Snake predators of bird eggs: a review and bibliography
 
Serpientes depredadoras de huevos de aves: una revisión y bibliografía

Jody M. Barends ¹  and Bryan Maritz ¹ 

• Abstract
• Introduction
• Methods
  • Data collection and inclusion criteria
  • Ecological traits of oophagous snakes and bird egg prey
  • Statistical analyses
• Results
  • Records of bird egg predation by snakes
  • Which snakes eat bird eggs?
  • Body lengths and habitat use of oophagous snakes
  • Sizes of consumed bird eggs
  • Relationship between snake-size and bird egg-size
• Discussion
  • What we know about bird egg feeding by snakes
  • Why don’t more snakes eat bird eggs?
  • Importance of identifying snake predators of bird eggs
• Responses to this article
• Acknowledgments
• Data availability
• Literature cited

INTRODUCTION

Predatory attacks by snakes on nesting birds and their offspring have been well-documented globally (for example, in Africa: Lloyd 2004, mainland Asia: Khamcha et al. 2018, Australia: Fulton 2018, North America: DeGregorio et al. 2014, and the Neotropics: Menezes and Marini. 2017). However, while many species of snakes are known consumers of nestling birds, chicks, and brooding adults, few species are reported consuming bird eggs. Predation of eggs by snakes can reduce recruitment of birds and impact bird population dynamics (Lavers et al. 2010). In addition, by preying on eggs, snakes have the potential to influence bird life history patterns by forcing them to re-lay and brood successive clutches (DeGregorio et al. 2014). Given that many species of birds provide important ecosystem services (Whelan et al. 2008, Whelan et al. 2015, Şekercioğlu et al. 2016), population fluctuations from reduced recruitment could potentially alter the functional integrity of a range of ecosystems (Mortensen et al. 2008, Gascon et al. 2015, Lowney and Thomson 2021). For example, extensive predation on birds and eggs by invasive brown tree snakes (Boiga irregularis) on the islands of Guam has fundamentally altered the local faunal community through extirpation of several species, ultimately causing trophic collapse (Wiles et al. 2003). Thus, by preying on bird eggs in large numbers, snakes have the potential to indirectly influence ecosystem functioning in many biological communities.

Quantifying the extent to which snakes affect ecosystems by consuming bird eggs is hindered by numerous challenges. Several facets of these trophic interactions are unclear, including knowledge of which species of birds lay eggs that are at risk of snake predation, as well as the extent to which predation of bird eggs by snakes varies spatiotemporally (Weatherhead and Blouin-Demers 2004, Lahti 2009, Menezes and Marini 2017). Identification of which snakes consume bird eggs offers a critical first step in understanding these dynamics. Knowing which species of snakes consume eggs allows researchers to formulate predator-specific hypotheses across a range of habitats and environments (Reidy and Thompson 2012, Ibáñez-Álamo et al. 2015). Additionally, avian conservation practitioners can use that information to produce anti-predator strategies for bird conservation efforts. Unfortunately, information on snake feeding is poorly catalogued (Grundler 2020, Maritz et al. 2021b) making the compilation of a robust list of oophagous species challenging.

Snake diets are diverse, compositionally complex, and often difficult to adequately quantify (Greene 1997, Glaudas et al. 2017, Maritz and Maritz 2020). Unfortunately, the natural history data required to systematically describe snake diets are often lacking, particularly for taxa that occur in poorly-studied regions. For most species, we know very little about their feeding habits apart from generalized characterisations of their diets inferred from a limited quantity of published information (Maritz et al. 2021b). For many others, we lack even a basic understanding of their feeding habits. A recent global synthesis of snake feeding records by Grundler (2020) highlights the incomplete nature of our understanding of snake diets. Of the 3921 species of snakes distributed across the globe (Uetz et al. 2021), less than a third (1248 species) could be included in that dataset and the majority of those species were only represented by fewer than ten records. Due to this paucity of feeding records, our understanding of which types of prey are, or are not eaten by different species of snakes is limited. Consequently, many species of snakes not currently known to eat bird eggs may be oophagous.

