Abstract
The Black Brant is a common inhabitant of the Western Artic American tundra, which migrates to Southern Pacific coasts during the winter season. Approximately, 31000 birds (31%) constitute the Mexican population of Brants at Guerrero Negro, Ojo de Liebre, and Exportadora de Sal lagoon complex; nevertheless, there is little information about the distribution patterns and zone usage. At Guerrero Negro Lagoon (GNL), Ojo de Liebre Lagoon (OLL, both natural sites), and at Exportadora de Sal (ESSA, artificial site) we determined by monthly censuses (from November 2006 to April 2007, 08:00-16:00h) and observed: (1) season and site effects on population structure (age groups), and (2) the tide level relationship with the abundance and proportion of feeding birds. Within a total of 150 observation hours and 98 birds, our results showed a general 0.68 proportion of adults, that was higher in winter than in spring. The statistics analysis showed no effects by site on the proportion of feeding birds, but we observed a temporal decrease at ESSA and at GNL. In contrast the proportion of feeding birds at OLL was constant. We observed an increase in the juveniles between winter and spring. This increase is related with the differential migration, which mentions that the juveniles are the last to leave the wintering area. In winter the relations of the tide level with the abundance of Brant were: direct at ESSA, inverse at OLL and no relation found at GNL. In spring, no relation was observed in the sites. The proportion of Brants feeding at OLL (the site with the higher abundance) was independent of the tide level. This is related with two possible behaviors of the geese: (1) they can move through the lagoon and take advantage of the tidal lag, which is up to four hours; and (2) they can modify their feeding strategies, more on floating eelgrass (Zostera marina).
Highlights
El Ganso de collar del Pacífico (Branta bernicla nigricans Linnaeus 1758) se reproduce en la tundra alta del Ártico occidental y migra hacia el sur para pasar el invierno por las costas del Pacífico; se ha reportado que alrededor del 74% de su población, inverna en el noroeste de México (Reed et al 1998)
De tal forma, se ha indicado que las estrategias y patrones migratorios de esta especie están modelados por gradientes latitudinales en la fenología del alimento y en las fluctuaciones mareales (Moore et al 2004, Ward et al 2005, Moore & Black 2006)
La alimentación del Ganso de collar en Laguna Ojo de Liebre (LOL) (el sitio de mayor abundancia) fue independiente del nivel de marea, lo que se puede relacionar con dos conductas: (1) que las aves se muevan a lo largo de la laguna, para aprovechar el desfase, de hasta cuatro horas en el nivel de marea o (2) que modifiquen sus estrategias de alimentación, para consumir el pasto flotante
Summary
El complejo lagunar Guerrero Negro-Ojo de Liebre-ESSA se ubica en la costa centrooccidental de la Península de Baja California, dentro de la Reserva de la Biósfera del Vizcaíno (28o13’9” - 27o34’24” N y 114o24’39” 113o53’28” W, Fig. 1). El área presenta menos de 100mm de precipitación anual y una temperatura anual promedio que oscila entre los Guerrero México Negro
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