What happened to the island populations of the first wave?
In the Southern Ocean around Antarctica, each island seems to house its own species of Blue-eyed Shag (genus Leucocarbo). A recent genetic study in the Journal of Biogeography found evidence for at least two waves of colonization, starting from South America where we can find the Rock Shag (L. magellanicus) and the Guanay Cormorant (L. bougainvillii). The first wave occurred during the Late Pliocene or Early Pleistocene (around 2 million years ago) and reached all the way to New Zealand, including the Chatham, Bounty, Auckland and Campbell Islands. The second wave was dated to less than one million years ago and resulted in the colonization of several island groups, such as South Georgia, the South Orkney Islands, Marion, Crozet, Kerguelen, Heard, and Macquarie Islands. The main colonization history of the Blue-eyed Shags seems clear, but many details remain to be studied.
As explained above, the first colonization wave eventually arrived in New Zealand. But the shags probably did not directly fly from South America to New Zealand. They could stop at numerous islands on the way. But what happened to these pioneering island populations? The researchers speculate that:
It is possible that South Georgia, the South Orkney Islands, the Antarctic Peninsula, and Marion, Crozet, Kerguelen, Heard, and Macquarie Islands were all originally colonized by the same wave of dispersal that founded the taxa comprising the New Zealand clade, but that the resulting populations were repeatedly extirpated during glacial maxima later in the Pleistocene.
This scenario can be tested with ancient DNA from shag bones that have been found beneath the current breeding colonies on these islands. Exciting!
It is also possible that some island populations from the first wave came into contact with birds from the second wave. Hybridization between the resident birds and the visitors could have resulted in gene flow, leaving traces in the genomes of present-day populations. This phenomenon – known as ghost introgression – can be detected with the latest genomic techniques. The current study relied on a handful of mitochondrial and nuclear genes and could not directly test this hypothesis. A genomic perspective – perhaps combining recent and ancient DNA – can lead to new insights into the evolutionary history of the Blue-eyed Shags. Whether this history involves ghost populations remains to be determined.
Rawlence, N. J., Salis, A. T., Spencer, H. G., Waters, J. M., Scarsbrook, L., Mitchell, K. J., … & Kennedy, M. (2022). Rapid radiation of Southern Ocean shags in response to receding sea ice. Journal of Biogeography, 49(5), 942-953.
Featured image: Imperial Shag (Leucocarbo atriceps) © Calyponte | Wikimedia Commons