Variation and hybridization in the Green Heron (<em>Butorides virescens</em>) and Striated Heron (<em>B. striata</em>) in Trinidad and Tobago, with comments on species limits

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Floyd E. Hayes

Abstract: The rufous-necked Green Heron (Butorides virescens) of North America and the Caribbean hybridizes with the gray-necked Striated Heron (B. striata) of South America in Panama and southern Caribbean islands. I analyzed historic and current variability of the two taxa in Trinidad and Tobago by comparing museum specimens and live individuals in the field with a color photograph of nine voucher specimens used as a hybrid index. In Trinidad, the population is dominated by B. striata, whereas B. virescens and intermediate individuals are rare. In Tobago, B. virescens is the predominant form, with small numbers of B. striata and intermediate individuals. The increased variability and intermediacy of individuals in Tobago strongly implies hybridization, and the higher proportion of intermediate individuals among museum specimens suggests a shift within the past century toward relatively “pure” phenotypes. Neck color was unrelated to clinal variation, seasonality, or habitat. The distribution of phenotypes differs markedly between populations of Butorides in Trinidad and Tobago, which are separated by only 36 km, suggesting that competitive exclusion may preclude B. virescens from colonizing Trinidad and B. striata from colonizing Tobago. Because the two taxa may often have opportunities to interbreed on Tobago but tend to mate assortatively, they appear to have achieved essential reproductive isolation, thus supporting their current treatment as distinct species.


Keywords: Butorides striata, Butorides virescens, Green Heron, hybridization, species limits, Striated Heron, Tobago, Trinidad, variation


Resumen: Variación e hibridización de la Garcita Verde (Butorides virescens) y la Garcita Estriada (B. striata) en Trinidad y Tobago, con comentarios acerca de los límites de la especie- La Garcita Verde (Butorides virescens) con cuello rufo de Norteamérica y el Caribe se hibridiza con la Garcita Estriada (B. striata) con cuello griz de Suramérica, en Panamá y las islas caribeñas del sur. Se analizó la variabilidad histórica y actual de ambos taxas a través de la comparación de especímenes de museo y animales vivos con una fotografía a color de nueve de nueve ejemplares utilizados como patrones del híbrido. En Trinidad la población está dominada por B. striata, mientras que B. virescens e individuos intermedios son raros. En Tobago, B. virescens es la forma predominante, con pequeños números de B. striata e individuos intermedios. La variabilidad incrementada incrementada y los caracteres intermedios en los individuos de Tobago sugieren fuertemente hibridización y la mayor proporción de individuos intermedios entre los especímenes de museo sugiere un cambio en el siglo pasado hacia los fenotipos “puros”. El color del cuello no estuvo relacionado a variaciones clinales, estacionales o de hábitat. La distribución de los fenotipos difiere marcadamente entre las poblaciones de Butorides en Trinidad y Tobago, que están separadas tan solo por 36 km, lo que sugiere que la exclusión competitiva evita la colonización de Trinidad por B. virescens y la de Tobago por B. striata. Como ambos taxas tienen frecuentes oportunidades de entrecruzarse en Tobago, pero tienen un apareamiento asociativo, parece ser que han alcanzado un aislamiento reproductivo esencial que apoya su tratamiento actual como especies diferentes.


Palabras clave: Butorides striata, Butorides virescens, Garcita Estriada, Garcita Verde, hybridización, límites de especies, Tobago, Trinidad, variación


Résumé: Variabilite et hybridation entre le Heron vert (Butorides virescens) et le Heron strie (B. striata) a Trinite et Tobago: commentaires sur les frontieres d’especes- Le Héron vert (Butorides virescens) à cou roux d’Amérique du Nord et de la Caraïbe, se croise avec le Héron strié, à cou gris d’Amérique du Sud, du Panama et des îles du sud des Antilles (B. striata). J’ai étudié la variabilité historique et actuelle et deux taxons à Trinité et Tobago en comparant des spécimens de Muséum et des individus vivants en liberté avec des clichés en couleur de neuf spécimens utilisés comme index hybride. A Trinité, la population est dominée par B. striata, alors que B. virescens et les individus intermédiaires sont rares. A Tobago, B. virescens est la forme prédominante, avec un nombre faible de B. striata et d’individus intermédiaires. La plus variabilité et situation intermédiaire croissante à Tobago implique fortement la présence d’hybridation et la plus grande proportion d’individus intermédiaires dans les spécimens de Muséums suggère une dérive pendant le siècle passé vers des phénotypes relativement « pures ». La couleur du cou n’est pas liée à une variation clinale, ou de saisonnalité ou d’habitat. La distribution des phénotypes diffère nettement entre les populations à Butorides de Trinidad et de Tobago, séparées de seulement 36 km, suggérant qu’une exclusion compétitive empêche B. virescens de coloniser Trinidad et B. striata de coloniser Tobago.


Mots clés: Butorides striata, Butorides virescens, Héron strié, Héron vert, hybridation, limites d’espèces, Tobago, Trinité, variabilité

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