Despite the above limitations, published records of snakes consuming bird eggs have accumulated in the literature (Weatherhead and Blouin-Demers 2004, Ibáñez-Álamo et al. 2015). Over the past few decades, using camera monitoring systems, some snake species have been documented eating eggs for the first time (Cutler and Swann 1999, Pierce and Pobprasert 2007, Ribic et al. 2012, Khamcha et al. 2018). Moreover, novel feeding records published in natural history publications and online community science portals continue to confirm additional species as bird egg predators. However, because studies and platforms vary in their objectives, records are scattered in the literature and online. In some cases, reports may be difficult to access or are completely inaccessible to researchers or conservationists interested in using such data.

We compiled a comprehensive list of confirmed snake predators of bird eggs. We collated records of snakes consuming bird eggs from a range of sources of information and used the details within those reports to broadly summarize trends of bird egg predation by snakes globally. We also analysed several traits of the identified snake species and egg prey to test hypotheses regarding why those species consume bird eggs but many others do not. Specifically, we tested if the inclusion or exclusion of bird eggs in the diets of snakes is associated with 1) differences in snake body size, 2) variation in snake habitat use, and 3) taxonomic relatedness between snake taxa. To contextualize which bird species are at risk, we also compared the size distribution of consumed bird eggs to that of a sample of bird eggs not reported in the diets of snakes. Lastly, we investigated the sizes of eggs consumed by snakes of varying body lengths.

METHODS

Data collection and inclusion criteria

Between August 2020 and July 2021, we searched for and collected data from reports of bird egg predation by snakes. Our main sources of data were formal publications (i.e., peer-reviewed journal articles and books) found on the online indexer Google Scholar, JSTOR, and SquamataBase (Grundler 2020) - an online natural history repository containing close to 11,000 records of predator-prey interactions across 1248 snake species. We also searched the literature cited within those publications to identify additional sources. Additionally, we collected data from unpublished academic theses and personal communications from researchers. Lastly, we collected data from community science records published on the online platform iNaturalist (https://www.inaturalist.org) and the social media network Facebook. Facebook records were obtained from the groups “Predation records - reptiles and amphibians (sub-Saharan Africa)” (published in Maritz and Maritz 2020), “Snakes of South Africa” (https://www.facebook.com/groups/snakesofsouthafrica), and “Wild snake predation records” (https://www.facebook.com/groups/wild.snake.predation.records).

We restricted our data collection to include records of snakes unambiguously eating bird eggs. We did not include reports with vague descriptions of snakes attacking nests unless eggs were directly specified as the prey rather than nestlings, chicks, or adult birds. Conservatively, we excluded records without clear evidence of snakes eating eggs. For a record of a snake species to be included it needed to meet these criteria: 1) snakes were observed eating, attempting to eat, or having eaten (shells in digestive tracts) eggs and 2) records were of snakes in the wild consuming eggs they found without human intervention. We included cases in which the eggs of captive or domesticated birds were consumed if those predatory attacks met the above criteria.

For each reported predation event, we identified the snake and bird species to the finest taxonomic level possible, and we noted the number of eggs involved. Geographic coordinates were noted from the original record or estimated using Google Maps. We updated snake species names to match their current taxonomic nomenclature as per Uetz et al. (2021). We provide a summary of these records detailing the taxonomic diversity of oophagous snake predators and their bird egg prey, as well as geographic biases in these trends.

Ecological traits of oophagous snakes and bird egg prey

Although the primary goal of this study was to compile a list of known snake predators of bird eggs, we were also interested in examining traits of those species that might explain why those snakes consume bird eggs but others do not. Differential prey use within a particular snake species is facilitated by several factors, chief among which include varying body size constraints (Arnold 1993, Greene 1997, Maritz and Alexander 2014) and variable encounter rates of different prey (Alencar et al. 2013, Mori and Nagata 2016). Accordingly, we chose to examine and compare the body lengths and primary habitats of the snakes on our list to snakes not known to consume bird eggs. Snake body lengths correlate with their diet breadth as larger snakes can typically consume bulkier and heavier prey than smaller ones, and can therefore hunt a broader range of prey (Arnold 1993, Maritz et al. 2021c, Barends and Maritz 2022). Habitat use largely influences the probabilities at which snakes encounter different prey (for example, arboreal snakes are more likely to encounter arboreal prey; Harrington et al. 2018). Taken together, these traits are likely major limiting factors towards bird egg consumption by snakes.

