Chasing the Gray-cheeked Thrush

A Gray-cheeked Thrush; Photo by Darroch Whitaker
A Gray-cheeked Thrush; Photo by Darroch Whitaker

The Newfoundland Gray-cheeked Thrush is a songbird found breeding across the island, part of the broader breeding range of the Gray-cheeked Thrush which extends across North America along the northern fringes of the boreal forest, and even into Siberia. These island birds have a story which stretches from Gros Morne National Park all the way to Sierra de Perijá National Park in Venezuela, where some spend the winter near the border with Columbia. But their story has not been so happy since the 1980’s, when thrush numbers began to decline on the island – unlike their mainland cousins who seem to be doing okay. Dr. Ian Warkentin, a Grenfell Campus environmental scientist, is working with collaborators, such as Gros Morne National Park’s Dr. Darroch Whitaker, to try and find out why their numbers have dropped.

“Darroch was key to getting this project going, because he was the one who recognized that the thrush population seemed to be declining,”

explains Dr. Warkentin,

“and then, because we have mutual interests and have worked together in the past, we began working [on this project] and have now been trying to figure things out for about ten years now.”

Forty years ago, the Gray-cheeked Thrush could be commonly found across the island of Newfoundland by those who sought it out, but it is now increasingly found only in high elevation areas and on certain small islands off the coast of Newfoundland. Indeed, according to recent estimates, the island population has declined by as much as 95% since 1975. Understanding why this is happening is key to the future preservation of the species.

Blood samples of the Gray-cheeked Thrush; Photo by Alyssa FitzGerald
Blood samples of the Gray-cheeked Thrush; Photo by Alyssa FitzGerald

Putting the ‘Newfoundland’ in ‘Newfoundland Gray-cheeked Thrush’

Primarily feeding on insects during the breeding season, the Gray-cheeked Thrush arrives in Newfoundland each spring, builds its nest low to the ground in a forest stand, and reproduces. During autumn it journeys some five thousand kilometers to reach its wintering grounds. Migration takes these thrushes down the US eastern seaboard and then either south from Florida directly to the northern coast of South America or island-hopping through the Caribbean to north-eastern South America. In its wintering habitat its diet expands to include berries.

Not much is known about the species and it is very reclusive, Dr. Warkentin adds:

“It’s been described by some as shy, not the sort of bird that’s sitting up there at the top of the tree singing loudly all day… it’s not very flashy.”

So, what makes the Gray-cheeked Thrush in Newfoundland unique?

The island thrushes had been recognized as being slightly different in plumage and size than other Gray-cheeked Thrush. However, it was not known whether this difference in appearance was also linked to being genetically distinct from other populations of the species.

It just so happened that Dr. Jeremy Kirchman and Dr. Alyssa FitzGerald of the New York State Museum were interested in the genetics of the Catharus thrushes, the genus to which the Gray-cheeked Thrush belongs. Together the group struck up a collaboration to determine if the Gray-cheeked Thrush in Newfoundland was unique, says Dr. Warkentin:

“We knew they were distinctive based on size and appearance, but no one had done the genetic work to prove that… they actually have different genes.”

Blood samples were collected from across the island and in Labrador for comparison with the DNA of thrushes found across their distribution in North America. Thanks to this work we now know that these differences in size and plumage are part of a package of genetic and other differences that make Gray-cheeked Thrush on the island of Newfoundland unique. As a result it has become even more urgent that efforts be taken to preserve them.

Dr. Ian Warkentin
Dr. Ian Warkentin

Clues and Correlations

The decline in populations of the Gray-cheeked Thrush had many possible explanations. But finding the right explanation meant eliminating the possibilities through careful research.

The possibility that forestry operations, to harvest trees, were intruding on the habitat of Gray-cheeked Thrush here in Newfoundland was one avenue investigated by Dr. Warkentin and his partners:

“Are Gray-cheeked Thrush affected by forestry practices? Are they affected by the way we cut timber? The size of the cut blocks?”

Analyses of data from broad-based surveys of both intact forests, and areas with harvesting activity, suggested that these small birds actually select areas with a mixture of mature forest and previously harvested areas. This is probably due to the abundance of opportunities to forage for insects in the nearby harvest patch.

“What we see is that Gray-cheeked Thrush seem to like to be in patches of forest next to cut blocks, that they’re a species that does well there.… So, forestry practices in the areas that we’ve worked don’t seem to be the reason for the declines we’ve seen.”

Another possibility was that something was happening to the Thrush’s winter habitat in South America that was causing the declines. Bird species that share the same breeding areas sometimes share the same wintering areas as well. If Gray-cheeked Thrush from Newfoundland were all wintering together in the same place then anything damaging their habitat there could lead to population wide declines like those seen on the island.

Dr. Warkentin and his partners attached GPS tags to thirty of the birds and were able to recover tracking information on the movements of four of them (from tags retrieved the following summer). From this information they were able to locate what they suspect is the approximate location of the Newfoundland Gray-cheeked Thrush’s wintering grounds; in a region seeing deforestation for agriculture and the increasing use of shade-grown coffee farming.

Shade-grown coffee farms leave higher canopy trees intact while clearing out lower vegetation to make room for coffee plants. It’s possible this practice may be having a negative effect on the Thrush since shade-grown coffee, while beneficial to many other songbird species, may reduce the availability of low-growth berry bushes that provide an important part of the Thrush’s diet during winter.

Confirming whether or not land use practises in its winter habitat may be contributing to the Newfoundland Gray-cheeked Thrush’s decline will require further research. Based on this research, it hasn’t yet been eliminated as the cause of the Thrush’s decline.

Caught Red-handed, So Far at Least

The likely culprit, as it turns out, may be the red squirrel which was first introduced to Newfoundland in the late 1960’s. Thanks to these animals being brought to the island, and then additionally moved from place to place over time, squirrels spread rapidly at just about the same time as Gray-cheeked Thrush populations began to decline.

“Various ideas [as to why the squirrels were introduced] have been floated around,”

Dr. Warkentin elaborates.

“One is that they were meant to be food for endangered pine martin. Another is that it was done to help with the development of a pelt harvest.”

These squirrels largely depend upon tree cones for food, which tend to grow most abundantly at the lower altitudes where the Thrush populations have seen the highest declines. But squirrels are also predators that will eat bird eggs and chicks if they find them. This answer would also explain why Gray-cheeked Thrush populations may be surviving on smaller islands off the coast of Newfoundland, where the squirrels may not yet have arrived.

Dr. Warkentin says that work by a graduate student at Grenfell, Jenna McDermott, clearly showed the disappearance of thrushes and a high density of squirrels at lower elevations of her study area. The thrushes are also frequent occupants of survey points at higher elevations where squirrels are absent.

But this possible connection between the introduction of squirrels and the decline in Gray-cheeked Thrush is only a series of correlations at the moment, not proof of causation. To establish a direct link between squirrels and Thrush will require further collaborations; to monitor nests and see if squirrels are actually taking eggs and young from the nests, and to explore the smaller islands off our coast where Gray-cheeked Thrush are still prominent, determining if they are also home to squirrels.

Dr. Warkentin is hoping to pursue more research on these topics in the near future and emphasizes the importance of working with others:

“All the research I’ve ever done, right from the beginning of my career, has been collaborative. It’s important in all the things I have done and all the things I continue to do… the basis for my success as a researcher has been working with groups of people who can bring different skillsets.”

Written and edited by Conor Curtis; Additional editing by Dr. Ian Warkentin

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