Imagine you’re trying to find out where someone lives – someone who doesn’t want to be found. You might start by lurking around their favorite hangouts and getting a feel for their movement patterns. Once you spot them, you might follow them around, hoping they’ll lead you to their house. But they’re not as oblivious as you’d hope. The closer you get to the right address, the sneakier and angrier they become. You might end up with just a street name or just a neighborhood and have to check every house one at a time.
This is a very creepy (but fairly accurate) analogy for nest searching. We locate and monitor as many Cape Sable Seaside Sparrow nests as we possibly can on our study plots during the breeding season. The more nests we find, the better idea we have of how successful birds are at raising young.
Cape Sable Seaside Sparrow nests are not easy to find. They are cup-shaped, only a few inches across, woven from warm-season grasses and suspended low above the ground, usually in a clump of grass with good cover from the sun. That’s why we rely on behavioral cues from the nest parents to find young. Sometimes it’s as easy as watching a sparrow fly back and forth to the same spot with food items, like insects, in its beak. More often, it’s a combination of subtle clues that contribute to a gut feeling that there may be a nest nearby. What comes next is the heart of nest searching: trudging through part of a bird’s territory and picking through clumps of grass looking for the nest. Often, one or both parents will perch nearby and chip with indignation.
We record data for each nest we find: GPS coordinates, parent identification, even the distance from the bottom of the nest to the ground. We also bury nickel-sized temperature sensors, called iButtons, in the bottom of the nest. The way nest temperature fluctuates can tell us things like what time of day the babies fledged.
It takes just under two weeks for sparrow eggs to hatch. Over the next 10 days, the pink-translucent nestlings open their eyes, grow feathers, beg for food… and that’s about it. Baby birds still have a lot of growing to do once they hatch, and that takes a lot of energy; so during this time, parents deliver food to the nest round the clock.
We usually check on nestlings once or twice during their development to make sure they are alive and well. We determine nestling age based on feather development, which means we usually need to gently remove at least one chick to get a good look. This 3-day-old nestling, for example, is starting to develop tiny wing feathers, and its eyes are nearly open:
Once the nest is empty, we use context clues to decide whether we think the nest fledged or failed. A big, stretched-out nest with droppings in it tells us that the fledglings took flight. They will spend the next few weeks following their parents around and learning how to take care of themselves. And after that, Mom and Dad will leave the juveniles to fend for themselves, and if we’re lucky, start building their next nest and starting all over again.