According to the Cornell Lab of Ornithology Birds of the World (https://birdsoftheworld.org/bow/species/grycat/cur/introduction), Gray Catbirds tend to leave the nest, or fledge, between 8 – 12 days. At this point they should be ~80% of their adult mass and you can see that their feathers already look much more complete than they did yesterday! Fledglings will usually leave the nest with developed flight feathers but clumsy flight, and they’re still reliant on parental care (for about 12 more days in catbirds). This is a very vulnerable stage in their lives.
Back in 2019, Rosenburg et al. published research in Science using decades of standardized bird surveys and weather radar to show that North America’s breeding bird numbers have shrunken by an estimated three billion since the 1970s. That means that more than 1 in 4 birds has disappeared in the past 50 years. While the biggest declines were in grassland birds, there are substantial losses everywhere and habitat loss is a huge reason. But cats are another.
A study of Gray Catbirds in a suburban area (much like this nest) found that predation accounts for almost 80% of fledgling mortality and 47% of the known predation came from domestic cats. Outdoor and feral cats are believed to kill about 2.4 billion birds annually (that’s four times more than collisions with windows and 10,000 times more than wind turbines). I know I’ve seen many outdoor cats wandering my neighborhood, so we can only hope that these birds go unnoticed. While we’ve watched these nestlings grow with the incredible care of the parents, the hardest may be yet to come.
At this point, the nestlings are at about 75% of their adult body mass and their rate of growth slows a bit as energy goes to the fun stuff: feathers!
As adults, birds don’t grow new feathers all at once. They have a programmed molt schedule to drop old feathers and grow new ones in an orderly fashion so they avoid the naked exposure we see in these nestlings. After all, they still need to fly to avoid predators! (Sea ducks, like eiders, molt all of their flight feathers at once and they look pretty awkward flap-running away along the surface of the water!)
But growing feathers is also energetically expensive. Feathers are mostly protein (like keratin, the stuff of hair and fingernails) and make up about a quarter of a bird’s total protein. This makes molting a very demanding period of their lives, and even when they do it gradually it still takes raises their energy demand by 10% or more. And that’s when it’s a few feathers at a time…imagine all of them at once!
In early July I noticed a Gray Catbird eyeing an existing nest right outside my front door. This was unexpected because the nest was left over from last year, and it was only uncovered with some shrub-trimming last fall. Interestingly, the Cornell Lab of Ornithology Birds of the World species account for the Gray Catbird mentions that a “new nest is usually built for each brood and for renests, but there is one observation of reuse of a Gray Catbird nest between years (Marshall et al. 2001).”
Because of this, I thought maybe she was just checking out the existing nest, especially since it was relatively late in the season and it would likely be a renest or second brood. But by July 12 she was incubating two beautiful blue eggs! I wasn’t sure how many eggs she’d lay, since clutch sizes vary from 1 – 6 eggs, but I wanted to keep an eye on her without disturbing the nest. I mounted a GoPro camera above the nest to remotely take videos and monitored her progress, but she stuck with just the two eggs. Stay tuned to see what happened!