On August 1 I peeked out the window at dawn and even through the darkness and the dense shrub I could still see the little catbird eyes peering out of the nest. After a cup of coffee, I went back to fire up the GoPro camera…and they were gone!
Gray Catbirds tend to leave the nest in the morning on their fledging day, so this was expected. As I mentioned in my previous post, this is a very vulnerable time for these birds, so they will usually hunker down in some dense vegetation to keep from being noticed. Unfortunately, this was also the day I was leaving for Maine, so I didn’t have a chance to relocate the fledglings. I can only hope they evaded the neighborhood predators and are flapping around as successful juveniles now!
Thanks for following along with this catbird story!
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.
Birds don’t chew, and as you can see this can appear pretty risky, but (usually) they know what they’re doing!
Birds have some pre-stomach food storage called a crop. This lets them stuff their faces in risky areas before retreating somewhere safe to digest. You may have seen this when birds swarm feeders at dusk in the winter to fill up so they have calories to sustain them through the long, cold night.
From the crop, food goes to the inhospitable proventriculus, where VERY potent stomach acid dissolves food. Shrikes (awesome predatory songbirds known as “butcherbirds”) can digest a whole mouse in three hours! Next is the gizzard, where strong muscles grind the food, often with the help of small ingested rocks. Then the small intestine extracts as many nutrients as possible from the food before reclaiming water from the large intestine. Finally, it reaches the cloaca, the opening where digestive, urinary, and reproductive systems all meet for excretion (and, as you’ve seen, occasionally becomes a fecal sac snack!).
Feathers can actually give a snapshot of the bird’s nutrition at the time of growth, and a small sample can tell us a lot. For example, nutritional stress can appear in stress hormones deposited in the feathers. Amounts of deuterium (the heavy isotope of hydrogen) can tell us roughly where the bird came from based how it’s distributed in freshwater around the world. Birds may even be able to detoxify their body tissues of toxins, like mercury, by concentrating it in their feathers!
Feathers grow from follicles on the skin, emerging from within a protective sheath before the vane we know and love expands. You can see that most of the feathers on these nestlings are only just starting to emerge from these tube-like sheaths.
But you can see that the feathers don’t grow everywhere. They grow in distinct tracts over a bird’s body called pterylae (pteron for “feather,” and hulé for “forest”). So while a bird appears completely covered, the feathers are actually growing from specific areas on the body to cover all the gaps. At the base of the feathers are muscles that allow birds to raise them—fluffing up to trap air and stay warm or release heat trapped under the feathers when it’s hot—or lower them to maintain aerodynamics and protect the body.
Because these pterylae leave bare skin in between (apteria, “without feathers”), this has its advantages for research. Birds have such thin skin that when we catch one and hold it carefully with its belly up, we can blow lightly on its belly to part the feathers and see the muscle and fat stores below the skin! This is one way for us to gauge the condition of birds non-invasively, particularly on migration when they’re fattening up in preparation for flight or depleted after one. Take a look at the second video to see this in action as I check out the fat and muscle of a post-flight Blackpoll Warbler!
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!
Okay, we know that the adults are working tirelessly at their own expense to feed the nestlings. As in this video, you’ve probably noticed in several earlier ones that the adult typically feeds the chicks then waits around for a moment before grabbing something white from a nestling’s behind and eating it. You probably thought, “hey, did she just eat that nestling’s poop?” You’d be right.
Songbird nestlings produce what’s called a fecal sac: a mucous membrane surrounding the poop (and urine, since birds mix it all up before it leaves their all-purpose hole—the cloaca). This keeps it all contained like a diaper.
They likely do this for a few reasons. First, it keeps the house clean. You may have noticed that the nest isn’t disgusting, right? Fecal sacs make it easier for the adult to collect the poop from the nestlings and carry it away from the nest, preventing unsanitary conditions at home.
We also talked about predation risk from begging nestlings. But imagine how much easier it would be to locate a nest if there was a bunch of bird poop all over the place! Birds may carry fecal sacs away to ensure that the nest goes unnoticed.