Unfortunately, most accounts of snakes consuming bird eggs do not include linear measurements of the sizes of the individual snakes in question. To compensate for this, we instead used maximum body length data (i.e., length from snout to tail) of each species on our list (Electronic dataset 1) collected from Feldman et al. (2016). We also collected these data for all other species in the Feldman et al. (2016) dataset (N = 3529) for use in comparisons (Electronic dataset 2). Similarly, we gathered information on snake habitats to classify species as either aquatic, arboreal, fossorial, semi-arboreal, or terrestrial. We gathered these data for as many species as we could (N = 2646) from field guides and published datasets, including Pizzatto et al. (2007), Lawing et al. (2012), Feldman and Meiri (2014), Bars-Closel et al. (2017), Cyriac and Kodandaramaiah (2018), and Harrington et al. (2018).

We were similarly interested in examining traits of the consumed bird eggs that could provide insight into which bird eggs are at risk of predation by snakes. Because prey bulk (i.e., the cross sectional-diameter of prey) relative to snake size is an important consideration of dietary selectivity in snakes (Greene 1997) we chose to quantify the diameters of consumed eggs. Snakes typically ingest bird eggs length-wise (Gans 1952), and so the diameter of the eggs acts as the main dimensional constraint on ingestion. However, as before, most reports did not include measurements of the dimensions of the eggs consumed. We thus gathered information on average egg diameters for each of the bird species on our list (Electronic dataset 1). We gathered these data from resources detailing the reproductive traits of birds that breed in Australia (Garnett et al. 2015), Asia (Tsai et al. 2020), Britain and Europe (Harrison and Castell 2002, Storchová and Hořák 2018), Micronesia (Brandt 1962), North America (Baicich and Harrison 2005), South America (Mason 1985, Auer et al. 2007, Marques-Santos et al. 2015), and southern Africa (Tarboton 2011). For comparative purposes, we also gathered egg diameter data for a geographically and phylogenetically diverse sample of 2326 species of birds (~25% of all birds; Electronic dataset 2).

Statistical analyses

We analysed geographical trends of bird egg predation by snakes by comparing the numbers of 1) feeding records, 2) identified snake species and 3) identified bird egg prey species across major geographical regions. We demarcated regions as Africa, Asia, Australia, Central America, Europe, Micronesia, the Middle East, North America, and South America. We also examined the elevation (in metres above sea level) of each area where predation events were observed. We gathered elevation data where predation events occured (N = 350) at a resolution of 30 arc seconds from the Worldclim global elevation dataset (Fick and Hijmans 2017).

We evaluated the ecological traits of oophagous snakes by first analysing patterns of their body length distributions. We used a Kolmogorov-Smirnov test to compare the relative distribution of the maximum body lengths of oophagous snakes to all snakes included in Feldman et al. (2016). We then used a phylogenetic ANOVA to test for differences in average log-transformed maximum body lengths of snakes that do and do not consume bird eggs while accounting for the effects of phylogenetic autocorrelation caused by species relatedness. We performed this test with the "Geiger" package (Pennell et al. 2014) in R software v.4.1.1 (R Core Team 2021) using a pruned version of the phylogeny of squamate reptiles published by Tonini et al. (2016) (N = 3503 species) as the input phylogenetic tree. We similarly summarized oophagous snake habitat use and then compared body lengths (log10 transformed) by habitat use controlling for phylogeny via phylogenetic ANOVA.

