Animals often perform conspicuous, and sometimes curious, courtship behaviours, which attract the attention of evolutionary ecologists to study the origin and maintenance of these behaviours (Andersson, 1994). Nestling-like courtship displays in birds are one such behaviour: courting males (and females) often perform nestling-like behaviours, including wing fluttering and/or nestling-like vocalisation in many bird species (Bradbury and Vehrencamp, 1998). The resemblance to young may have evolved as a sensory trap to exploit the parental care of potential mates (Christy, 1995; Stålhandske, 2002), although there is only one series of experimental tests of the sensory trap hypothesis in birds (see below).

Hasegawa and colleagues experimentally demonstrated that, during the courtship period, female barn swallows Hirundo rustica were, similarly, attracted to playback of two structurally similar calls: male enticement calls (i.e., nestling-like courtship calls that are emitted from nest sites to attract females) and nestling food-begging calls, supporting the sensory trap hypothesis [e.g., Hasegawa et al., 2013; Hasegawa and Arai, 2016; reviewed in Hasegawa (2018); also see Hasegawa and Arai, 2018 for close adult-young resemblance of appearance in parental, but not non-parental, birds]. This explains why male barn swallows emit enticement calls rather than courtship songs when attracting females to their nest sites (e.g., see Supplementary Figure 1 for sonograms; see Hasegawa et al., 2016a for videos). In other words, as cuckoos exploit the parental care behaviour of the host species by mimicking the begging behaviour of their nestlings, males might exploit conspecific female parental care for nestlings (Hasegawa et al., 2013). Males utter the enticement calls only when attracting females to their nest sites, which seems necessary for males to pair (and use complex songs in other timings; Hasegawa et al., 2013).

Female attraction, however, might not be the sole function of courtship signals. It is now well-known that male courtship traits such as songs can be dual-function signals, by attracting females and stimulating rival males [e.g., see Berglund et al., 1996 for a general pattern; see Galeotti et al., 1997; Wilkins et al., 2015 for the song of the barn swallow; reviewed in Turner (2006)]. Even when courtship displays are solely directed at females, these displays can be exploited by eavesdroppers, including neighbouring males, which can affect the design of courtship signals (e.g., see Reichard and Anderson, 2015 for the use of low-amplitude song to avoid eavesdroppers). Because nestling-like courtship displays attract females as a sensory trap to exploit female parental care for nestlings (i.e., approaching to the vocalisation; see above), males might also have a sensory bias toward nestling-like behaviours, behaving less aggressively toward characteristics typical of immature birds (sensu Foster, 1987; see Turner, 2006 for tolerance of adult swallows toward food-begging young). In other words, male agonistic response to neighbouring males would be cancelled out by their tolerant response to immature birds. Nestling-like traits might therefore function to avoid interference from neighbouring males.

Here, I experimentally examined male responses to the playback of male enticement calls, male courtship songs, and nestling food-begging calls in the barn swallow. I predicted that, if nestling-like traits function to avoid interference from neighbouring males, there would be fewer male responses to male enticement calls and nestling food-begging calls than to male courtship songs. Although males would have limited intrinsic responsiveness to nestling calls compared with females due to the uncertain paternity [e.g., Møller, 1988; reviewed in Turner (2006)], I also predicted that male responses to male enticement calls and nestling food-begging calls would increase when approaching the nestling period, during which parents respond to nestling food-begging calls (e.g., Saino et al., 2000).

During the incubation period, vocalisation type had a significant effect on the number of male responses (GLMM with a Poisson error distribution: n_males = 17, n_total = 51, χ² = 15.30, P < 0.001). The number of male responses to male enticement calls was significantly lower than that to male songs (Figure 1 and Table 1; the number of male responses to nestling food-begging calls was also significantly lower than that to male songs: difference ±SE = −1.01 ± 0.38, z = 2.69, P < 0.01). Similar pattern was found during the pairing period (Supplementary Figure 2 and Supplementary Table 1), though sample size was limited (n_males = 8, n_total = 24).

No significant correlations were found between the numbers of male responses to each treatment (Spearman’s rank correlation coefficient, |r_s| < 0.44, n = 17, P > 0.07), although the sample size was small. When the male responses to each vocalisation were analysed separately, the number of male responses to the playback of each treatment increased with the progress of the incubation period, although this relationship was not significant in the analysis of male responses to male songs (Figure 2 and Table 2). Rather, male responses to male songs marginally increased with body condition (Table 2 and Supplementary Figure 3).