Okay, so my catbirds occasionally carried the fecal sacs away, but they were mostly eating them. Since we also just discussed how exhausting this must be for these busy adults, can we also appreciate that they might want a snack? Nestling digestion isn’t 100% efficient, so there are still a lot of nutrients in their poop (like from this blueberry in the video below). Parents may be eating the fecal sacs for a little snack on the go!
These demanding little nestlings are growing fast. According to the Cornell Lab of Ornithology, catbirds that hatched at 3 grams should now be over 20 grams, and supporting this growth spurt clearly requires a lot of effort from the parents!
Interestingly, studies on songbirds like Great Tits and Purple martins seem to show a cap on energy expenditure, even when these birds are clearly working hard. So what’s going on? Some of the coolest work I’ve seen of this actually comes from seabirds. Using some nifty techniques to measure both total daily energy expenditure and resting metabolic rate (RMR, i.e. self-care energy), researchers compared breeding and non-breeding Black-legged Kittiwakes. They found that breeding birds had higher energy expenditure but actually lowered their estimated RMR (Welker et al. 2014). So while there may be a cap on energy expenditure, these birds may instead allocate more of that energy to the chicks than to themselves.
In other words, breeding birds were reducing their self-care in order to put more energy toward raising the chicks! (Human parents reading this: “duh.”)
With that in mind, it makes sense that parents would sneak a quick snack whenever they can. You’ve probably seen this in the previous videos…Any guesses?
Nest predation is one of the most common reasons that breeding attempts fail, and predators can eavesdrop on begging babies to locate nests. Plus, the nestlings make a lot of mistakes; especially if it’s been a while since the last feeding, they’ll start begging as soon as something comes near the nest.
So why do they do this? Here are a few of the ideas:
“I need food the most!”: nestlings that are hungry will beg more, while those that aren’t will beg less, so the parent distributes food based on need. This works evolutionarily because if close relatives (i.e. siblings) survive then more of your shared genes are also passed on.
“Give me the food, not them!”: each bird is more related to itself than a sibling, right? So why let them get the food? In this scenario, begging is simply a way to get food at every opportunity even if it doesn’t need it.
“Feed me, I’m clearly the best!”: the nestling that can beg vigorously for the longest time is a better investment for the parent because it’s more likely to survive. So why not bet on the winner?
A study by Caro et al. (2016) found that nestlings would beg more honestly when they were alone in the nest, and other studies have shown that begging may not be all that energetically costly (so it probably isn’t a good signal of quality). So while the jury is still out and it’s likely a mix of a lot of factors, a lot of it seems to depend on the brood size. Having lots of other begging birds makes things more competitive, so chicks are less likely to be honest with their begging!
You may be wondering whether the mother or father is feeding the babies. Catbirds can’t be sexed just by appearance, so we can’t actually tell. In the breeding season we can catch birds and inspect them for a brood patch on a female—a smooth, featherless portion on the breast to facilitate heat transfer to eggs—or a cloacal protuberance on a male—which is basically what you think it is.
For birds outside of the breeding season we often take a small blood sample and use molecular techniques to figure out the sex based on separation of DNA fragments on a gel (called gel electrophoresis). If you look at the image of a completed gel below, you can imagine 5 columns: the first column is a standard of DNA fragments so we know what sizes to look for, then in the next two columns you can see that two bands separated while the other two columns only show one band in each. Just like how human males have XY chromosomes but females have XX, we are just looking for bands of different sizes. But it’s actually swapped in birds: males have ZZ chromosomes and females have ZW. So the columns here with two bands are actually females!
You can see in this video how quickly the chicks have been growing. If you look very closely you can see that the older nestling is just starting to open its eyes!
According to growth charts for catbirds from the Cornell Lab of Ornithology Birds of the World (https://birdsoftheworld.org/bow/home), nestlings are about 3 grams when they hatch (a bit more than a penny). But by day 4 they should be close to 13 grams (a bit more than a AAA battery)! So those little meals really add up, and at this point the mother and the father will be starting to spend less time brooding and more time feeding, and by day 5 they should be feeding the chicks roughly equally.