We tested for the presence of a phylogenetic signal associated with bird egg consumption by snakes by calculating Blomberg’s K (Blomberg et al. 2003). We considered a Blomberg’s K value less than one to indicate that oophagy occurs randomly across our tree under Brownian motion evolution whereas K values greater than one suggest oophagy is more prevalent between closely related snake taxa (Blomberg et al. 2003). We performed this test using the "Phytools" package (Revell 2012) in R.

Similar to our analyses of snake body lengths, we performed the same comparative tests between consumed eggs and other eggs. We used a Kolmogorov-Smirnov test to compare the relative distributions of egg diameters of eggs eaten by snakes and all other eggs. We then looked for differences in average log-transformed diameters of consumed eggs and other eggs (N = 2326) via phylogenetic ANOVA. We used a pruned version of the phylogeny of extant birds published by Jetz et al. (2012) as the input tree for this test. Finally, we visually inspected the relationship between bird egg diameters and snake body lengths across all predation events by creating a Sankey plot depicting the flow between egg diameters (in mm) and snake length (in meters). For bird egg diameter size classes, we used bins of 10 mm, and for snake body length size classes we used bins of 1 m.

RESULTS

Records of bird egg predation by snakes

Our search produced a total of 471 records of confirmed predatory interactions between snakes and bird eggs across the globe (Table 1). Bird eggs were consumed by 123 different snake taxa (114 species and nine subspecies) belonging to 59 genera and seven families (Boidae, Colubridae, Elapidae, Psammophiidae, Pseudaspididae, Pythonidae, and Viperidae). Of these, Colubridae (70% of all 123 taxa) and Elapidae (13% of all 123 taxa) were most frequently reported (Fig. 1). The eggs of at least 210 species of birds across 159 genera, 71 families and 21 orders, including passerines and several non-passerine orders, were consumed. In 26 cases, bird eggs were only identified to genus, family, or order levels (seven cases, 14 cases, and five cases respectively). In 63 cases, bird eggs were not identified beyond the class level, or the exact identity of the species was ambiguously reported in the source material (for example, “the eggs of land birds”).

Predation of bird eggs by snakes was reported on all continents on which snakes are distributed as well as on several archipelagos and small islands (Fig. 2). The majority of these observations (~75%) occurred at low elevations < 500 m above sea level. Sampling frequencies of feeding records varied between geographical regions (Fig. 3) as most predation events were observed in North America (37% of all records) and Africa (24% of all records). At the national level, most records disproportionally represented the relatively well-studied United States of America (35% of all records) and South Africa (14% of all records) respectively. Species richness of snake predators and bird egg prey also both varied regionally and were similarly proportioned to the spread of predation records (Fig. 3). Approximately 29% of recorded snake predators were from North America, 20% from Asia, and 17% from Africa. Similarly, 31% of identified bird taxa whose eggs were consumed were from North America, and 23% were from Africa.

Which snakes eat bird eggs?

In Africa, the common egg-eater (Dasypeltis scabra), was responsible for most reports of egg-eating and was most reported for any snake species (N = 53, 11% of all records, Table 1). Common egg-eaters consumed the eggs of at least 40 species of birds throughout southern and East Africa, ranging from the southernmost regions of South Africa to Uganda. Other important oophagous African snakes included various species of cobras (Naja spp.), boomslang (Dispholidus typus), and mole snakes (Pseudaspis Cana) that were predominantly from southern Africa. Southern and East African pythons (Python natalensis and Python sebae) were also confirmed as bird egg consumers.

In North America, various rat snakes (Pantherophis spp.) were the principal consumers of bird eggs, collectively accounting for 15% of all records (Table1). Other frequently reported species included several species of bullsnakes (Pituophis spp.), kingsnakes (Lampropeltis spp.), and eastern racers (Coluber constrictor). Collectively, snakes from the above genera consumed the eggs of at least 66 species of bird across the USA (Fig. 2). In particular, these snakes were most frequently observed raiding hen-houses for the eggs of Domestic Chickens (Gallus gallus domesticus) and often consumed the eggs of Black-capped Vireos (Vireo atricapilla), Field Sparrows (Spizella pusilla), Northern Bobwhites (Colinus virginianus), and several species of ducks and geese. In Florida, the invasive Burmese python (Python bivittatus) consumed the eggs of Limpkins (Aramus guarauna), American White Ibises (Eudocimus albus), and introduced Helmeted Guinea Fowl (Numida meleagris). Other notable North American oophagous snakes included common garter snakes (Thamnophis sirtalis), eastern indigo snakes (Drymarchon couperi), and massasaugas (Sistrurus catenatus), the only viperid from North America included on our list.

