Birds rarely hybridize: A citizen science approach to estimating rates of hybridization in the wild*

Abstract

The rate of hybridization among taxa is a central consideration in any discussion of speciation, but rates of hybridization are difficult to estimate in most wild populations of animals. We used a successful citizen science dataset, eBird, to estimate the rates of hybridization for wild birds in the United States. We calculated the frequency at which hybrid individuals belonging to different species, families, and orders of birds were observed. Between 1 January 2010 and 31 December 2018, a total of 334,770,194 species records were reported to eBird within the United States. Of this total, 212,875 or 0.064% were reported as hybrids. This estimate is higher than the rate of hybridization (0.00167%) reported by Mayr based on impressions from a career studying museum specimens. However, if the 10 most influential hybrid species are removed from the eBird dataset, the rate of hybridization decreases substantially to about 0.009%. We conclude that the rate of hybridization for individuals in most bird species is extremely low, even though the potential for birds to produce fertile offspring through hybrid crosses is high. These findings indicate that there is strong prezygotic selection working in most avian species.

An evidence-based overview of hybridization in tinamous

“…Hybridization refers to the situation where species "that are distinguishable on the basis of one or more heritable characters overlap spatially and temporally and cross to form viable and at least partially fertile offspring" (Arnold 1997). Although this phenomenon is rare on an individual level, hybrids between numerous bird species have been documented (Justyn et al 2020;Ottenburghs and Slager 2020). Indeed, according to the latest estimates, about 16% of bird species has hybridized with another species (Ottenburghs et al 2015).…”

How common is avian hybridization on an individual level?*

“…In this issue, Justyn et al. (2020) provide the first data‐based estimate of individual hybridization rate in birds using data from the citizen science project eBird.…”

Do phylogeny and habitat influence admixture among four North American chickadee (family: Paridae) species?

“…Grava et al (2012) noted all of the admixed chickadees identified in their study were phenotypically mountain chickadees. Justyn et al (2020) used phenotypic analysis only and these results combined with our own suggest that rates of hybridization may be higher than currently estimated as hybrids are not always phenotypically distinguishable or easy to detect based on phenotype alone (Ottenburghs and Slager 2020).…”

“…We documented admixture among five of the six chickadee species pairs examined; we did not detect any admixture between mountain and chestnut‐backed chickadees. The prevalence of admixture among chickadee species is not surprising given Justyn et al (2020) found that among passerines, hybridization is most frequent in family Paridae. mountain × black‐capped chickadee and boreal × black‐capped chickadee hybrids were previously documented by Grava et al (2012) and Lait et al (2012), but our study provides genetic evidence to suggest admixture occurs between black‐capped × chestnut‐backed, boreal × chestnut‐backed and boreal × mountain chickadees and admixture rates vary based on geographic location.…”

“…Among North American parids, hybridization is well established for Carolina Poecile carolinensis and black‐capped chickadees P. atricapillus (Reudink et al 2006, 2007, Taylor et al 2014, Wagner et al 2020), tufted Baeolophus bicolor and black‐crested titmouse B. atricristatus (Curry and Patten 2014), and less frequently black‐capped and boreal chickadees P. hudsonicus (Lait et al 2012), and mountain P. gambeli and black‐capped chickadees (Grava et al 2012). Recent analysis of phenotypic data suggests that hybridization occurs in 0.19% of all wild parids and that among passerines, Paridae hybridize most frequently (Justyn et al 2020). Given how widely distributed parids are in North America, and the frequency of hybridization within this family, species within family Paridae are ideal for examining patterns of admixture in the context of ecological and habitat variation (Curry 2005).…”