Linked paper: Diel fledging patterns among grassland passerines: Relative impacts of energetics and predation risk by C.A. Ribic, C.S. Ng, N. Koper, K. Ellison, P.J. Pietz, and D.J. Rugg, The Condor: Ornithological Applications 120:4, October 2018.
We know that human kids grow, mature, and gradually move towards a life that is independent of their parents’ home. The same is true for baby birds: they also have to decide when the time is right to leave the nest and start on their journey to independence. This seems to involve a balancing act between making sure they are big and healthy enough to survive independently, while leaving the nest quickly to avoid predators. Nests are busy places where chicks beg for food and parents are constantly coming and going with food deliveries. All of this activity could easily draw predators to the nest! The timing of chicks leaving the nest (fledging) isn’t well understood, particularly for birds that live in grasslands, many of which are threatened or endangered due to habitat loss.
Our new research focused on a variety of grassland songbirds, such as meadowlarks, sparrows, and longspurs. We found that the time baby birds leave the nest has more to do with having enough food (energetics) than avoiding predators. This is surprising because research on birds nesting in shrubs says that risk of predation is the most important thing affecting when chicks leave the nest. This suggests that nests in grasslands (hidden on the ground with protective cover from surrounding grasses and a few low shrubs) face different risks than nests placed in shrubs.
We found that grassland chicks can start to leave anytime throughout the day and when they leave depends on what species they are. Some chicks, like Clay-colored Sparrow and Grasshopper Sparrow, usually left the nest in the early morning, while Eastern Meadowlark and Chestnut-collared Longspur left closer to mid-morning. But sometimes chicks delayed leaving until the afternoon, with their siblings waiting until the next day to depart. The time it takes for all the chicks to leave a nest can be several hours to more than a day! Maybe some chicks are taking advantage of their siblings’ early departures to get more food and attention from mom and dad before they finally leave, too.
Measuring fledging time can be tricky because chicks run in and out of the nest multiple times before leaving for good. We don’t know why they do this; maybe they are exploring their world and gaining confidence before leaving to brave the world outside their home. Remember these birds have only been alive for a week and a half or so! Regardless, it’s a bit like kids going off for college but returning for school breaks … nestlings may leave and return repeatedly before fully fledging. Fledging is not nearly as simple as people think it is!
Understanding the fledging process allows us to better understand the biology of grassland birds. Learning about the pressures they face in their daily lives lets us understand what threats they face and how those threats may change as people alter grasslands. Grassland birds are declining more than birds of any other habitat type across North America. Research like this is part of understanding why they are declining and what we can do to help them recover.