Day 7: Feeling drained?

Gray Catbird Nesting, , , ,

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?

Feeding these hungry mouths takes a lot out of the parents!

Day 6: Nestling begging is risky. So why do it?

Gray Catbird Nesting, , , , ,

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.

Begging comes as second (or first) nature to these nestlings!

So why do they do this? Here are a few of the ideas:

  1. “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.
  2. “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.
  3. “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!

Day 5: Who’s feeding the babies??

Gray Catbird Nesting, , , ,

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.

Males and females feed the nestlings at similar rates at this point, so telling them apart is impossible without catching them first.

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!

Gel electrophoresis showing a DNA size ladder followed by female (2 bands) and male (one band) catbirds

Day 4: Post-storm growth

Gray Catbird Nesting, , , ,

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.

The parents communicate with the chicks with little “quirt” calls, especially when their eyes are still closed.

Day 3: Storms

Gray Catbird Nesting, , , , ,

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.

Even in bad weather, they still need to eat!

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!).

Sound on for the full storm effect!

Day 2: Another new beginning!

Gray Catbird Nesting, , , ,

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.

The males does most of the brooding when the nestlings first hatch.

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!

Especially when it’s hot out the parents brood the youngsters, fluffing out their feathers to keep them cool and cover the chicks as much as possible.

Day 1: A New Beginning!

Gray Catbird Nesting, , , , , ,

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.

Baby birds have very little strength at first, so it takes a lot of effort to receive food!

Gray Catbird nest!

Gray Catbird Nesting, , , ,

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).”

Catbird eggs are typically a nice shade of blue.

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!

A GoPro camera mounted above the nest will let me monitor their progress without disturbing the birds!