Baby Catbird Survival
The Perilous Life of a Fledgling Bird
The factors that determine the survival of recently fledged birds after the nesting period are poorly understood. To gain a greater understanding of such factors an intense study was launched on the breeding biology of gray catbirds inhabiting the backyards of several Neighborhood Nestwatch participants.
This study involved measuring events in the post-breeding period, a time when a fledgling learns to survive and become independent. Right after nestlings leave the nest, fledglings are immediately exposed to numerous factors crucial to their survival including high vulnerability to predators.
The lack of research on the post-fledging period is the result of the difficulties in monitoring fledgling birds. Fledgling songbirds are highly mobile and cryptically-colored, making them difficult to track.
Tracking Catbirds
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| Baby catbird outfitted with a radio transmitter so that its movements after leaving the nest can be followed. |
Recent advances in radio-telemetry have helped to facilitate a greater understanding of the dynamic nature of the post-fledging period. For the past two summers, we have been studying the post-fledging behavior of gray catbirds. They are an ideal species because they meet the standard size requirement of carrying radio-transmitters (small birds can comfortably carry a transmitter that is about 5 percent of their body mass).
Catbirds are a native, long-distance migratory species that is abundant in the Washington, DC region. Three suburban Neighborhood Nestwatch sites were chosen as study sites: two in Takoma Park, Maryland and one in Bethesda, Maryland. At each site, birds hatched in spring or summer were captured by mistnetting or by carefully removing them from the nest and then attaching a transmitter.
Fledglings were monitored every other day with radio tracking equipment until they died or departed their natal (birth) site. The results showed alarmingly high rates of nest predation and fledging mortality, especially at the Takoma Park sites.
Percent of successful nests (those fledging young)
The age at which fledglings died reveals the apparent vulnerability to life-threatening events young birds face upon fledging. At both sites, mortality was greatest during the first week after fledging, and furthermore, mortality was significantly higher at the Takoma Park sites.
Percent of young that survive to 8 weeks of age
Cats Clobber Catbirds
This was primarily due to nest predation and predation of fledglings by cats.
Domestic cats typically only decapitate birds and leave the carcass.
Censuses of domestic cats at our sites showed significantly higher number in the Takoma Park sites compared to Bethesda.
In addition, several guilty-looking cats were found in close proximity to dead birds.
Our guess is that closer examination would have revealed feathers in their whiskers.
One take-home lesson from this study thus far is that we could do wild birds a huge favor by keeping cats indoors. It's the only effective means by which to prevent cat-related bird death.
SMBC Investigators:
- Peter Marra
- Ann Balogh, graduate student M.S.
