Temporal stability of the hybrid zone between Calocitta magpie-jays revealed through comparison of museum specimens and iNaturalist photos

Abstract

Hybrid zones are natural experiments for the study of avian evolution. Hybrid zones can be dynamic, moving as species adjust to new climates and habitats, with unknown implications for species and speciation. There are relatively few studies that have comparable modern and historic sampling to assess change in hybrid zone location and width over time, and those studies have generally found mixed results, with many hybrid zones showing change over time, but others showing stability. The white-throated magpie-jay (Calocitta formosa) and black-throated magpie-jay (Calocitta colliei) occur along the western coast of Mexico and Central America. The two species differ markedly in throat color and tail length, and prior observation suggests a narrow hybrid zone in southern Jalisco where individuals have mixed throat color. This study aims to assess the existence and temporal stability of this putative hybrid zone by comparing throat color between georeferenced historical museum specimens and modern photos from iNaturalist with precise locality information. Our results confirm the existence of a narrow hybrid zone in Jalisco, with modern throat scores gradually increasing from the parental ends of the cline toward the cline center in a sigmoidal curve characteristic of hybrid zones. Our temporal comparison suggests that the hybrid zone has not shifted its position between historical (pre-1973) and modern (post-2005) time periods-a surprising result given the grand scale of habitat change to the western Mexican lowlands during this time. An anomalous pocket of white-throated individuals in the northern range of the black-throated magpie-jay hints at the possibility of prehistorical long-distance introduction. Future genomic data will help disentangle the evolutionary history of these lineages and better characterize how secondary contact is affecting both the DNA and the phenotype of these species.

Keywords: birds; gene flow; global change; habitat change; introgression.

FIGURE 1

(a) Black‐throated magpie‐jay (C. colliei) from Culiacán, Sinaloa, Mexico, photo: Juan Ramón Manjarrez (iNaturalist 68514766 CC‐BY‐NC license); (b) white‐throated magpie‐jay (C. formosa) from Tehuantepec, Oaxaca, Mexico, photo: Liam Wolff (iNaturalist 177100291 CC‐BY‐NC license); (c) ranges of the two species, five geographical zones of comparison, and 10 sites for cline analysis with average throat scores in our historical (left) and modern (right) datasets; (d) transition in throat scores in the four populations nearest to the contact area; and (e) example specimens representing each of the categorical throat scores.

FIGURE 2

(a) Comparison of cline centers and geographic clines for historical (gray) and modern (black) data; (b) Comparison of cline widths. Bars represent the 2‐log likelihood error around each point estimate.