Neotropical snakes from Central and South America that consumed bird eggs mostly included several species of colubrids (Table 1). Western indigo snakes (Drymarchon corais), several species of puffing snakes (Phrynonax spp.), and both species of chicken snakes (Spilotes pullatus and S. sulphureus) were the principal egg predators in these regions. Records involving those species were largely restricted to regions in Brazil and Peru but extended as far south as Chile and as far north as Costa Rica (Fig. 2). Collectively, Neotropical colubrids consumed the eggs of at least 20 species of birds. Large boas and anacondas of the genera Boa, Epicrates, and Eunectes were observed consuming the eggs of at least seven species of birds in various habitats in Brazil and Argentina. Similarly, in the Caribbean, several species of Antillean boas (Chilabothrus spp.) were notable bird egg predators.

In Europe, only five species of snakes were reported consuming bird eggs (Table 1). The most frequently reported species were the four-lined snake (Elaphe quatuorlineata) in Italy and the Montpellier snake (Malpolon monspessulanus) in Spain. The European adder (Vipera berus) in the United Kingdom, the Aesculapian snake (Zamenis longissimus) in Italy and Poland, and the ladder snake (Zamenis scalaris) in Spain were also confirmed as oophagous. Those snakes were frequently recorded consuming the eggs of Common Pheasant (Phasianus colchicus), Great Tit (Parus major), Common Linnet (Linaria cannabina), and Common Babbler (Argya caudata). We only found one record of bird egg predation in the Middle East which was of the Arabian tiger snake (Telescopus dhara).

Across the oceanic region of Asia, Australia, and Micronesia, cat snakes of the genus Boiga were the predominant bird egg predators. Records of these snakes accounted for 6% of our dataset (Table 1). More than half of those observations were of the invasive brown tree snake (Boiga irregularis; N = 16) on the island of Guam (Fig. 2). Predations by other cat snakes (B. cyanea, B. cynodon, B. dendrophilia, B. kraepelini, B. ochracea, and B. siamensis) were observed on several islands and coastal regions of South-East Asia. Asian rat snakes (Elaphe spp.) were important predators of bird eggs in habitats across China and offshore Japan. In India and surrounding areas, the bird egg specialist Indian egg-eater (Elachistodon westermanni) purportedly consumed the eggs of several species of birds similarly to African Dasypeltis. However, few feeding records for these snakes have been published. Lastly, while few observations were reported from Australia, at least two species of pythons (Liasus fuscus and Morelia spilota) and three species of elapid snakes (Denisonia devisi, Notechis scutatus, and Pseudechis australis) consumed bird eggs in this region.

Body lengths and habitat use of oophagous snakes

Oophagous snakes averaged 2057 mm in maximum length, ranging by an order of magnitude in size from 600 mm (Denisonia devisi) to 6000 mm (Python bivittatus). However, most of these species ranged between 1500 mm to 2000 mm in maximum length. The distribution of maximum body lengths of oophagous snakes differed significantly from snakes in general (D = 0.671, P < 0.001; Fig 4.A). Oophagous snakes were significantly larger in maximum length on average compared to other snakes (Phylogenetic ANOVA: F_{1, 3501} = 307.322, P < 0.001). Body size thus appears to be an important component of bird egg consumption by snakes.

