Report of an extra-pair copulation in the Rufous Hornero, Furnarius rufus (Aves: Furnariidae)

In the last decade, studies in bird breeding biology have shown that infidelity is prevalent in socially monogamous species. Here, we describe an extra-pair copulation (EPC) event in the Rufous Hornero (Furnarius rufus), a socially monogamous bird with year-round territoriality and low levels of extra-pair paternity. Before the EPC, a within-pair copulation (WPC) occurred inside the pair’s territory. The WPC occurred on the ground and between a banded male (ca.  6  years-old) and an unbanded female. Ten minutes later this breeding pair invaded a neighboring territory, presumably to forage. The territorial male was chased back to its territory by an unbanded male neighbor after being detected. The male neighbor was paired with an unbanded female that did not participate in the aggressive interaction. When flying back to its territory the male neighbor copulated with the territorial female on the ground (ie. EPC). The territorial male flew, vocalized, and perched above the male neighbor, interrupting the EPC. The aggressive interaction then ceased as each pair resumed foraging in their respective territories. These observations suggest that Rufous Horneros can use EPC to obtain immediate benefits (food access in a neighbor’s territory). Moreover, WPC may be detected by neighbors and physical mate guarding and/or frequent WPC may be necessary to prevent EPC in the Rufous Hornero.


INTRODUCTION
The study of copulatory behavior and fertilization patterns contributes towards understanding the evolution of mating systems (Griffith et al., 2002;Brouwer & Griffith, 2019). In socially monogamous species, extra-pair copulation (EPC) occurs when an individual copulates with another individual outside the pair bond, which may or may not result in extra-pair fertilization (EPF) and the production of extra-pair offspring (Kempenaers & Schlicht, 2010). Extra-pair paternity (EPP) has been detected in 76% of the socially monogamous bird species studied to date (Brouwer & Griffith, 2019). In contrast, little is known about EPC. The lack of data concerning copulatory behavior is due to the difficulty of observing copulations in the field, as they are usually short and cryptic in birds (review in Gill et al., 2020).
Mating strategies are influenced by the context in which EPC occurs. For example, vegetation density reduces an individual's ability to follow mates during daily activities (Mays & Ritchison, 2004) leading to low mate-guarding efficiency (Sherman & Morton, 1988;Westneat & Sherman, 1997;Westneat & Stewart, 2003). Thus, it is predicted that high levels of monogamy should occur in birds that occupy open habitats (Mee et al., 2004;Muck et al., 2009;Biagolini-Jr. et al., 2017) and when social mates are close (Osorio-Beristain & Drummond, 1998). From the female's perspective, when birds occupy open habitats, EPC should only occur when the territorial male is away from the territory because cues of infidelity can lead to reduced male paternal care of a forthcoming brood (Ewen & Armstrong, 2000;Harts et al., 2016). Moreover, females may be less willing to engage in EPC due to the high risk of retaliation (e.g., reduced care or aggressions) by their social mates (Valera et al., 2003;Westneat & Stewart, 2003).
The Rufous Hornero, Furnarius rufus (Passeriformes, Furnariidae) is a medium-sized passerine (16-23 cm; 31-65 g), commonly found ISSN On-Line: 1807-0205 ISSN Printed: 0031-1049 ISNI: 0000-0004-0384-1825 in second-growth scrub, pastureland, and urban areas in southern South America (Remsen-Jr. & Bonan, 2020). The Rufous Hornero is a socially monogamous, duetting species with typically slow pace of life-history: year-round territoriality, long-term pair bonding, and high nesting success (Fraga, 1980;Diniz et al., 2018). A previous study that applied molecular tools to assess the occurrence of extra-pair paternity shows that EPP levels are low in this species (3.33% of 120 offspring and 6.52% of 46 broods) and that one extra-pair nestling was sired by a male neighbor (Diniz et al., 2019). Field observations and images from citizen scientists suggest that females produce a copulation solicitation call (Fraga, 1980) and that copulations occur conspicuously on the ground (Paula, 2014) or cryptically inside the nest (Freitas, 2013) or on dense trees (Figueiredo, 2011) in the Rufous Hornero. Here, we describe an event of EPC that suggests that the low EPP recorded for Rufous Horneros could be explained by the constraints of females to perform EPC when they are close to their partners.

MATERIAL AND METHODS
We observed an event of EPC performed after a within-pair copulation (WPC) had occurred in the juncture of two territories, each defended by different Rufous Hornero mated pairs. The records were made during a one-day fieldwork to resight a few banded pairs in the Universidade de Brasília campus, Brasília, central Brazil (15°46′13.36″S; 47°52′12.21″W), at 16:43 h on 20 September 2020. This period is considered the beginning of the breeding season in the studied population when most females are probably fertile (Diniz et al., 2018). Individuals of this population were marked, from 2013 to 2015, with unique combinations of plastic color bands and/or one numbered aluminum band supplied by the Brazilian Bird Banding Agency (CEMAVE/ICMBio) (see tag protocol description in: Diniz et al., 2016). A previous study in this population did not record any EPC despite the big sampling effort (101 hours of observations on 12 breeding pairs; Diniz et al., 2019).
The pairing and territorial statuses of each pair were assigned by observing partners duetting and foraging closer to each other for approximately 20 minutes before and after the copulation records. One of the mated pairs was composed by a banded male (≥ 6 years-old) and an unbanded female ("Territorial Pair", hereafter). This male was observed on previous duetting studies conducted up to 2015, when it was still mated with a banded female and had its territory perimeter estimated (Diniz et al., 2018(Diniz et al., , 2019. Thus, the female of the Territorial Pair was replaced at least once in the last six years. The second mated pair was located in an adjacent territory and was composed of unbanded individuals that had not been studied previously ("Neighbor Pair", hereafter). Distances between birds and between birds and territories' borders were estimated visually by the observer (PD).

