Grey catbirds are a relatively common species of songbird found throughout much of North America. Their distinctive “cat-like” vocalizations are a familiar sound in many backyards, parks, and nature preserves during the spring and summer months. However, there are some key facts and statistics about the grey catbird that suggest they may be rarer than their wide distribution and familiar presence might imply.
The grey catbird (Dumetella carolinensis) is a medium-sized songbird belonging to the mimid family, which also includes mockingbirds and thrashers. Adults have a slate grey body, black cap and tail, and a distinctive reddish patch under their tail which is visible when they fly. Their name comes from their frequent cat-like “mewling” calls.
Grey catbirds breed in thickets, forest edges, parks, and suburban backyards across much of eastern and central North America. They winter in the southern United States, Mexico, Central America, and the Caribbean. According to the North American Breeding Bird Survey, there are estimated to be around 9 million breeding grey catbirds across their range. However, when looking at more localized data, we can see that grey catbird populations may actually be declining in many areas.
Population Declines
According to data from the North American Breeding Bird Survey, grey catbird populations declined significantly between 1966-2015 in the following regions:
Region | Population Decline |
---|---|
Eastern Breeding Bird Survey Region | -1.7% per year |
New England/Mid-Atlantic Coast | -2.4% per year |
Southern Ontario | -3.8% per year |
These declines equate to a cumulative loss of tens of millions of grey catbirds from these regions over the past 50 years. The causes of these declines are not fully understood, but likely involve threats on their breeding grounds, wintering grounds, and migration routes. Possible factors include habitat loss, predation, disease, pesticides, collisions with human structures, and climate change. More research is needed to pinpoint the exact causes and implement appropriate conservation actions.
In addition to population declines, grey catbirds have also disappeared as breeding species from some northern parts of their historical range. An analysis published in 2016 found that grey catbirds are among many bird species shifting their breeding ranges northwards in response to climate change. However, the northern shift has not kept pace with temperature changes. As a result, grey catbirds no longer breed in places like southern Manitoba and Saskatchewan, where they were once regular summer residents. Their range retraction indicates they may be vulnerable to the impacts of a warming climate.
Reduced Genetic Diversity
A genetic study published in 2003 analyzed DNA samples from grey catbird populations across North America. It found that genetic diversity was markedly lower in some peripheral populations at the northern edge of the catbird’s range, such as in Ontario and Quebec. These marginal populations were found to be genetically distinct from the core populations further south.
The researchers noted that the peripheral populations may be isolated and represent unique genetic lineages adapted to northern environments. However, their small size and low genetic diversity leaves them at greater risk of inbreeding depression and localized extinction. This underscores the importance of protecting subpopulations across the entirety of a species’ range in order to maintain overall genetic health. The loss of peripheral populations could deprive the species of important genetic variation.
Specialist Habitat Dependencies
Although grey catbirds utilize a diversity of habitat types across their broad range, research shows they are actually dependent on very specific microhabitats for nesting.
A study published in 2008 examined nest site selection in catbirds breeding in a nature reserve in Ohio. It found that catbirds overwhelmingly nested in multiflora rose bushes, an invasive shrub. Nests were typically placed 3-6 feet off the ground, concealed by dense vegetation. No nests were found in any other plant species despite a variety of suitable native shrubs being available.
The researchers speculated that the thorns and dense structure of multiflora rose offer greater protection from predators compared to native shrubs. However, this tight association with a single invasive species represents an ecological trap. Land management efforts that remove multiflora rose could inadvertently eliminate critical nesting habitat unless native species providing similar structure are also available.
Rarity Among Related Species
When assessing a species’ rarity, it can be instructive to compare it to closely related species. The grey catbird is a member of the mimid family of passerines, along with mockingbirds, thrashers, and tremblers. Of the 30+ species in this family that breed in North America, the grey catbird falls on the rare side based on the following metrics:
Population Size
The grey catbird is not particularly abundant compared to other mimid species. For example, the estimates from the North American Breeding Bird Survey put the total grey catbird population at around 9 million individuals. In comparison, other widespread mimid species have significantly higher total populations:
Species | Estimated Total Population |
---|---|
Northern Mockingbird | 43 million |
Brown Thrasher | 13 million |
Grey Catbird | 9 million |
Sage Thrasher | 3.5 million |
Bendire’s Thrasher | 730,000 |
While 9 million individuals is still a substantial number, the grey catbird ranks in the middle of the pack in terms of mimid abundance. Species like the Northern Mockingbird and Brown Thrasher are several times more populous over their respective ranges.
Range Size
The grey catbird also has a comparatively small breeding range relative to other mimids. The grey catbird’s breeding range encompasses approximately 4.5 million square miles in North America. In contrast, other widespread mimid species occupy considerably larger ranges:
Species | Breeding Range (million square miles) |
---|---|
Northern Mockingbird | 7.5 |
Brown Thrasher | 6.3 |
Grey Catbird | 4.5 |
Curve-billed Thrasher | 2.2 |
Sage Thrasher | 1.3 |
The Northern Mockingbird and Brown Thrasher again far exceed the grey catbird, occupying breeding ranges 66% and 40% larger respectively. This is likely due to the grey catbird’s more limited tolerance for arid and high elevation areas relative to other mimids.
Conservation Status
Unlike many other mimid species which are common and secure, the grey catbird is ranked as a Species of Concern under several state and regional conservation assessments. For example, it is listed as a Species of Special Concern in Connecticut, a Species of Greatest Conservation Need in Massachusetts, and a Priority Species in Bird Conservation Region 30 (New England/Mid-Atlantic Coast).
These designations recognize the grey catbird’s steep population declines, high threats, and disproportionate rarity relative to other mimid species in these areas. No other mimid species shares these heightened conservation designations.
Conclusion
In summary, while the grey catbird is widely distributed across eastern North America, a closer look reveals several indicators that it is rarer than we might expect based on its familiarity:
– Population declines of 1-4% annually across large parts of its breeding range over the past 50 years
– Disappearance from historical northern range areas due to climate change
– Low genetic diversity in peripheral populations
– Specialist nesting habits dependent on invasive plant species
– Comparatively small population size and range extent relative to other mimid species
– Official conservation status as a Species of Concern in multiple regions
Conserving remaining populations and habitat, restoring native nesting vegetation, and monitoring responses to climate change will be important for ensuring the long-term viability of grey catbird populations. Although still a common species, the grey catbird warrants extra attention as a vulnerable member of the mimid family.