Most snake species in our list were terrestrial (60% of all 123 taxa) rather than semi-arboreal (21% of all 123 taxa) or arboreal (17% of all 123 taxa). Only two species (Laticauda colubrina and Thamnophis hammondii) were aquatic (~2% of all 123 species), and none of the species in our list was fossorial. We found no differences in the body sizes of snakes of differing habitats (Phylogenetic ANOVA: F_{3, 105} = 2.117, P = 0.339). Thus, differences in body size of oophagous and non-oophagous snakes are unlikely driven by differences in habitat use. Additionally, we found a low phylogenetic signal for oophagy in snakes (Blomberg’s K value of 0.065; P = 0.504), indicating that this trait evolves independently of phylogenetic relatedness.

Sizes of consumed bird eggs

Consumed bird eggs snakes ranged between 10 mm (Zebra Finch, Taeniopygia guttata) and 58 mm (Domestic Goose, Anser domesticus) in average diameter. Approximately 64% of the eggs consumed by snakes were on average narrower than the mean of this range (24.38 mm, back-transformed from log widths). Overall, the relative distribution of egg diameters did not differ between consumed eggs and all other eggs (D = 0.061, P = 0.602, Fig. 4B). The same pattern was found when comparing 100 samples randomly drawn from each distribution (D = 0.091, P = 0.813). Moreover, average egg diameters of both groups were statistically similar in size (Phylogenetic ANOVA: F_{1, 2342} = 0.570, P < 0.723; Fig. 4B).

Relationship between snake-size and bird egg-size

With the exception of predation events involving the uniquely adapted, bird egg specialist Dasypeltis, snake species in the lowest size classes (i.e., < 2 m in length) mostly consumed narrow bird eggs (< 20 mm; Fig. 5). Larger-bodied species mostly consumed narrow and moderately-sized eggs but also consumed bulkier eggs inaccessible to most other smaller-bodied species.

DISCUSSION

Our search for reports of snakes consuming bird eggs produced 471 feeding records from 238 individual data sources. From those reports, we produced a global list of oophagous snakes spanning 123 species, 58 genera, and seven families. Our list greatly exceeds prior attempts at cataloguing predatory interactions between snakes and bird eggs but is similarly geographically biased to a few well-sampled regions. For instance, we compiled nearly five times more records of snakes consuming bird eggs than Grundler (2020), 98 records across 50 snake taxa, but 60% of our records were from North America and southern Africa together. Collectively, the snakes on our list consumed the eggs of at least 210 species of birds across a variety of different families and orders. Our examination of traits of identified snake species and bird egg prey revealed that most oophagous snakes are large-bodied terrestrial species and that narrow bird eggs are most frequently, but not disproportionally, consumed. We identified several trends in the data that we hope will form the basis for testable hypotheses and serve as indicators of sampling bias that needs to be addressed.

What we know about bird egg feeding by snakes

There are currently 3921 recognized species of snakes (Uetz et al. 2021) distributed across the globe, all of which are predators (Greene 1997, Cundall and Greene 2000). Excluding the 471 species of invertebrate specialist Scolecophidian snakes (i.e., the blind snakes and thread snakes), the vast majority of the remaining 3450 species feed on vertebrate prey. Our list of 123 snake taxa represents a meagre 4% of those species. Bird eggs thus appear to be an uncommon source of prey for snakes overall. However, our list is undeniably an under-representation of the full range of snakes that consume bird eggs. Many congeners and close relatives of several taxa in our list almost certainly also consume bird eggs but have yet to be directly reported as doing so. For example, despite all 16 members of the genus Dasypeltis being known as obligate bird egg specialists (Bates and Little 2013, Bates and Broadley 2018), we only found feeding records for four of these species.

Unsurprisingly, most of the species on our list were represented by only a handful of feeding records. Only ten species had ten or more records, and nearly half of the species were represented by only a single observation. This paucity of feeding records, of which a large proportion represent apparently novel observations, highlights our limited understanding of bird egg predation by snakes. Moreover, additional factors like method-specific biases in feeding data collection also limit the extent of this knowledge. Several studies have highlighted the propensity at which different sampling techniques can affect the quality and quantity of collected dietary information for snakes (Rodrigues-Robles 1998, Glaudas et al. 2017, Maritz and Maritz 2020). As a result, even the diets of species that are relatively “well-known” may be incomplete because the methods used to collect feeding data for those taxa may have been unfavourable towards detecting prey like bird eggs. From this perspective, it is clear that continued reporting of novel feeding records, increased publications of descriptive studies of snake diets, and especially investigations of nest predation will lead to additional identifications of species suitable for inclusion in our list.