RESULTS
The WPC occurred between the members of the Territorial Pair (Fig. 1A). Both individuals were foraging on the lawn in an open spot surrounded by trees and located between a sidewalk and a road (Fig. 1). The partners were close to each other (< 5 m) and close to the territorial border. The pair copulated for ~ 5 s and resumed foraging. Ten minutes later the Territorial Pair was foraging outside its territory (distance between partners: 5 m) and probably inside the Neighbor Pair's territory (distance from the border, male: 11 m, female: 6 m; Fig. 1B). The male neighbor vocalized and chased the territorial male for 30 m back to the territorial male territory, where both perched on the ground approximately 14 m from the territorial border (Fig. 1C). The territorial female followed both males during the chase and returned to its territory, but perched earlier, on the ground, closer (5 m) to the territorial border (distance to the males: 11 m). The female neighbor also followed the males but perched inside its territory (distance to the border: 6 m; perch heigh: ~ 2 m).
The EPC occurred between the territorial female and male neighbor inside the territory of Territorial Pair. After chasing the territorial male back to its territory, the male neighbor flew toward its territory, and perched nearby the territorial female on the ground (Fig. 1D). The territorial female did not produce copulation solicitation calls and did not show any aggressiveness towards the male neighbor. During the EPC, the territorial male flew, vocalized, and perched above the male neighbor, presumably to interrupt the copulation, which lasted for ~ 4 s. The male neighbor flew in the direction of its territory and perched closer to the territorial border and the female neighbor. Finally, both pairs resumed foraging and moved further apart within their respective territories.

DISCUSSION
The observation that Rufous Hornero performed WPC before an extra-territorial foray, followed by an EPC when the social male was physically distant, brings to light new interpretations concerning elements that may limit or promote EPP in this species. First, the territorial female was not aggressively assailed by the extra-pair male, suggesting that cases of EPP in Rufous Horneros should not be interpreted as forced copulation (Bukacińska et al., 1998;Gill et al., 2020). Moreover, as it is clear that the within-pair male was alive, low levels of EPP could not be explained by possibilities of establishment of a new pair bond after a previous partner dies (Petrie & Kempenaers, 1998) or divorces (Culina et al., 2015;Boucherie et al., 2018).
Our observations indicate that mate guarding behavior is the key component that determines the low rates of EPP in the Rufous Hornero. However, our observations indicate that mate guarding efficiency is reduced when males engage in costly territorial disputes since males cannot guard their mates and fight with a neigh-bor at the same time (Meek & Robertson, 1994;Low, 2006). Physical mate-guarding behavior is described as behavioral adaptations used to prevent EPP (Schamel et al., 2004), commonly expressed through two general components: (i) increased copulation frequency; (ii) following female during the fertile period, which can be an attempt to prevent females from extra-territorial forays or to dissuade other males from approaching (Westneat & Stewart, 2003).
Because the Rufous Hornero is a highly altricial species (Dial, 2003) with a great demand for parental care (Massoni et al., 2012), the use of social behavior to prevent EPP is determinant for male fitness (Gilbert et al., 1998;Valera et al., 2003;Hoi et al., 2013). By tracking the female during the breeding season and a possible reduction of parental effort in face of infidelity cues (Westneat et al., 2013;Reding, 2015;Ball et al., 2017), males might increase the cost of EPC to females, which could contribute to low levels of EPP in this species (Ewen & Armstrong, 2000;Matysioková & Remeš, 2013;Harts et al., 2016). In contrast to physical mate guarding (Valera et al., 2003), a previous study with Rufous Horneros showed that male duet responsiveness was not associated with the female fertile period, suggesting that males do not acoustically guard paternity (Diniz et al., 2018).
It is known that birds, in general, copulate more times than necessary for fertilization, which suggests that females obtain benefits by exploiting male interest in within and extra-pair copulations. These benefits can be immediate (e.g., food access in males' territories) or future (e.g., paternal care) (Velando, 2004). The observation that the Rufous Hornero Territorial Pair performed a WPC before an extra-territorial foray indicates that increased copulation frequency may be a mate guarding strategy. On the other hand, the EPC after the female's entry into the neighbor's territory could be associated with gaining access to resources in that territory. Females can seek EPC for other reasons (reviewed in Brouwer & Griffith, 2019), such as to ensure fertility when mated to an infertile male (Sheldon, 1994). Concerning the possibility of males chasing their social mate to prevent EPC, it is known that females are more likely to escape male surveillance when the vegetation is dense (Mays & Ritchison, 2004). Thus, we expect that mate guarding in urban environments (such as our study site) is facilitated because this habitat typically presents low vegetation cover (Samia et al., 2015).
We recommend that future studies evaluate the effects of short-term removal of pair males during the female fertile period on female behavior, territorial intrusion, and EPC in the Rufous Hornero (Dowling & Webster, 2018). Then, we could get insight into the role of female infidelity and mate-guarding in the occurrence of EPC in this species (Hall & Peters, 2009). Unfaithful females are expected to engage in off-territorial forays and singing more solos to attract extra-pair males in the absence of its mate; also, the lack of mate-guarding may increase extra-pair male intrusions and EPCs (Brylawski & Whittingham, 2004;Johnsen et al., 2008;Hall & Peters, 2009;Dowling & Webster, 2018).