THE DENSITY OF LAND BIRDS ON THE TRES MARÍAS ISLANDS IN MEXICO: I. NUMBERS AND BIOMASS

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Abstract
Repeated surveys of the land birds in a 10-ac census area were carried out on María Magdalena and in a comparable mainland area near Tepic. The density of birds is about 35% greater on the island than on the mainland because of the extreme abundance of the two most common species on the island. Island endemics are slightly more abundant than the non-endemics. These two results are partly explicable in terms of the greater ecological opportunity on islands than in mainland regions due to the paucity of species in the former.Standing crop biomass of birds per unit area is greater on María Magdalena than on the mainland by about 35%, although the average SCB per individual bird is the same in the two regions. It is suggested that the greater biomass may be due to greater productivity on the island than in an equivalent area of mainland.