It's Catbirds vs. Cats…and the Cats Are Winning
By Tina Gheen
Scientists from the Smithsonian Migratory Bird Center and Towson University began a study to determine how well birds are surviving in suburban areas. Specifically, they wanted to determine the success of gray catbirds (Dumetella carolinesis) in areas with concentrated populations of people. They looked at nest success, the probability the baby birds will survive through incubation and as hatchlings, and post–fledgling survival rates, the period after the birds leave the nest, but before they migrate.
From May to September, with the help of ordinary citizens participating in the Smithsonian’s Neighborhood Nestwatch program, the team studied catbird nests in 3 suburban neighborhoods in Maryland: Spring Park, Opal Daniels Park, and Bethesda.
Catbirds typically build large, recognizable nests in the middle of dense shrubs or trees, so they were fairly easy to locate. The nests were monitored every 2 to 4 days both during incubation, which lasts about 12 days, and the nestling stage, which lasts about 11 days, until the nests either failed or the baby birds fledged.
The chicks were banded and weighed, and some of the nestlings and juvenile catbirds were fitted with tiny radio transmitters. These transmitters allowed the researchers to locate and track the birds after they began to fly. It also allowed them to locate the transmitter after a bird had died.
This gave the researchers important insights as to how the birds died and revealed tell-tale clues about any predators involved. For example if the transmitter was found underground, the bird was most likely taken by a rat in the suburban environment.
Sadly, predators were responsible for 79 percent of the mortalities of the juvenile catbirds in the study. Of those deaths, nearly half were attributed to cats in Opal Daniels Park and Spring Park. Predation was highest the first week after the birds had fledged.
Since the baby birds are noisy and constantly receiving attention from the parents during that first week, domestic cats are most likely intensely monitoring and hunting the inexperienced birds during this time.
Most scientific studies attribute predation to native animals such as hawks, snakes, and chipmunks, but the D.C. study found that novel, or new, predators such as cats may be driving the survival rates of juvenile birds in suburban areas instead.
Since domestic cats can thrive in large numbers in suburban environments because they aren't under the usual environmental pressures of limited food resources, disease, and competition for survival, they are in a position to dramatically influence the success of bird populations.
After examining the results from each neighborhood, the scientists discovered something else. The results showed that habitat suitability for the catbirds varied from neighborhood to neighborhood in the suburban environments. The birds in the Bethesda neighborhood were much more successful than catbirds in the other 2 neighborhoods.
In particular, the nestlings in Bethesda had a very high survival rate. The study showed that although each neighborhood site provided the right kind of habitat, predators, especially cats, often tipped the balance against the young birds' survival.
This article summarizes the information in this publication:
Balogh, Anne L., Ryder, Thomas B., and Marra, Peter P. 2011. Population demography of Gray Catbirds in the suburban matrix: sources, sinks and domestic cats. Journal of Ornithology.
Understanding factors that limit the productivity and survival of birds in rapidly changing human-dominated landscapes are key to managing future population persistence. To date, few studies have quantified both nest success and post-fledging survival for birds breeding within the suburban matrix. Here, we estimated nest success and juvenile post-fledging survival for Gray Catbirds (Dumatella carolinensis) and used those site-specific parameters to model source–sink dynamics at three sites in suburban Washington DC (USA). Cumulative nest success probability varied substantially among suburban sites and indicated that in some cases suburban habitats may provide suitable breeding sites for passerine birds. In addition, we documented the effects of sex and brood size on postfledging survival rates and determined the role of predation on dispersing fledglings. Like nest success, estimates of post-fledging juvenile survival also varied among sites and highlight the importance of site-specific demographic estimates in urban habitats. Predation accounted for 79% of all mortalities, with 47% of known predation events attributable to domestic cats (Felis catus). Our models of source–sink dynamics underscore the importance of seasonal recruitment parameters for calculating population growth rate and subsequent persistence. This study provides parameter estimates for two critical life history stages in the avian annual cycle in the suburban matrix and posits that predation drives differential nest and postfledging survival within human-dominated environments.
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