Leaving home on a wing and a prayer
By Jim Cox, Stoddard Bird Lab Director
Watching your kid stepping out the front door for good can break a parent’s heart. The big step can also be cause for subdued celebration, depending on the kid of course.
Animals don’t have such emotional fits during this major milestone in life, but knowing how young animals leave home and settle within the surrounding landscape is a critical part of wildlife management. Dispersal affects population growth, genetic integration, colonization of available habitat, and the stability of small populations. It’s also one of the least studied aspects of population dynamics for small songbirds.
Songbird enthusiasts would love to know as much as game bird biologists know about this critical period in life, but the radio transmitters needed to follow juvenile movements exhaustively are simply not yet available. Songbird enthusiasts suffer from radio envy as a result. For example, the transmitters available for brown-headed nuthatches last only 16-24 days before the battery goes dead. That’s about 2% of the normal lifespan of these birds, and this brief snapshot may provide as many false leads as it does useful information, not to mention the fact that the transmitters are expensive.
The Stoddard Bird Lab has devised some new field procedures to help assess dispersal in nuthatches. Nuthatches respond to recorded calls strongly and will fly in from 100 m away to investigate a fake nuthatch blasting from an audio speaker. Once they’re close, trained eyes can usually get a look at the legs and determine whether or not the individual is banded. If you then look at the legs in a spotting scope, you can actually any color bands that were applied and know exactly which juvenile nuthatch is responding to the recorded lure.
We’ve been performing this type of sampling within territories that had successful nests and then at hundreds of random points that are distributed through Tall Timbers and at least 200 meters away from successful nests. The sampling near successful nests lets us know which individuals are staying close to home, while the random points allow us to pick up individuals that have made the decision to launch off in search of new beginnings.
It turns out that the decision to leave comes surprisingly early in life for a nuthatch. In 2012, we found a marked juvenile female in early May at a random point that was more than two miles south of her nest site. She fledged in late March, which means she’d made the decision to leave the home turf with about 30 days of experience under her wings. Many young are still being fed by their parents at this at this point, but this young nuthatch had decided that a better future lay somewhere else. (See video: http://youtu.be/foyxk4eQyV0)
Other observations of dispersing females piled up during the first few years of monitoring and suggested it was rare to find females in their natal territories more than two months after the individuals fledged. As Springsteen might say, if he ever sang about birds, these babies were born to fly.
Females also seemed to make a sudden jump from their natal territories and then settle in new areas with other nuthatch family groups. We keep encountering some dispersing females at different random points within a 20-acre area that may be a mile or so from their original nests. These females are mixed in with marked adults and juvenile males that have stayed closer to home, and the females seem to be shifting among neighboring territories assessing the local conditions.
We also have observed marked males physically grooming the dispersing females, perhaps as a means of inviting them to stay. The grooming consists of using the beak to scratch around the head and neck of the recipient. The birds being scratched close their eyes and often tilt their beaks skyward to expose additional areas under the neck. The activity can go on for five minutes in some cases, and though we can’t say the birds really “enjoy” it, we pretty sure they don’t hate the massage, either.
Unlike females, many juvenile males decide to stay home for extended periods and frequently help mom and dad raise young the following breeding season. This type of cooperative breeding behavior was well known and represented a reason for studying nuthatches in the first place, but we also documented more than a dozen young males in random points away from successful nests. Unlike some of their brothers who stay to help, these males decided to fly the coop.
When we compared the nestling weights of the males who stayed versus the weights of those who fled, there was a tendency for the dispersing males to have lower nestling weights. Just as with human wrestlers, nestling weight is a good indicator of an individual’s social status and competitive abilities. We have not yet secured enough information on how brothers interact with one another after they leave the nest, but the differences in weight suggest that larger males may out-compete their smaller siblings in some way, so the smaller males decide to leave.
Thanks to these field techniques, we are monitoring one of the interesting phases of the annual cycle with precision, and finding out that the social life of this small songbird is extremely complex. There are lots of decisions being made in some really dynamic social settings. At times, there seem to be parallels with human societies, but we’re also very confident that we’ll never see a tear in the eye of a proud nuthatch parent when a son or daughter leaves home.