The main finding of the current study is that, although both male enticement calls and songs function as courtship signals, male songs elicited frequent male responses, while male enticement calls (and nestling food-begging calls) elicited fewer responses. This indicates that males emitting enticement calls rather than songs would be selected for by avoiding interference from early-breeding, neighbouring males. Because males ignored male enticement calls near their territory boundaries (see section “Materials and Methods”), male enticement calls might allow for high density breeding in the barn swallow, which often breeds in nests that are close to (though visually hidden from) other nests (Fujita and Higuchi, 2007).

Previous studies on courtship signals have often focussed on the positive responses of potential signal receivers, in which signals that enhance the receiver’s attention should be effective. An exception is the “soft song,” a low-amplitude, short-range song, which confers a mating/reproductive advantage to males by not attracting the attention of conspecifics simply because it is inaudible from a distance (e.g., Vargas-Castro et al., 2017; reviewed in Reichard and Anderson (2015)]. Male enticement calls might have similar functions but, unlike the soft song, male enticement calls are loud, long-range vocalisations, with which males attract females from nest sites even when the females cannot see them (Hasegawa et al., 2013). By using vocalisations that are effective at drawing a response from potential mates, but not neighbouring males, males can attract females from a distance without the cost of reduced communication with the signal receivers.

Because the enticement call is emitted during the courtship sequence for pair formation (Hasegawa et al., 2013), the tolerance of male enticement calls increases the possibility that other males will breed nearby. Thus, as is the case for the male courtship song, neighbouring males should keep signallers away, even outside of their own mating periods, from their territories to reduce nest competition for current and future clutches (Turner, 2006; also see Arai et al., 2009 for frequent re-clutches due to nest predation in Japanese barn swallows). An explanation for why neighbouring males rarely responded to male enticement calls is that males would be less aggressive toward characteristics typical of immature birds [sensu Foster, 1987; reviewed in Hawkins et al. (2012); see Turner, 2006 for tolerance of adult swallows toward food-begging young]. As females confuse male enticement calls with nestling food-begging calls (Hasegawa et al., 2013; Hasegawa and Arai, 2016), males might have similar sensory or perceptive bias toward nestling calls (i.e., non-hostile behaviour toward nestlings). Such male behaviour would be maladaptive but can have persisted through evolutionary time scale as was the case with other sensory trap systems even when mimics and models were temporally separated: for example, in some insects, male penis mimics egg ovulation to elicit females consuming the other males’ sperm stored [reviewed in Christy (1995) and Arnqvist (2006)]. In consistent with this perspective, male responses toward enticement calls increased with progress of the incubation period, i.e., when approaching the nestling period (Figure 2 and Table 2), supporting this sensory trap explanation, or more specifically, nestling-mimicry hypothesis. A previous study showed that less-competitive males with drab throat colouration mimic nestlings more than do colourful males (Hasegawa and Arai, 2016). This finding further supports the nestling-mimicry hypothesis, given that competitively inferior males should avoid, rather than compete with, dominant and colourful males that breed early in high-quality territories when they attempt pair formation among these competitive males (Hasegawa et al., 2014; Wilkins et al., 2015).

Clearly, this is not the sole explanation for the observed pattern, and males might tolerate nestling-like calls for other reasons. For example, males might ignore nestling-like calls and concentrate on songs to simplify their sensory pathways, which can effectively prevent most neighbours’ breeding attempts nearby, because courting males often, but not always, use songs before/after emitting enticement calls (see section “Introduction”). However, this possibility alone cannot explain the reason why male responsiveness increased when approaching the nestling period. The actual mechanism remains to be shown in future studies.

In conclusion, I demonstrated here that the male enticement call, a nestling-like courtship behaviour of barn swallows, elicited few responses from neighbouring males. Together with a previous study showing female attraction to nestling-like calls, the current study indicates that male enticement calls have a dual function. However, unlike male song, the nestling-like courtship call is unique in attracting females while being tolerated by neighbouring males. Males as well as females would have sensory bias (but have contrasting behaviour) toward signals with nestling characteristics, simultaneously selecting for nestling-like courtship signals. This could explain why species with parental care exhibit immature-like behaviour.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

The animal study was reviewed and approved by the Ishikawa Prefectural University.

MH conducted all field work and wrote the manuscript.

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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