It’s good to be fat

Banded siskins

Dan took advantage of the wonderfully mild weather we’ve been enjoying the past few days to open our feeder net to band some birds. Dan obtained his Master bander permit back in the fall, with the intention of undertaking a number of studies in our area. One of them was to look at the demographics of our winter bird population. He explained it in this October post, excerpted here:

The project will be initiated to benchmark the demographics of birds inhabiting this particular neck of the woods during the cold months (November-March). Data on abundance, diversity, survivorship, condition, and age/sex will be gathered to index and monitor the health of resident bird species in the area. These kinds of projects are important, particularly today with the threats posed by climate change to the ecology of even the most common bird species.

Banded siskin

So we set our net up, situated near the feeder so that birds coming or departing hat the chance to be caught in it. Over the course of two hours in the afternoon we caught and banded 33 birds. Most of these were the redpolls and siskins coming to the nyger feeder. We had a lot of chickadees, those boldest of the feederbirds, first to find new seed sources. We also happened to catch our Red-breasted Nuthatch, as well as a White-breasted Nuthatch and a Hairy Woodpecker, all nice birds to see in the hand, although Hairys, with their strong beak designed for drilling, can really do a number on a bander’s hands.

The last couple of days we’ve seen a lot of our banded birds at our feeders, not the least put off by the whole experience. For most species, banding is a mild inconvenience. It takes them out of their routine for a short period, but generally doesn’t result in any ill effects, providing the bander is conscientious and doesn’t keep them longer than necessary. In the winter, this means speedy (though still careful!) work, since the birds need to spend most of their time eating or looking for food, to maintain energy levels. Good thing our net is right next to our house – not only can we keep an eye on it and go retrieve a bird as soon as it’s caught, but we can also transfer them indoors for quick banding.

Banded redpoll

One of the things banding can tell us is how many birds we have in our area. Banding of course can only sample a portion of them, but the premise of mark-recapture studies is that if you mark a set of individuals, and then measure how many of them appear in subsequent samples, you can extrapolate the size of the population. For instance, if one marked and released 100 fish, and then came back and took a bunch more 100 fish samples, of which there was an average of 10% marked fish per sample, you’d know that your original 100 fish represented about 10% of the total population – which was, therefore, about 1000 individuals.

Similarly, we banded 33 birds a couple days ago. In watching the feeders since then I would estimate that between 1 in 8 or 1 in 10 individuals was banded. Through extrapolation, that would mean that our 33 birds represented between 10% and 12.5% of the total number of birds visiting the feeders, which would mean a total of between 264 and 330 birds coming to our seed. No wonder we’re going through it so quickly!

Banded redpoll

Another measurement that one can take from birds in the hand that’s impossible to get with simply observing birds in the trees is how much fat they’re carrying. This is extremely important simply for assessing health (for example, a Boreal Owl up near Ottawa a couple weeks ago had looked fine in the tree, and had even been seen with a rodent, but was found dead a few days later and was severely emaciated – a fact that was hidden by its feathers), but it also tells you a lot about their travel plans.

Birds only lay down fat if they’re migrating. You and I and Fido lay down fat as a security against lean times, but wild birds are always on the move, and their metabolism is too high to accumulate much if any fat during regular activities. This is especially true during the breeding season, when they’re busy collecting food for all their young as well as themselves, but it’s also true in the winter, when keeping warm requires a lot of energy. Come spring and fall, though, their daily time budgets change. While flying during migration there’s no opportunity to eat, so they need to make sure they have enough energy stored to get them through the long haul to their next pit stop. This is done in the form of fat.

Banded redpoll

Birds have translucent skin, and because their feathers grow in tracts, rather than uniformly, it is possible to part the feathers between the tracts and look through the skin. The fat appears yellowish or orangeish beside the red tones of the muscle, so it’s easy to see how much fat a bird has put on. Extremely fat birds are probably preparing to depart in the next couple of days. Birds with just a little bit of fat are either residents that are finding so much to eat they are actually storing a bit, or, more likely, are migrants that have just arrived, having exhausted all their fat reserves in the just-completed leg of their trip, and haven’t had a chance to put more fat on again. They may stay for four or five or more days while they do this, and they may need to do it a dozen or more times over the course of their trip.

The Hairy Woodpecker had no fat. This was no surprise, since we would expect him to be a permanent resident. The same was the case with the nuthatches. The siskins and redpolls all had fairly heavy fat deposits. They may start to peel out over the next week, to be replaced, perhaps, by birds moving in from farther south. But the ones that surprised me the most were the chickadees. The finches are irruptive birds, they don’t breed around here, so we would expect them to be leaving. But chickadees do breed here, so it was a surprise to discover them all with full fat deposits as well.

Banded chickadee

Dan and I actually noted that there was a recent influx of chickadees at the feeders, and it was hard to tell if they were just birds that had exhausted their winter food caches and were now relying on the feeders, or what the story was. The banding data, however, may provide a clue to the answer. Chickadees, in years of high summer breeding success and low winter food availability, will irrupt south just like redpolls and other winter birds. It may be that the chickadees we have at our feeders, fat as they are, are irruptive birds that had come south back in the fall, spent the winter, and are now preparing to head north again. It could be that they either started coming to our feeder in order to more easily and quickly build up their fat stores prior to departure, or that they are birds from further south who have just arrived and are stopping to fuel up before their next leg.

Since they can’t tell us, we can only hypothesize on the reason. However, we can see, come spring, just how many banded chickadees we still have coming to visit the feeder then.