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!
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!
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.
Open-cup songbird nests, like the one occupied by these catbirds, can be absolute marvels of weaving intricacy, but they’re still exposed to the elements. Gray Catbirds typically choose dense shrubs that both conceal the nest from predators and provide some degree of shelter. But when extremes come along—such as hot temperatures and direct sun or windstorms and heavy rain—the adults will do their best to cover the helpless chicks. This is especially important because the nestlings have not yet developed the contour and flight feathers that adults keep waterproof with preen oil.
On this particular afternoon a huge system of thunderstorms moved in. This was only mid-afternoon, but as you can see from the second video the clouds blocked almost all light. Buckets of rain poured down and lightning flashed all around, giving me a great opportunity to see this excellent parent sheltering the nestlings through the weather (make sure your sound is on for the full effect!).
On July 21, the second egg in the nest hatched!! Songbirds are altricial, meaning they’re helpless when they hatch and require parental care in the nest (contrasting with precocial birds, like shorebirds, that are relatively mature and can start running around soon after hatching). As you can see, the helpless nestlings’ eyes are still covered and they’re mostly naked, so they still require brooding from the adult to maintain stable temperature. Meanwhile, the adults also have to bring small prey items to the nestlings. But with all the brooding these feeding trips aren’t too frequent yet.
According to the Cornell Lab of Ornithology, during these early days the male does most of the feeding (I think the female deserves a break, don’t you?). These tiny beakloads with lots of time brooding in between may not seem like much, but providing protein-packed little invertebrates like grubs and arthropods can grow these little chicks quickly, as you’ll see!
Gray Catbird incubation lasts about 12 days. During that time, this mom was brave and vigilant. As this was right outside our front door (and we don’t have the luxury of alternative exits) I knew she’d be dealing with our comings and goings throughout this whole breeding ordeal. Since the shrub had been recently trimmed, I was lucky that at one point while walking past you could see straight through a gap to see her quietly watching you.
Embryos require a fairly stable and narrow temperature range, so her vigilance is important. Carryover effects of suboptimal temperatures during incubation range from slower nestling growth (Ospina et al. 2018, Ecol Evol) to more fearful behaviors in the offspring (Bertin et al. 2018, Scientific Reports). In catbirds, the female does all the incubating and will typically sneak off to eat around sunrise. I never directly saw the male coming to feed the female on the nest during the day, but that behavior is also typical for these songbirds.
On July 20, we had our first nestling! (It was only 8 days after I first noticed the eggs, so the first must have been laid a few days earlier as it usually takes about 12 days to hatch.) It doesn’t immediately start begging for food, so if you listen in the video you can hear the adult give several “quirt” calls that eventually trigger the open-mouthed begging response.