Breeding origins of a uniquely regular migrant songbird in the Galápagos Islands

Abstract

Little is known about the causes and consequences of alternative pathways flown by long-distance migratory birds. Bobolinks (Dolichonyx oryzivorus) breed in grasslands across northern North America and migrate from their breeding grounds toward the eastern Atlantic Coast and then proceed through the Caribbean to South America. However, a small but regular number of Bobolinks have been recorded on the Galapagos Islands. We collected genetic samples from nine Galapagos Bobolinks and performed double-digest restriction site-associated sequencing. We compared them with samples from seven locations across their breeding distribution to determine their population of origin. Galapagos Bobolinks shared the genetic structure of a cluster in the eastern portion of the breeding range that includes New Brunswick and Ontario, Canada, and Vermont, United States. Genetic assignment tests largely corroborated this finding, although slightly different results were obtained for the two methods. All individuals were assigned to the Ontario breeding population using AssignPop, while Rubias assigned six of the migrants to Ontario and three to a Midwest breeding population. Low average relatedness among Galapagos individuals indicates that they are not more related to one another than to individuals within a breeding population and are therefore likely not from a single, small isolated population. Our results do not support the probability hypothesis-that Galapagos Bobolinks originated from the region that includes the greatest proportion of their breeding range (Great Plains)-or the vagrant hypothesis-that migrants are displaced onto Galapagos due to weather events. Instead, our findings support the proximity hypothesis, where migrants originate from the geographically closest-breeding populations.

Keywords: Bobolink; Charles Darwin; ddRAD; songbird.

FIGURE 1

Our seven sampling locations, including New Brunswick (NB), Vermont (VT), Ontario (ON), North Dakota (ND), Nebraska (NE), British Columbia (BC), and Oregon (OR). The Bobolink breeding distribution is represented in tan.

FIGURE 2

Bar plot from STRUCTURE analysis of Bobolink breeding sampling locations and migrants caught on San Cristobal Island, Galápagos, at K = 4 population structure identified in previous studies (3226 SNPs). Each vertical bar shows individual membership to each genetic cluster (shown here in east–west geographical manner).

FIGURE 3

Discriminant analysis of principal components (DAPC) scatter plot with the seven breeding sampling locations of Bobolinks and the migrants caught on San Cristobal Island, Galápagos. Each dot represents an individual with each color denoting where the samples were collected in 95% inertia ellipses. Insets show DA and PCA eigenvalues. A total of 78 PCSs and 7 discriminant functions were retained, and 61.3% of variation was explained.

FIGURE 4

Results of the Monte–Carlo cross‐validation performed in AssignPop to evaluate the baseline Bobolink data (reference dataset of 174 individuals from seven breeding locations). Assignment accuracy was estimated through resampling random training individuals using all loci (3234 SNPs). Each inset shows assignment accuracy in the baseline data across each sampling location and overall at varying proportions of individuals used in the training dataset (0.5, 0.7, and 0.90).