Disturbance sources, human predation and reproductive success of seabirds in tropical coastal ecosystems of Sinaloa State, Mexico

From March to July 2000, four seabird colonies in coastal ecosystems of Sinaloa, north-west Mexico were visited: on islets, a sandy beach and a long-abandoned salt-panning flat. There were partial and total breeding failures, most of them due to human activity. Amongst colonies of nine species, the breeding success of three was severely affected. All 250 Royal Tern Sterna maxima eggs were taken for direct consumption; 50% (75) of Blue-footed Booby Sula nebouxii chicks were killed for crab (Portunidae) fishing; and in a Least Tern Sterna antillarum colony (97 pairs), 50% failed due to disturbance from a fishing championship. Some of these species have priority status within Mexican and international conservation regulations. Certain aspects of these problems are discussed, and actions are suggested to balance conservation and the development of economic activities.

Habitat Characteristics and Nest Success of Snowy Plovers Associated with California Least Tern Colonies

Nest success of Snowy Plovers (Charadrius alexandrinus) was estimated at six sites in San Diego County, California, to determine the effects of habitat characteristics and social factors on predation risk. Egg predation was expected to be lower for plovers nesting among Least Terns (Sterna antillarum) because of benefits associated with nesting under the “protective umbrella” of a colonial species. Snowy Plovers nested adjacent to objects and in areas with more vegetation cover than random sites in beach and fill habitats and in areas with more debris cover in beach and salt pan habitats. Estimated nest success from 1994–1997 was 50%, and most predation was attributed to corvids, primarily Common Ravens (Corvus corax). Social factors, including distance to nearest Least Tern and Snowy Plover nests, predicted nest success whereas habitat characteristics did not. Nest success was greatest for plovers nesting closest to an active tern nest and nesting at intermediate distances to conspecifics. Nest success also varied among years and sites, with two sites consistently less successful over the four years. Nests within tern colonies received some mitigation of egg predation. Plover nests initiated prior to the arrival of Least Terns were less likely to hatch eggs than later nests; nevertheless, Snowy Plovers in southern California initiated nesting well before Least Terns.Características del Hábitat y Éxito de la Nidada de Charadrius alexandrinus Asociados a Colonias de Sterna antillarumResumen. Para determinar los efectos de las características del hábitat y factores sociales sobre el riesgo de depredación, estimé el éxito de las nidadas de Charadrius alexandrinus en seis sitios del Condado de San Diego, California. Se esperaba que la depredación de huevos fuera menor para los C. alexandrinus que anidan junto a Sterna antillarum debido a los beneficios de anidar bajo un “paraguas protector” de una especie colonial. En playas y hábitats rellenados, C. alexandrinus anidó con mayor frecuencia en lugares adyacentes a objetos y en áreas con mayor cobertura de vegetación que en sitios aleatorios y en áreas con mayor cobertura de desechos vegetales en playas y salinas. El éxito estimado de las nidadas entre 1994–1997 fue de 50%, y la mayor tasa de depredación fue atribuida a córvidos, principalmente a la especie Corvus corax. Factores sociales como distancia al nido más cercano de S. antillarum y C. alexandrinus predijeron el éxito de la nidada, mientras que las características del hábitat no lo hicieron. El éxito de la nidada fue mayor para los individuos de C. alexandrinus que nidificaron más cerca de un nido activo de S. antillarum y que nidificaron a distancias intermedias de conespecíficos. El éxito de la nidada también varió entre años y sitios. Dos sitios presentaron consistentemente un menor éxito durante los cuatro años de estudio. Los nidos ubicados dentro de la colonia presentaron una leve disminución en la depredación de huevos. Los huevos de nidos de C. alexandrinus iniciados antes de la llegada de S. antillarum tuvieron una menor probabilidad de eclosión que huevos de nidos más tardíos. Sin embargo, C. alexandrinus comenzó a nidificar bastante antes que S. antillarum en California del Sur.