition for nesting cavities by tree swallows (Tachycineta bicolor), a competitive pressure that is relatively new for North Carolina bluebirds (∼35-40 years) and is absent in Alabama populations. We found that bluebirds in North Carolina are more aggressive to simulated territorial intrusions compared to their Alabama counterparts. Behavioral similarity (here, in aggression) between partners is strong and similar in both populations. These results suggest that bluebirds in North Carolina may have to maintain higher baseline aggression during territory establishment and nest construction to co-occur with highly aggressive tree swallows, but that, in both populations, behavioral similarity between partners may be adaptive. Finally, we acknowledge the preliminary nature of this study and the need for expanding the behavioral studies to other sites in the southeastern United States. Greater regional coverage would exclude the possibility of alternative drivers of the observed behavioral differences between the North Carolina and Alabama populations.

Interspecific Density Influences the Adaptive Significance of Provisioning Coordination Between Breeding Partners

“…Tree swallows are a relatively new nest competitor in the southern Appalachian Mountains population and prior to ∼1980 were uncommon and only observed during migration (Lee, 1993). Today, competition from tree swallows is fierce as 45% of the early season bluebird nestboxes were usurped by tree swallows (Albers et al, 2017). The swallow density had clear effects on parental provisioning strategies and how provisioning influenced nestling mass.…”

“…Yet, a recent study compares cooperation (alternation and synchrony of partner provisioning) We have excluded the 61 nests that failed during the egg laying and incubation stages, 48 which were usurped by tree swallows during the egg laying and incubation stages. If we observed tree swallow harassment followed by nest failure or if a tree swallow nest was built immediately after nest failure in the same nest box, we labeled the nest as usurped (Albers et al, 2017).…”

“…We focus on parental coordination of eastern bluebirds (Sialia sialis), a species that experiences high competition for nesting sites with tree swallows (Tachycineta bicolor). In the year the data were collected (2015), 44% of early nesting bluebird partners were evicted by tree swallows (Albers et al, 2017). Finally, because tree swallows preferentially settle near water, we test whether distance to water or tree swallow density has a greater influence on bluebird behavior and reproductive success.…”

Nesting strategy shapes territorial aggression but not testosterone: a comparative approach in female and male birds

“…Focusing on the early spring period of territorial establishment, we hypothesize that competition for a limited cavity resource drives higher territorial aggression. This hypothesis stems from species-specific observations that aggressive competition over nesting cavities can escalate to injury or death (Duckworth, 2008; Leffelaar and Robertson, 1985), presumably because aggression has some adaptive value in obtaining or maintaining access to limited cavities (Albers et al, 2017; Duckworth and Badyaev, 2007; Krieg and Getty, 2018; Rosvall, 2008; Sandell and Smith, 1997; Szász et al, 2019). One prediction is that cavity nesters will have higher aggression and elevated testosterone secretion, regardless of sex.…”

Competitive events during nest‐building influence offspring growth and behaviour, but not testosterone deposition in the egg yolk in Western Bluebirds (Sialia mexicana)

“…Furthermore, inconsistent results regarding maternal effects triggered by increased competition among species or populations from the same species have been reported (e.g. Navara et al 2006, Bentz et al 2016a, Albers et al 2017, suggesting that differences in life histories and ecological conditions among populations may influence the occurrence of testosterone-mediated maternal effects as a response to increased competition over key resources.…”