The limited reports on nest predation by the Great Kiskadee confirm, at the very least, the existence of this behavior in the species. Nevertheless, uncertainties persist regarding its frequency of occurrence and other crucial aspects, such as preferences for specific species and the environments where it is more prevalent. Drawing upon the gathered information, I present three hypotheses to address the scarcity of information on nest predation behavior in the Great Kiskadee. These hypotheses not only shed light on the current knowledge gap but also pave the way for future research to delve deeper into this aspect of the species. They are: 1) nest predation is a rare and sporadic behavior, indicated by the low number of documented cases; 2) the behavior is common and frequent, but has been poorly documented 3) its occurrence is influenced by environmental factors that may limit its observation. Hypothesis 1: Nest predation in the Great Kiskadee occurs sporadically The initial hypothesis suggests that the scarcity of available records mirrors the rarity of this behavior in the species. As an uncommon phenomenon, the probability of its observation diminishes. Certain aspects of the species' biology could substantiate this hypothesis. Research on the Great Kiskadee's diet reveals a prevalent consumption of invertebrates(van Rossem, 1914; Beltzer, 1983; Argel-de-Oliveira et al., 1998; Latino and Beltzer, 1999; Munin et al., 2012)while others found fruits as the major food source (Gorena, 1997). This dietary preference aligns with the general pattern observed in Neotropical passerines, which have not specialized in preying on vertebrates (Lopes et al., 2005). In fact, vertebrates constitute a small fraction of their diet, despite this behavior being widely present among the Neotropical passerines with, at least, 11 families with species preying on bird nests (Menezes and Marini, 2017). Frogs and lizards are the most commonly consumed vertebrates by passerines, with only about 18% of the records involving other vertebrate groups (Lopes et al., 2005). The few studies performed in the urban environment that described the Great Kiscakee’s diet did not provide evidence of nestlings in it. While Argel-de-Oliveira et al. (1998) had monitored only one single nest, Pereira and Melo (2012) analyzed the content of 218 regurgitated pellets. In both studies, the only vertebrate found was the Common House Gecko (Hemidactylus frenatus). Although analyzing only the diet of the Great Kiskadee’s nestlings may not offer a complete understanding of its predatory behavior, a study conducted by Gorena (1997) focusing on adult individuals found that fish constituted the main vertebrate prey (95%), without further description of other taxa. Similarly, Martins-Oliveira et al. (2012) did not provide information on the specific vertebrate taxa preyed upon. The same lack of evidence regarding the Great Kiskadee preying on nests was observed in studies conducted in wild habitats, monitoring nests of various species (e.g., França et al., 2009; Galetti et al., 2009; co*ckle et al., 2016). In these cases, the presence of a more diverse range of prey options, including potential competitors and predators of the Great Kiskadee, or variations in methodological procedures could be speculated as potential causes for the lack of findings. In methodological terms, a possible explanation may lie in the species selected for nest monitoring in those studies. Given that predator-prey relationships can be species-specific or constrained by different factors (Patten and Bolger, 2003), and considering a possible existence of a Great Kiskadee's preference for certain species (nests of small-sized species – I discuss about this later – and in specific strata and locations, i.e., edge or interior), the lack of Great Kiskadee records could be a methodological artifact. Hypothesis 2: The behavior is common and frequent, but has been poorly documented The second hypothesis, in contrast to the first, suggests that the behavior is common in the species but has been insufficiently documented. The broad spectrum of vertebrates preyed upon by the Great Kiskadee allows one to speculate that this behavior can be prevalent. The literature review showed that the Great Kiskadee preys on a broad spectrum of vertebrates, showcasing the species' versatility as a generalist predator, possibly one of the most versatile among passerines (Fitzpatrick, 1980; Sick, 1997). This versatility stems from its medium size (20-25 cm) and its ability to forage on various substrates and strata in both forested and open areas (e.g. tree foliage and branches, on the ground, in the air, above and under the water surface), feeding also on fruits, flowers, seeds, and invertebrates (Fitzpatrick, 1980; Beltzer, 1983; Richard, 1986; Cintra, 1997; Gorena, 1997; Latino and Beltzer, 1999; Gabriel and Pizo, 2005; Fischer et al., 2010; Martins-Oliveira et al., 2012). Its size and prolonged searching capabilities enable it to target larger prey (Schoener, 1971; Fitzpatrick, 1981; Gabriel and Pizo, 2005). These generalist characteristics make the Great Kiskadee a broad-spectrum omnivore, common in both wild and human-modified environments (Ruszczyk et al., 1987; Argel-de-Oliveira, 1995; Cintra, 1997; Sick, 1997) and, probably as a consequence, widely distributed from the southwestern United States into the central Argentina (Sick, 1997; Brush and Fitzpatrick, 2002). Moreover, the Great Kiskadee possesses reproductive and abundance characteristics that can heighten its predation potential. With a breeding season lasting up to nine months, this species exhibits high reproductive rates and success within urban ecosystems (Pereira et al., 2009; Santos et al., 2019). It also prefers to nest in anthropic structures distant from forested areas in urban centers, probably to reduce the risk of nest predators (Corrales-Moya and Sandoval, 2021). Despite potential threats from predators, the Great Kiskadee's assertive behavior towards larger birds, coupled with its robust medium size, contributes to effective nest defense (Sick, 1997; Smith, 2006), potentially enhancing its overall reproductive success. Furthermore, to support this hypothesis, the species biology and ecology resembles other widely recognized birds known for preying on nests, particularly in urban environments.These species can enhance their survival and abundance in urban spaces by limiting the recruitment of potential competitors, potentially exerting additional pressure on other species. For instance, the Pied Currawong (Strepera graculina) has been implicated in over 50% of nest predation cases, leading to a decline in species recruitment in urban areas of Australia (Major et al., 1996; Debus, 2006). Similar patterns have been observed for magpies (Pica pica) (Jerzak, 2001), Western Scrub-Jay (Aphelocoma californica) (Reidy and Thompson III, 2012), and crows (Corvus sp.) (Marzluff et al., 2001; Rodewald and Kearns, 2011), recognized as major nest predators in urban habitats, among others. In support of these observations, it has been noted that the foraging versatility of the Great Kiskadee more closely resembles that of the Corvidae family rather than a typical suboscine passerine (Fitzpatrick, 1980). Similar to them, the Great Kiskadee also demonstrates remarkable adaptability to diverse habitats, ranging from natural to highly urbanized environments. Numerous studies conducted in various urban areas across Brazil consistently identify the Great Kiskadee as the most abundant and widespread species (Ruszczyk et al., 1987; Argel-de-Oliveira, 1995; Matarazzo-Neuberger, 1995; Krügel and Anjos, 2000; Scherer et al., 2005; Galetti et al., 2009; Braga et al., 2010; Cruz and Piratelli, 2011; Vasconcelos et al., 2013; Sacco et al., 2015) The scarcity of records of this behavior, assumed to be common by the hypothesis, may be attributed to the unique characteristics of nest predation. The swift and instantaneous nature of predation poses challenges for observation without the use of video recording devices near the nests (Cox et al., 2012). However, these devices can be expensive and often yield small sample sizes, being mostly used in developed countries (Fischer et al., 2012). As a result, there is a lack of knowledge about the ecology of nest-predators and prey relationships especially in developing countries (Ibáñez-Álamo et al., 2015), which make up the majority of the Great Kiskadee's distribution. For example, Perrella et al. (2017) found a predation rate of 46% in nests of the Purple Euphonia monitored in an urban area, but the predators were not identified. Similarly, Rivera-López and MacGregor-Fors (2016) observed a high rate of nest predation in a Mexican city, but they were unable to accurately identify the predators. Additionally, researchers may be less motivated to publish such occasional events, potentially due to perceived lack of novelty or previous journal rejections, overlooking the importance of filling knowledge gaps in the phenomenon. However, as emphasized by (Anderson, 2017), naturalistic observations at the organismal level are crucial for a comprehensive ecological understanding and should be