Most of the observed predations between snakes and bird eggs took place in the USA. However, at similarly high latitudes east of the Atlantic Ocean, exceedingly few records were reported. Moreover, there were no records at latitudes exceeding 60° N. The paucity of records at high latitudes regions can likely be explained by the limited numbers of snake species that occur in those regions. Snake species richness at high latitudes is relatively low compared to regions closer to the equator and in the southern hemisphere. For example, while there are around 200 species of snakes distributed across the USA there are fewer than 30 species in Canada (Ernst and Ernst 2003). Similarly, in most of northern Europe, there are fewer than 10 species of snakes, and in Russia, there are fewer than 45 species (Uetz et al. 2021). The lack of records from these areas is therefore not surprising given that egg consumption is uncommon in snakes and even in areas with high species richness, there are few records.

External factors unrelated to snake occurrences may also have inhibited records from being published. Several regions with high snake species richness are represented by only a few records of egg consumption (for example, West Africa, North Africa, India, and China). In some of those areas, the financial constraints on publishing may make it difficult to report on observations (see Mekonnen et al. 2021) since it may simply be too expensive to publish, especially for standalone observations like dietary feeding records. Additionally, sampling biases caused by a lack of interest in avian or reptile ecology may also have hindered observations of oophagy being reported.

Why don’t more snakes eat bird eggs?

While detailed dietary records are not available for many species (Grundler 2020), the feeding habits of most snakes are either at least generally known or can be inferred from life-history traits and the diets of their close relatives (Greene 1997, Cundall and Greene 2000). While not without exception, such inferred generalized assertions of snake feeding habits are often supported by detailed dietary studies (Bates and Little 2013, Maritz et al. 2019, Maritz et al. 2021a). Many species of snakes can be ruled out as consumers of bird eggs because they occur in areas where other prey types may be more abundant, easier to forage, or less difficult to consume. Alternatively, these snakes may lack the necessary morphology or physiology to consume eggs. Egg-specialist species like Dasypeltis possess unique adaptations that facilitate egg swallowing such as reduced teeth and vertebral modifications (Gans 1952) that most other snakes do not have. Factors like limitations in gape size, active selection of different prey, differences in encounter rates, and variable habitat use each contribute to the selectivity of different prey types, including bird eggs (de Queiroz and Rodríguez-Robles 2006).

Our results demonstrate that most snakes that consume bird eggs are large-bodied, exceeding 2000 mm in maximum length. Comparatively, the average maximum length of snakes overall is only ~800 mm (Feldman et al. 2016). Snake body size appears to play an important role in the inclusion of bird eggs in snake diets. Longer snakes tend to have larger gapes, and as a result, larger snakes are generally able to consume bulkier and heavier prey than smaller snakes (Arnold 1993, Cundall and Greene 2000). The ovoid shapes and wide cross-sectional diameters of eggs relative to snake head dimensions make them difficult for snakes with narrow gapes to handle and ingest (de Queiroz and Rodríguez-Robles 2006). Some small-bodied species like those in the genera Dasypeltis and Elachistodon overcome these mechanical constraints using specialized morphological features (Bates and Little 2013, Dandge and Tiple 2016) but most other small-bodied snakes are morphologically ill-equipped to ingest this type of prey (Gartner and Greene 2008).

The relationship between snake body size and bird egg prey sizes further illustrates the importance of relative prey bulk in facilitating these interactions. Most snakes, including several large-bodied boas and pythons, consumed relatively narrow eggs compared to their own lengths. This pattern reflects the findings of Gartner and Greene (2008) who quantified the egg-eating performance of Lampropeltis getula and found that adult specimens could only ingest modestly sized eggs relative to the dimensions of their feeding apparatus whereas juveniles could not ingest eggs at all. Those results highlight the body-size mediated mechanical difficulty of bird egg consumption for snakes and support the general predator-size, prey-size pattern found here. However, this pattern is not without exception given that several snakes consume bulky chicken, duck, and goose eggs.

