The song sparrow is one of the most polymorphic species of birds in North America, though genetic evidence suggests that geographic subspecies are more connected than initially thought. The song sparrows on Mandarte Island are year-round residents of the rocky island, which is about six hectares. The habitats on Mandarte include sea island scrub, grassland, and open cliffs. Most song sparrows breed among the scrub, and prime territories are established in these areas. Since the 1960’s, the song sparrows on Mandarte Island have been studied in depth almost annually, and researchers summering on the island have many publications on these birds and even a book, published in 2006, Conservation and Biology of Small Populations: The Song Sparrows of Mandarte Island, where much of this information comes from.
Breeding can begin as early as March, and researchers on Mandarte have found pronounced effects of breeding date on the resulting number of fledglings. The earlier a pair breeds, the more offspring that will survive to fledgling. Moreover, the age of the parent causes significant changes in the total number of offspring, with both males and females peaking in reproductive success at age 3, displaying a bell curve distribution.
Another ingredient of song sparrow life history on Mandarte is their density dependent reproduction. When the population is high, the juvenile survival is very low, and vice versa. Thus, during population crashes on the island due to weather or other unknown causes, the population quickly recovered with high fecundity while population highs saw the likely effects of competition acting on the birds.
Baked into those population crashes are the effects of inbreeding, and researchers found pronounced effects from inbreeding. Reproductive success declined with higher coefficients of inbreeding, and in a trial of song sparrows experimentally injected with a parasite, those with a lineage of inbreeding exhibited a lesser immune response than heterozygous song sparrows. Overall, the birds resulting from inbreeding were less fit than other birds on Mandarte.