Polymorphism of Melanin-based Pigments in Barn Owls in California, USA
The barn owl group (Family Tytonidae, including Tyto alba and T. furcata) displays within-species variation in plumage, ranging from reddish to whitish and from heavily spotted to having no spots at all. One explanation for the persistence of polymorphism in raptors is the habitat matching choice hypothesis, which posits that different morphs exploit the different spatial or temporal environments to which they are best adapted. For example, in heterogeneous landscapes in Europe and the Middle East, reddish Western Barn Owls (Tyto alba) inhabit territories with proportionally more arable fields and consume proportionally more voles than mice, in contrast to their whiter counterparts (Journal of Evolutionary Biology 25:103–114; Biological Journal of the Linnean Society 106:447–454). We here sought to quantify polymorphism in female and male American Barn Owls (Tyto furcata) in Napa Valley, California, USA, and to test predictions about the relationships among landscape composition around nests, prey composition, and degree of reddishness in this population. We photographed adult owls and used MATLAB software to analyze plumage characteristics. We analyzed pellets to determine the proportion of mice (Peromyscus, Reithrodontomys, and Mus), voles (Microtus), and gophers (Thomomys) in the owls’ diet. Analyses showed mixed support for the habitat matching choice hypothesis. As predicted, redder females occupied nest boxes with less forested area within their hunting area, but unlike the barn owls from Europe and the Middle East, they provisioned proportionately fewer voles than whiter females. The results provide further evidence for the existence of polymorphism in the barn owl group in heterogeneous landscapes, in this case within and surrounding vineyards. POLIMORFISMO DE PIGMENTOS BASADOS EN MELANINA EN TYTO FURCATA EN CALIFORNIA, EE.UU. El grupo de las lechuzas comunes (Family Tytonidae, que incluye a Tyto alba y T. furcata) presenta variación intraespecífica en el plumaje, que va desde rojizo a blanquecino y desde muy moteado hasta completamente sin manchas. Una explicación para la persistencia del polimorfismo en aves rapaces es la hipótesis de elección por coincidencia de hábitat, que plantea que los distintos morfos explotan los diferentes entornos espaciales o temporales a los que están mejor adaptados. Por ejemplo, en paisajes heterogéneos de Europa y Medio Oriente, T. alba habita territorios con proporcionalmente más campos de cultivo y consume proporcionalmente más topillos que ratones, en contraste con sus contrapartes más blancas (Journal of Evolutionary Biology 25:103–114; Biological Journal of the Linnean Society 106:447–454). En este estudio buscamos cuantificar el polimorfismo en hembras y machos de T. furcata en el Valle de Napa, California, EUA, y evaluar predicciones sobre la relación entre la composición del paisaje alrededor de los nidos, la composición de las presas y el grado de coloración rojiza en esta población. Fotografiamos lechuzas adultas y utilizamos el software MATLAB para analizar las características del plumaje. Analizamos egagrópilas para determinar la proporción de roedores pertenecientes a los géneros Peromyscus, Reithrodontomys, Mus, Microtus y Thomomys en la dieta de las lechuzas. Los análisis mostraron un apoyo mixto a la hipótesis de elección por coincidencia de hábitat. Como se predijo, las hembras más rojizas ocuparon cajas nido con menor área boscosa en las cercanías, pero a diferencia de las lechuzas de Europa y Medio Oriente, aportaron proporcionalmente menos Microtus spp. que las hembras más blancas. Los resultados aportan evidencia adicional de la existencia de polimorfismo en el grupo de las lechuzas comunes en paisajes heterogéneos, en este caso dentro y alrededor de viñedos. [Traducción del equipo editorial]ABSTRACT
RESUMEN
Map of Napa Valley, California depicting seven land cover types (from Coro 2021 and Huysman and Johnson 2021), with inset locator maps of Napa County and California. Locations of 22 vineyards (jittered to preserve anonymity) with number of American Barn Owl nest boxes sampled per vineyard in this study (total n = 54 boxes).
Partial response plots for relationship between female degree of reddishness and proportion of each land cover variables at nest site and hunting radius levels (square root transformed) in Napa Valley, California, in 2021. Each plot represents effect of the independent variable as others are held constant. Asterisk indicates confidence intervals that did not overlap zero.
Relationship between female degree of reddishness and an estimated index of the proportion of prey species found in pellets and brought back by females (n = 29) in Napa Valley, CA in 2021. Top: Female prey contribution was estimated at 100% (top row)75% (middle row) and 50% (bottom row). Asterisks indicate significance (P < 0.05).
Raw photograph of male Barn Owl’s ventral aspect. The rectangle corresponds to the 60 × 40 mm area used to measure red, green, and blue (RGB) values; count the number of spots; and measure the maximum vertical dimension of each spot (referred to as “diameter”).
(a) Cropped 60 × 40 mm area of male Barn Owl’s ventral aspect. (b) Spots (white polygons) selected for counting and measuring.
Contributor Notes
Associate Editor: Christopher W. Briggs