First off, happy World Sparrow Day! Did you know such a day exists? Well, surprise, now you do! This international day of sparrow appreciation was originally designed to raise awareness of the population declines of the house sparrow (it’s true, many places on Earth are seeing significant declines). While the house sparrow is an introduced species in our part of the world, we can still appreciate and work to conserve our many native sparrow species, including but not limited to the delightful Song Sparrow. Read on!
In the theater of evolution, Darwin’s finches and Hawaii’s honeycreepers arise as the preeminent examples of adaptive radiation. These island lineages, shaped through natural selection, have undergone intense speciation.
On the Galápagos Islands, James Smith worked with Peter and Rosemary Grant to band and track the life history of the finches on the islands. The insights from work in the Galápagos have made incredible contributions to evolutionary biology. Years later, Smith would conduct a similar study as he and armies of others have banded song sparrows on Mandarte Island in British Colombia.
Mandarte Island, BC, Canada. Photo courtesy of mandarte-island.weebly.com
All of this interesting research is built on the work of Margaret Morse Nice, who banded song sparrows in her backyard in Ohio starting in the 1930’s, changing the study of birds forever. Although she was not a classically trained biologist, Nice became a prolific researcher and published over 250 papers on birds in her lifetime. Banding the sparrows allowed her to study individual life histories, and learn about the intricacies of sparrow ecology.
Today, we will look at the song sparrow and its fascinating life history, the whims of selective pressures, and the dynamic processes of an island population.
Song sparrow. Photo courtesy of mandarte-island.weebly.com
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.
Banded song sparrow. Photo courtesy of mandarte-island.weebly.com
Numbers of females on the islands fluctuated wildly from 4 during the population crash of 1989, to over 70 1986. During these population crashes, it might have been expected that high inbreeding and low population could lead to the extinction of the song sparrows on Mandarte, but this was not the case. There was one effect sprinkled in that helped to save the song sparrow: immigration. On average, 2.8 song sparrows immigrate to Mandarte each year. While a small number, during population crashes, these immigrants bolstered the gene pool of Mandarte song sparrows and contributed to a population rebound. However, sparrows since the early 1990’s have seen less vigorous rebounds after population crashes, which may be a result of decreased immigration—the island only averaged .7 immigrant sparrows per year from 1993-2002.
Another interesting variable began sneaking its way onto Mandarte Island in the 1960’s: brown-headed cowbirds. This brood parasite lays its eggs in another bird’s nest, where the resident bird will often raise the outsized cowbird young. Cowbirds are migratory, and typically arrive on Mandarte after song sparrows start breeding. The proportion of nests parsitized is low when cowbirds first arrive but steadily increases into June and July, when over half of all nests are parasitized. With a high population of sparrows, the cowbirds found plenty of nests to parasitize, thus cowbird eggs correlated with the number of breeding females. In a similar vein, the proportion of failed nests also increased with a higher female population, and reached almost 50% of nests failing when there were over 70 females on the island. Overall cowbirds depressed population size and increased the likelihood of extinction on the island.
Perhaps the most obvious thing to study in the song sparrow resides in its name, the song. Song sparrows exhibit variation in song length, and while females do sing in early spring during the establishment of territories, males dominate singing during the breeding months. As a male’s song repertoire increases, his probability of mating increases and his number of offspring increases. One of the more pronounced effects of inbreeding in song sparrows is the effect of inbreeding on song repertoire; males with higher coefficients of inbreeding showed simpler songs.
Jonathan Weiner, in his book The Beak of the Finch, terrifically summarizes the work on Mandarte:
Summed over years, the effects of natural selection were invisible. But at each stage of their lives and each year of their lives the sparrows on that little island [Mandarte] had been “daily and hourly scrutinized” by the hand of nautral selection, much as Darwin had imagined, only in fast motion. The population on Mandarte is still being pushed every year…What seemed most striking to Dolph [a Mandarte researcher] as he studied the selection of events hidden in Smith’s data was the boring uniformity of Smith’s birds. Compared with Darwin’s finches, the sparrows of Mandarte might have been turned out by a cookie cutter. They showed only the very slightest of variations from one bird to the next in length of beak and length of tarsus... Yet even these varitaions, trivial as they seemed, had helped to decide who lived and who died. “That is pretty amazing to me!” Dolph says. It means populations don’t need to be excessively variable in order to experience natural selection.
“Selection doesn’t happen just in the Galápagos,” Dolph concludes. “It happens in your backyard. “
Song sparrow living up to its name at Faville Grove Sanctuary. Photo by Drew Harry
From Margaret Nice’s backyard, to Mandarte Island, and even back here at Faville Grove, the song sparrow shows the tremendous variation and selective pressures present in nature, and what a remarkable view we have of these processes in one of our most common birds.
Written by Drew Harry, Faville Grove Sanctuary land steward
