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.