- By monitoring grassland bird nests with small video cameras, researchers are learning why and when chicks first venture into the outside world.
- A recent study using video footage showed that nestlings of different species left their nests at different times of the day and over varying lengths of time, from less than one hour to three days for nests with multiple chicks.
- Smaller and more affordable video equipment is allowing scientists to study events, such as bird nest predation and fledging, that happen quickly, generally when humans are absent.
Every year, millions of baby birds leave the security of their nests, flying off into the uncertainty of the outside world.
A single chick’s fledging takes just a moment, and all chicks in a nest typically fly off within an hour or two, so people, even dedicated birdwatchers, rarely witness it.
A team of researchers installed mini video cameras at 206 nests of 17 species of grassland birds in Canada and the northern United States to find out when and why birds fledge. They published their findings last week in The Auk: Ornithological Advances. Their analyses of the video footage suggested that chicks balance leaving the nest before a predator finds them with staying in the nest to take advantage of food provided by their parents.
Researchers began employing video cameras to improve their ability to monitor bird nests some 20 years ago. They have learned that young birds in forest and shrub vegetation tend to fledge in the morning, that well-developed birds tend to fledge rather than wait for their smaller siblings, and that chicks in nests under greater predation risk typically fledge earlier in the day and over a shorter period than broods living safer nests.
Nevertheless, scientists still have only limited understanding of how, when, and why baby birds in different ecosystems leave their nest. Most grassland birds, for example, build cup-shaped nests on the ground that are well concealed, making direct observation by people particularly difficult.
The authors of the recent study incorporated video camera data into their assessment of factors that could explain the time needed for all the birds in a nest to fledge, which sometimes extends into multiple days. It also considered how day length, which varies across the breeding season, might affect the timing or duration of fledging.
Lead author Christine Ribic from the U.S. Geological Survey and her colleagues collected and analyzed video camera data to examine two competing hypotheses about how baby birds decide when to fledge. The threat of predation by nocturnal raccoons, foxes, and coyotes might encourage all nestlings, regardless of species, to fledge as soon as possible and as early in the day as possible. Alternatively, once older siblings in a clutch have fledged, younger birds might stay in the nest to take advantage of food provided by parents, which would extend the fledging period for any given nest.
“Considerable research attention has focused on the breeding biology of birds, but until recently some events have been difficult to observe,” said bird nesting behavioral scientist T.J. Benson of the University of Illinois, who was not part of the study, in a statement. “Luckily, decreases in the size and cost of video equipment have allowed researchers to study these hard-to-observe events, such as the brief moments when a predator causes a nest to fail. This study took things a step further to begin exploring the point in time when young birds fledge from the nest.”
To monitor the nests, the researchers chose cameras that were small, had weatherproof housing, and incorporated infrared light-emitting diodes (LEDs) to allow them to record under low-light conditions. They installed the cameras when the nest contained either eggs or recently hatched nestlings and kept the cameras running throughout the nesting period.
The researchers set up the cameras between 10 and 50 cm, or between 4 and 20 inches, away from nests. If needed, they raised cameras up 40 cm (16 in) above the ground on a dowel to avoid being blocked by thick grass around the nest.
They connected the cameras with cables to both a battery and either a videocassette recorder or digital video recorder, which remained at least 25 meters (82 feet) from the nests. The cassette recorder systems typically captured four images per second and required a daily videotape change and a battery change every three to four days. The digital recorder systems captured around 30 images per second and required a battery and SD card change every three to four days.
The scientists analyzed the video camera data, covering the period between mid-May and late July, for the seven species with at least 10 nests each and which represented most of the nests. They scrutinized the footage for trends in fledging behavior, specifically when the process started in each nest and how long it took, and they assessed several possible drivers of fledging.
They examined how the species, number of nestlings in a nest, and time within the breeding season (representing day length difference from that of the North American summer solstice) affected the time of day the first nestling fledged and the amount of time it took for all the siblings to leave.
Building strength while avoiding predators
Unlike birds nesting in tree cavities, ground-nesting birds suffer heavy predation pressure by both daytime hunters, such as snakes and hawks (and even cows), as well as nighttime hunters. The footage showed that the chicks of these ground-nesting grassland birds left the nest at different times of the day, rather than uniformly leaving early in the morning to find a safe place before nightfall.
“Basically, anything that moves in the grassland can kill those birds,” Ribic told Smithsonian, by disturbing the nestlings and forcing them to evacuate, so no time is completely safe for them to venture into the outside world.
In fact, the species of the chicks in a nest influenced the time of day the first bird fledged more than the number of nestlings or day length. The times ranged from 3.3 hours after sunrise by clay-colored sparrows to 5.8 hours after sunrise by chestnut-collared longspurs.
They found that 165 nests had at least two chicks, and for more than 20 percent of these, it took two or three days for all the babies to fledge. In these cases, the siblings leaving later tended to leave the nest earlier in the day than the first to fledge. The researchers suggest this difference means that the parents may need less time in the morning to provide the fewer remaining nestlings with enough energy to fledge.
The video data indicated that nests with more chicks took longer for all of them to fledge, which suggested that younger chicks might stay in the nest to build up their strength from food brought by their parents.
“It was exciting to see events naturally occurring in an area of avian biology where very little is known, and was only possible due to the use of video surveillance systems,” Ribic said in the statement. “It seems fledging is more complex than we previously thought. We were surprised by the span of time over which grassland bird species fledge, with some species starting to fledge in the early morning and others closer to noon, and by the frequency of fledglings that spanned multiple days.”
Their project video, compiled by Smithsonian.com, compares nestlings preparing to fledge to human teenagers, in that they like to sleep late and wait for mom and dad to bring them food.
Curiously, the video footage also indicated that various nestlings left and returned to the nest several times before fledging, a behavior that is feasible for ground-nesting birds but not for birds nesting in tree cavities.
The researchers actually watched the videos from end to beginning. This helped avoid recording false fledging dates by nestlings that took these brief trips outside the nest before they fledged permanently. By counting the increasing number of chicks remaining in each nest on each earlier date, the researchers could determine the timing of each true fledgling.
The study shows how evolving video technology has been critical for scientists trying to better understand what drives these young birds’ decisions to move from one high-risk environment into another.
Ribic, C. A., Ng, C. S., Koper, N., Ellison, K., Pietz, P. J., & Rugg, D. J. (2018). Diel fledging patterns among grassland passerines: Relative impacts of energetics and predation risk. The Auk, 135(4), 1100-1112.
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