Avian breeding systems display remarkable diversity across species and populations (Argüelles-Ticó et al., 2016; Barber et al., 2024; Billerman et al., 2025; Gonzalez-Voyer et al., 2022). Understanding the origins and maintenance of such diversity remains a major challenge in evolutionary biology (Fromhage Royle et al., 2016). In monogamous systems, both parents typically cooperate in caregiving, whereas in polygamous systems, caregiving is often skewed toward one sex or shared among multiple individuals, resulting in distinct patterns of parental care (Kempenaers, 2022; Mock, 2022; Royle et al., 2012). Occasionally, within predominantly monogamous populations, individuals may adopt polygamous strategies (Chen et al., 2023). Such a switch in mating strategy can lead to contrasting patterns of parental care. Investigating how care adapts to these transitions and its impact on reproductive success offers insight into the diversity and adaptive flexibility of avian breeding strategies. Monogamous species that adopt polygyny may follow two alternative reproductive strategies: segregated brooding (split-nest polygyny), where females lay eggs in separate nests, requiring the male to divide his care effort (Evens et al., 2024; Newell et al., 2013), and joint nesting (same-nest polygyny), where multiple individuals care for a shared clutch (Heg Kitagawa, 2011). These two strategies involve different caregiving patterns, yet the ways in which care allocation and coordination influence breeding efficiency and reproductive success remain unclear. Here, we report a case in which a male formed a stable triadic group with two females in the obligatorily monogamous white-faced plover (Charadrius dealbatus) at Paisha Island (20.91559° N, 110.49945° E), Zhanjiang, Guangdong Province, China. The triad included an α-pair (α-male and α-female) that engaged in monogamous biparental breeding in 2023 and later incorporated an unmarked β-female for two breeding attempts in 2024 (Figure 1). Kinship analysis revealed that all three individuals were unrelated (Appendix S1: Table S3). The first breeding attempt began at the end of March. Genetic analysis confirmed that both females laid three eggs each into a single nest during a short window (March 26–30; Figure 1e; Appendix S1: Table S3), all of which were sired by the α-male and successfully hatched in an incubator. During the second breeding attempt (May–June), the α-male maintained bonds with both females, who laid eggs in separate nests (Figure 1f). The α-female laid three infertile eggs, while the β-female laid two fertile eggs sired by the α-male. However, the β-female and α-male subsequently abandoned them. This is the first documented case of polygyny in this population over 10 years of monitoring. Such behavioral oddities offer valuable opportunities to explore transitions in parental care strategies. To understand the patterns and outcomes of these two breeding attempts, we quantified parental effort and coordination to evaluate potential hypotheses. Compared to split-nest polygyny, same-nest polygyny may offer more caregivers, thereby reducing individual energy expenditure and improving breeding efficiency. Alternatively, an increase in caregivers may lead to coordination challenges, particularly when competition among same-sex individuals causes aggression or poor synchronization (Hartley Downing et al., 2020). White-faced plovers generally exhibit a strong diel incubation rhythm, with males incubating at night and females during the day. Around midday, when temperatures peak, both sexes reduce shift durations, and males often return to assist in cooling the eggs (Figures 2a and 3a, Appendix S1: Figure S1). Surprisingly, this rhythm persisted in same-nest polygyny and was primarily maintained by the α-pair, while the β-female contributed irregularly throughout the day and night (Figures 2b and 3b). In split-nest polygyny, both sexes continued the diel pattern. The male concentrated his efforts on the α-nest, contributing most of the night and midday incubation, while leaving the β-nest largely to the β-female (Figures 2c,d and 3c,d). We found that in the same-nest polygyny scenario, the six-egg clutch achieved an average daily nest attendance of 90.4%, higher than the α-pair's monogamous breeding attempt in 2023 (81.8%) and comparable to biparental monogamous 3-egg nests (86.7%; Figure 4a). Notably, individual contributions decreased: the β-female invested the least (mean = 24.1%), compared to the α-female (mean = 35.2%) and α-male (mean = 31.1%; n = 7 days; Figure 4b). During the second breeding attempt in 2024, the α-male was unable to maintain regular attendance at both nests. His combined effort across both nests resembled the daily attendance observed in monogamous males (mean = 39.5%, n = 14 days vs. 37.7%, n = 18 days from five nests), with a clear bias toward the α-nest (mean = 28.3%, n = 18 days vs. 12.3%, n = 16 days; Figure 4b). Despite this reduced investment, neither female increased her attendance. Both maintained levels similar to females in monogamous pairs (Figure 4b). Consequently, daily nest attendance was lower in both the α-nest and β-nest than in monogamous biparental nests (78.1%, n = 18 days; 60.3%, n = 16 days vs. 86.7%, n = 18 days; Figure 4a), with particularly low values at the β-nest. Despite the lack of a clear incubation rhythm in same-nest polygyny, shift transitions were tightly coordinated, with an average exchange gap (Sládeček et al., 2019) of 3 min (range: 0–50 min; see