encouraged. Hypothesis 3: The behavior occurrence is influenced by environmental factors that may limit its observation The third hypothesis can be considered a blend of the two preceding ones. The behavior in natural settings may be assumed as occasional, but it could become frequent in urban environments due to various factors, some of them that may be unique to cities – yet to be understood (Faeth et al., 2005). It is based on propositions suggesting that passerine predation on vertebrates may be influenced by environmental factors, even exhibiting regional patterns across the Neotropics (Poulin et al., 2001; Lopes et al., 2005). Similarly, nest predation rates can vary depending on habitat, nest type, and nest-site characteristics (Martin, 1988, 1993; Thompson and Burhans, 2003; Marini, 2017). Analyzing the urban environment specifically, nest predation is known to vary according to the characteristics of the urban environment (Stracey, 2011). Moreover, the foraging behavior of the Great Kiskadee has been demonstrated to vary in response to urban settings (Martins-Oliveira et al., 2012), influenced by the surrounding environment (Robinson and Holmes, 1982). Additionally, the Great Kiskadee's tendency to forage in tree canopies (Martins-Oliveira et al., 2012) might heighten its likelihood of encountering nests. The urban landscape displays a gradient of increasing intensity of use from the periphery to the more urbanized and verticalized central regions, exhibiting a negative correlation with green coverage: as the intensity increases, the green coverage diminishes (Blair, 1996; Lin et al., 2015; McDonald et al., 2023). Such changes have effects on the composition, abundance, and richness of the bird assemblage, increasing the number of exotic species and decreasing overall species richness and diversity (Blair, 1996; de Toledo et al., 2011; Kurucz et al., 2021), leading to a hom*ogenization of bird assemblages (Blair, 2001; McKinney, 2006; Luck and Smallbone, 2011).Moreover, the urban forest environment resembles highly fragmented forests (Soulé, 1991; Marzluff and Ewing, 2001; Chace and Walsh, 2006), where nest predation rates tend to be higher than in less fragmented landscapes (see Hartley and Hunter, 1998). This urban gradient may also influence the types of available prey through trophic cascade effects (e.g., Melo et al., 2016). Invertebrate availability, a key component in the Great Kiskadee diet, tends to be lower in urban environments compared to natural areas (e.g., Zapparoli, 1997; Teglhøj, 2017; Seress et al., 2018), albeit with some divergencies regarding these findings (see Kurucz et al., 2021). Could this potentially lead to an increase in bird nest predation? In fact, studies have shown that the amount of urbanization in the landscape tends to increase nest predation (e.g., Phillips et al., 2005; Reidy and Thompson III, 2012), but the effects are not always straightforward (see discussion in Rodewald et al., 2013; Vincze et al., 2017). As demonstrated in urban landscapes, the abundance of predators is heterogeneous, resulting in both the relaxation and intensification of nest predation (Major et al., 1996; Gering and Blair, 1999; Rodewald and Kearns, 2011; Rivera-López and MacGregor-Fors, 2016; Vincze et al., 2017). Studies indicate that opportunistic, medium-sized predators can attain higher densities in urban habitats compared to rural areas, a phenomenon known as mesopredator release (Soulé et al., 1988; Prugh et al., 2009; Fischer et al., 2012). While much attention has been given to mammal mesopredators and exotic species (e.g., domestic and feral cats) in urban environments (e.g., Crooks and Soulé, 1999; Fischer et al., 2012), omnivorous birds can also act as mesopredator species, contributing to increased pressure on other species by typically preying on the young (Fischer et al., 2012), as exemplified by the case I witnessed. Mesopredator bird species typically exhibit a positive response to urbanization (Jerzak, 2001; Fischer et al., 2012). This trend seems to be applicable to the Great Kiskadee as well, as evidenced by its status as one of the most abundant species in the urban and its shared characteristics with notable nest predators, as discussed earlier. Similar to the second hypothesis, inherent challenges in observing nest predation events and the still limited research in urban ecosystems, particularly in the Neotropical region, could account for the lack of recorded predation events. Given the positive association between the abundance of the Great Kiskadee and urbanization showed before, coupled with the