Apart from body size and gape size limitations, specific predispositions towards hunting particular prey also preclude several species of snakes from predating bird eggs. In snakes, the habit of eating the eggs of an animal tends to arise from first eating the corresponding laying animal (de Queiroz and Rodríguez-Robles 2006, Maritz et al. 2021c). This is thought to be because the eggs of an animal share chemical cues with the parent animal, and so 1) this allows snakes to recognize the eggs as suitable food, and 2) leads to greater encounter rates of those organisms (de Queiroz and Rodríguez-Robles 2006). As a result, because relatively few species of snakes consume birds (Greene 1997, Cundall and Greene 2000), few species consume the eggs of birds because they do not associate them as appropriate prey.

Snakes may also actively exclude bird eggs from their diet in favour of other prey. Relative to their size, bird eggs are filled with calories, lipids, proteins, and water (Sotherland and Rahn 1987) but because of their size and associated high handling costs offer lower energetic payoffs to most other vertebrate prey (Greene et al. 2013). Snakes that prey on bird eggs can compensate for this by eating multiple eggs in a single meal, a trend that our data suggests occurs often. However, most species of birds lay small clutches with few eggs (Baicich and Harrison 2005, Tarboton 2011). Moreover, bird eggs are sedentary and nests are often difficult to locate (Nalwanga et al. 2004). For many species of snakes, the energetic costs of searching for nests with eggs likely outweigh the costs of foraging other, more easily detectable and energetically profitable prey. As a result, it is likely optimal for most snakes to exclude bird eggs in favour of other prey. In particular, large snakes should theoretically prefer singular, heavy prey items that provide a surplus of energy whereas smaller-bodied snakes probably prefer less bulky prey that are easier to consume (Shine 1991a).

Differences in foraging mode (i.e., active foraging versus ambush foraging) and lifestyle habits between snakes also greatly affect the chances of species encountering sedentary prey like bird eggs (Greene 1997, Alencar et al. 2013). Sit-and-wait foraging snakes probably only rarely encounter nesting birds and even less so bird eggs. Similarly, aquatic and fossorial species will encounter bird eggs considerably less often than arboreal and terrestrial species. Surprisingly, the majority of the species in our list were terrestrial rather than arboreal or semi-arboreal. However, we suspect that this is likely an artefact of sampling bias rather than a reflection of true biological patterns as terrestrial snakes are easier to detect than arboreal species (Pizzatto et al. 2007). Additionally, most occurrences of egg predation took place in habitats at low elevations (< 500 m above sea level) which could also be indicative of biased sampling efforts since high altitudes are generally difficult to access.

Importance of identifying snake predators of bird eggs

Identifying the snake predators of bird eggs is a key first step toward understanding the extent of their roles in nest predation and the potential implications thereof (Weatherhead and Blouin-Demers 2004, Lahti 2009; Menezes and Marini 2017). By knowing which snakes occur in a given area and which of those species eat bird eggs, researchers can consider species-specific hypotheses informed by existing knowledge of the demographics, ecologies, and natural histories of those particular species (for example Barends and Maritz 2021). Ultimately, this will lead to investigations that further our understanding of the relative importance of different snakes towards avian breeding success and more broadly, their impacts on ecosystem functioning (Reidy and Thompson 2012, DeGregorio et al. 2016a). Importantly, these investigations can also inform conservation strategies that seek to manage or protect endangered or vulnerable species of birds (Carter et al. 2007, Thompson and Ribic 2012).

Our primary objective of this review was to compile a comprehensive list of snake species unambiguously categorized as predators of bird eggs. We hope that this list can act as a baseline for further research seeking to understand patterns of nest predation by snakes and their impacts on avian ecology. By searching through the literature, citizen science reports, and social media, we provide a summary of accounts of bird egg predation by snakes that can act as a foundation for a consolidated database for further research.

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