Appendix S1: Supplementary methods), which was significantly shorter than in biparental monogamous pairs (5 min; range: 0–47 min; t test, p = 0.016). Infrared video showed that the α- and β-females shifted harmoniously without aggression (Videos S1 and S2). In split-nest polygyny, the α-male retained his diel contribution pattern, focusing on the α-nest. The β-nest experienced prolonged nocturnal absences, reflected in extended exchange gaps (mean = 46 min; range: 0–499 min). When the α-male left the α-nest to assist at the β-nest, the α-nest also experienced occasional long gaps (mean = 10 min; range: 0–450 min). Rare polygynous associations in white-faced plovers provide a unique lens through which to understand the ecological and behavioral factors that enable or constrain such transitions and their consequences for reproductive outcomes. Several mechanisms may explain the emergence of this triadic system. First, opportunistic mate replacement may have occurred if the α-female was temporarily absent, allowing the α-male to pair with the available β-female, as observed in other plover species (Székely et al., 1999). Second, male-driven polygyny is plausible, as the α-male exhibited superior morphological traits that could enhance attractiveness and competitive success (Appendix S1: Figure S4). Comparative analyses in other shorebirds support this link (Székely et al., 2000, 2004). Third, female tolerance likely facilitated the coexistence of two females who laid in the same nest without rejecting each other's eggs. These mechanisms underscore that mating opportunities and social interactions may set the stage for polygyny in socially monogamous species. The shift from monogamy to polygyny necessitates a reorganization of parental care, yet care in this species appears to have limited flexibility, particularly regarding shift patterns and nest attendance. First, in same-nest polygyny, although three individuals were involved, the diel rhythm was still largely maintained by the α-pair, and the β-female failed to establish a consistent schedule. Whether this irregularity imposes additional costs remains unclear, but the progressive reduction in her incubation time suggests potential stress on her rhythm regulation. Second, in split-nest polygyny, the male was unable to effectively incubate both nests, and the β-female's limited compensation likely contributed to clutch failure. Unlike other biparental shorebirds (Bulla et al., 2017), white-faced plovers in this population rarely succeed in uniparental incubation. Over nearly 10 years, no successful uniparental hatching was observed. All nests failed following mate abandonment, with the remaining parent either abandoning shortly after or simultaneously (n = 29 abandonment events recorded by cameras between 2021 and 2025). This reliance on biparental care likely explains the rarity of split-nest polygyny and may represent a key force maintaining biparental care in birds (Long et al., 2022; Thomas Kitagawa, 2011). Thus, the benefits of cooperative incubation may only be realized under certain conditions, such as high ambient temperatures. In our case, the successful hatching of six eggs may be attributed to warm seasonal temperatures (mean: 30°C; range: 23–51°C), which exceeded the physiological zero for avian embryos (24–26°C; DuRant et al., 2012; Mainwaring, 2015), facilitating ambient incubation and potentially offsetting developmental issues caused by uneven heating (Griffith et al., 2016). In conclusion, the emergence and prevalence of polygyny in white-faced plovers appear constrained by their strong dependence on biparental care and rigid incubation rhythms. This study highlights how behavioral anomalies, such as the observed triad, offer valuable insights into the evolution of breeding systems. Unlike typical polygamous systems characterized by skewed care, this case suggests that transitions from monogamy to polygyny involve trade-offs in care allocation, coordination, and reproductive success. Boya Xie and Xi Lin led field monitoring and manuscript writing. Xiaotong Niu and Heiman Ho conducted the kinship analysis. Xin Lan and Xinlei Liu contributed to infrared data processing. Yachang Cheng and Yang Liu conceived the study and designed the methodology. Zitan Song assisted with manuscript revision. Yang Liu provided funding for the study. All authors read and approved the paper. We thank the members of the Avian Ecology and Evolution group at Sun Yat-sen University for their discussions and suggestions on an earlier version of the manuscript. We also thank Lifeng Zhuang for providing clear photographs of monogamous pairs. Additional thanks go to Tong Mu and Chung-Yu Chiang for their advice on age identification of plovers. We are especially grateful to Dr. Martin Bulla and an anonymous reviewer for their constructive suggestions on an earlier version of the manuscript. The authors declare no conflicts of interest. All aspects of the fieldwork complied with the Law of the People's Republic of China on the Protection of Wildlife and were authorized by the Guangdong Provincial Forestry and Grassland Administration. Birds were ringed and handled by trained individuals, with the aim of causing as little disturbance as possible. Data and code (Xie et al., 2025) are available in Zenodo at https://doi.org/10.5281/zenodo.17668571. Appendix S1. Video S1 Metadata. Video S1. Video S2 Metadata. Video S2. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
Xie et al. (Sun,) studied this question.