limited examination of the urbanization gradient in urban studies (Fischer et al., 2012), it is not possible to dismiss this possibility. Potential impact on other bird species The limited reports on nest predation by the Great Kiskadee (e.g., Sick, 1997; Francisco, 2006), combined with my own observations, indicate a preference for preying on nests of small-sized birds (approximately 9-12 cm). This preference may stem from the convenience of handling smaller nestlings. However, considering its broad spectrum of vertebrate prey, as discussed earlier, it is also plausible that the Great Kiskadee preys on nestlings of varying sizes. Further field observations would provide valuable insights into this aspect. Nevertheless, what outcomes might we anticipate regarding its impact on urban bird species susceptible to its voracity? If we presume that the low number of reported cases serves as an indicator of the behavior's commonality, the impact could be deemed low and not distinguishable from more preserved environments. Additionally, the conjecture of a low impact aligns with theories currently under examination in the urban ecosystem. The Prey-switching hypothesis, for instance, posits that the presence of anthropogenic food in the urban environment may diminish predation rates (Stracey, 2011). As a dietary generalist species, the presence of anthropogenic food could influence the species' behavior. However, the extent to which this hypothesis applies to the Great Kiskadee remains uncertain. Conversely, in situations where alternative food sources are scarce, it is unclear whether the Great Kiskadee would redirect its attention to bird nests and under what specific conditions such behavior might occur. The threshold for this potential shift is yet to be determined. These inquiries remain unanswered, necessitating further investigation into the matter. On the other hand, the Nest Predation Paradox (Stracey, 2011; Fischer et al., 2012) posits that, despite the presence of a high abundance of predators, nest predation rates may vary and could be lower than initially anticipated. This theory highlights the paradoxical relationship between elevated rates of nest predation and the sustained existence of bird populations. Despite the pronounced threat posed by nest predators, numerous bird species exhibit the ability to uphold stable populations. This paradox implies that birds have evolved diverse strategies, including nest-site selection, camouflage, and mobbing behavior, to mitigate the impact of nest predation events. In this scenario, even if the Great Kiskadee is one of the most abundant species in the urban environment and nest predation behavior were commonplace, its impact would have limited effects on shaping the structure of the urban bird assemblage. Final Remarks While the ideas presented here are speculative, they are grounded in a robust theoretical background. They clearly allow to speculate that the Great Kiskadee's role as a nest predator may have been underestimated and overlooked in the urban ecosystem. It is plausible to hypothesize that nest predation in the urban environment is influenced by all the myriad of variables discussed here (the presence/absence of apex predators, the structure of bird assemblages, the urban gradient, the availability of anthropogenic food, and the richness and abundance of arthropods and other small vertebrates as prey) – and probably more (Thompson III, 2007; Reidy and Thompson III, 2012). The interplay of these and other factors highlights the complexity of nest predation dynamics in urban environments. Understanding how these factors interact and contribute to the ecological dynamics of bird assemblages in the urban ecosystem is crucial for assessing the role of the Great Kiskadee – if any – in structuring local bird populations, food webs, and in the overall biotic hom*ogenization of urban biodiversity (Blair, 2001; Faeth et al., 2005; McKinney, 2006; Sacco et al., 2015; Santos et al., 2019). Understanding how urbanization influences the abundance of the Great Kiskadee, for example, and its interactions with other species is necessary to comprehend patterns of nest predation and bird assemblage structure in urban ecosystems. As stressed by Argel-de-Oliveira et al. (1998), it is crucial to further study the biology of the Great Kiskadee in urban settings and compare it to its behavior in less human-impacted environments. This knowledge will inform management strategies aimed at promoting bird diversity in urban landscapes, particularly in light of the challenges posed by climate change in the cities.
