Introduction
Here, we use mitochondrial DNA to track the diversity and population structure of the European starling (Sturnus vulgaris ) in three geographically independent invasive ranges throughout the world. European starlings are native to Eurasia, yet over the last 163 years have been strikingly successful at living outside of its native range in North America, South Africa, and Australia (Jenkins, 1977). They are part of the family Sturnidae and order Passeriformes(Feare, 1984). Although this group is historically found in the Palearctic, their success as an invasive species and their multiple introductions have led to their existence on every continent, except for Antarctica (BirdLife International, 2016; Sulliven, 2009). Their invasion success likely results from a suite of life-history and behavioral traits that facilitate their ecological flexibility. For example, they are often classified as diet generalists, preferring insects, but they will eat most other foods depending on availability of resources (Cabe, 1993). There is some indication that they are not as successful in urban areas and not complete diet generalists but have been able to adapt to surroundings in order to exploit the environment (Mennechez and Clergeau, 2006). They are cavity nesters utilizing natural features such as crevasses and trees but can also nest in man-made structures allowing them to breed in rural and urban environments (Cabe, 1993). Starlings benefit from associations with other species; they are commensal with humans and are often found around livestock that disturb the soil with their hooves, exposing invertebrates to starlings (East and Pottinger, 1975). They normally lay one or two broods a year, which include between three and six eggs (Cabe, 1993). Another unique feature that likely plays a role in the ability of the European starlings to expand into new localities is their ability to migrate (Cabe, 1993). Although not all starlings are migratory (e.g. in Australia and New Zealand, Higgins et al. 2006), it has been shown that there is a great deal of variation and that individuals can be differentially migratory from year to year (Cabe, 1993; Feare, 1984).
European starlings were introduced to North America in 1890 as part of an American Acclimatization Society initiative to populate Central Park with the birds from Shakespeare’s plays (Cabe, 1993). There were many species of birds mentioned in Shakespeare, but not all were able to survive and thrive in the new North American climate. The initial introduction consisted of approximately 60 individuals released in 1890 and 40 more in 1891, leading to a total of ~100 individuals released into Central Park in New York City (Cabe, 1993). From this founding population, starlings have expanded their range across all of North America where their current population exceeds 200 million individuals, over one-third of the global population of this species (Feare, 1984). This range expansion that has taken place in the last 128 years, demonstrating that they are able to rapidly adapt to new surroundings and are incredibly versatile.
Other starling introductions from the 19th century have been previously studied using genetics, including the mid-19th century Australian introduction (Rollins et al, 2009; Rollins et al, 2011; Rollins et al, 2016) and the late 19th century South African introduction (Berthouly-Salazar et al, 2013). During introductions, the often small number of founding individuals may result in a genetic bottleneck and, therefore, these populations are likely to have lower genetic diversity than those in the native range. This was demonstrated using data from the UK and Australia (Rollins et al, 2011). However, because multiple introductions were made to Australia (Jenkins 1959), and these occurred prior to and had a greater number of propagules than the New York introduction, we predict that the genetic diversity of the North American population will be lower than that of Australia. It is also of note that populations of North American and Australian birds increased exponentially following introduction (Bitton and Graham, 2014; Long, 1981), which may mitigate loss of genetic diversity from the founder population. In Australia, population expansion has been limited by large expanses of arid environment, which may have affected population growth and expansion on this continent.