The age when young birds leave their nest is the evolutionary compromise between parents, who want their chicks to leave as early as possible, and offspring, who want to leave as late as possible
Adult gray-headed junco (Junco hyemalis caniceps) enticing one of its youngsters to leave the nest.... [+] Parents hold food away from nest and tempt the young come out to get it. This picture captures a young bird that was just fed outside of the nest. (Credit: T. E. Martin, doi:10.1126/sciadv.aar1988)
Thomas E. Martin
Major life changes can be dangerous, even fatal. Probably the most dangerous life transition is when young animals, such as fledgling birds, begin to move about on their own and to make their own decisions. Predictably, when baby birds -- nestlings -- transition from dependency to their new life as fledglings living outside of the nest, their first few weeks of exploring the landscape and learning to fly are fraught with extraordinary dangers.
When nestlings leave the nest too early, they fly poorly, or not at all, because their wings are small and underdeveloped. Fledging too early is usually a fatal decision: it is in a nestling’s best interests to remain in its nest for as long as possible to allow its wings the time necessary to develop more fully.
But remaining in the nest for “too long” is tremendously dangerous for many bird species because predators are always searching their territories for something to eat, and upon discovering an occupied nest, a predator usually kills all the nestlings in one go. Since bird nests are stationary objects, it’s simply a matter of time -- sometimes just hours or even minutes -- before a nest filled with chicks on the verge of transitioning to fledglings is discovered and transformed into lunch. This is especially true for birds that build open-cup nests on or near the ground.
A young gray-headed junco (Junco hyemalis caniceps) is captured leaving the nest, with its sibling... [+] still in the nest in the background, illustrating the under-developed nature of wings when this species leaves the nest. (Credit: T. E. Martin, doi:10.1126/sciadv.aar1988)
Thomas E. Martin
Predictably, predation plays an important role in driving the evolution of optimal fledging times for birds. Songbirds that experience higher daily rates of predation -- species like towhees and juncos that build open-cup nests on the ground or in low bushes -- have evolved younger ages of fledging to deal with this pressure. In contrast, this pressure to fledge early is relaxed for birds that enjoy a relatively low risk of nest predation -- as seen in cavity-nesting birds, like chickadees and bluebirds.
Cavity-nesting birds, like this mountain chickadee (Poecile gambeli), about to feed its young, have... [+] safer nests that allow young to stay in nests longer and develop their wings for improved flight at leaving. (Credit: T. E. Martin, doi:10.1126/sciadv.aar1988)
Thomas E. Martin
“Predation pressure has a huge influence on the capacity of birds to fly,” said Bret Tobalske, a professor who works at the intersection of biology and physics to study animal locomotion at the University of Montana, and Director of the Field Research Station at Fort Missoula. Professor Tobalske was a co-author of the recently published study. “Our study shows this for the developmental phase from nestling to fledgling.”
For example, some species of songbirds lose only 12% of their young, mostly to predators, in the first 3 weeks after they leave the nest, whereas other species lose as many as 70% (for example; ref and ref). This is typical: similarly high or highly variable mortality rates due to predation in the first weeks of juvenile life are common across a wide variety of other animal species, too (ref).
A research team, headed by avian ecologist Thomas Martin, Assistant Unit Leader and Senior Scientist in the Montana Cooperative Wildlife Research Unit at the University of Montana, investigated how predation influences the transition from nestling to fledgling in different species of songbirds. These songbirds included species that build open-cup nests either on the ground, low down in bushes or higher up in trees, as well as species that nest in cavities. Dr. Martin and his colleagues measured nest predation rates, wing growth rates, fledging ages and they used high-speed videography to record and examine flight performances of newly fledged birds of 11 songbird species to see if this may explain differences in their fledgling mortality rates.
As expected, Dr. Martin and his collaborators found that songbird species with higher nest predation rates produced fledglings that left their nests earlier, and they had smaller, more underdeveloped wings, and poorer flight abilities.
Dr. Martin and his collaborators tested the effect of older fledging age on survival -- what would happen if the researchers delayed fledging time? To do this, they built a small enclosure around the nests of gray-headed juncos, Junco hyemalis, a species that builds open-cup nests on or near the ground, to delay fledging for three days, whilst leaving other junco nests unprotected to serve as experimental controls. The enclosures were high enough to keep predators out, but had an open top to allow the parents access to feed their nestlings.
They found that all young juncos had nearly identical masses (Figure 6A) regardless of experimental treatment, but the wing lengths of the delayed fledging juncos were substantially longer (Figure 6A and B) than controls, as expected. Further -- and most important -- the scientists found that mortality decreased for individual junco fledglings as their wing lengths increased (Figure 6C and D).
Fig. 6. Wing length and mass with respect to fledgling mortality rates. (A) Mass and wing length as... [+] a proportion of adult size in control versus experimentally enclosed nests for gray-headed junco. Control nests fledged at normal age (11 to 12 days), whereas enclosed nests prevented young from leaving for 3 days after fledging naturally to create a delayed fledge age. (B) Photos of typical wings of junco young from control versus experimentally delayed nests on fledging day versus release day, respectively. (C) Daily mortality rate (±1 SE) decreased among fledglings with increasing wing length at fledging in juncos. (D) Mortality rate of junco fledglings for the first week after fledging in nests where fledge age was experimentally delayed had substantially lower mortality rate than fledglings from control (normal fledge age) nests and comparable to other species based on wing length. (E) Daily mortality rate of fledglings and nestlings when based on estimates per offspring versus per brood across eight species. The line represents equal fledgling and nestling mortality rates. (F) Nest predation influences evolution of fledging age and growth rates of offspring with consequences for relative development when young fledge, which thereby influences locomotor performance and fledgling mortality. Fledgling mortality, in turn, feeds back to further influence evolution of the age of fledging and traits that affect performance and mortality, but parents and offspring conflict on the optimal fledging age. (doi:10.1126/sciadv.aar1988)
doi: 10.1126/sciadv.aar1988
It is predicted that natural selection should favor fledging at a time when mortality for remaining in the nest is the same as mortality for leaving the nest, but this is not what Dr. Martin and his collaborators found. Instead, they found that daily mortality is higher for junco fledglings (orange balls above the line for equal mortality rate in Figure 6E) than for junco nestlings. Whilst it is true that when nestling juncos leave later, the risk of nest predation increases, but delayed leaving allows greater wing development and thus, reduces overall individual fledgling mortality. This indicates that junco nestlings are leaving the nest sooner than they should.
“Songbird species differ in rates of mortality of young after leaving the nest due to differences in their relative stage of development caused by risk of predation in the nest,” Dr. Martin elaborated in email. “But the age of leaving is a compromise between offspring and parents, where parents want young to leave earlier than young want.”
“It fits into a broader pattern [that] predation pressure has been (and continues to be) a major driver of the evolution of flight,” Dr. Tobalske said in email.
Source:
Thomas E. Martin, Bret Tobalske, Margaret M. Riordan, Samuel B. Case, and Kenneth P. Dial (2018). Age and performance at fledging are a cause and consequence of juvenile mortality between life stages, Science Advances, 4(6):eaar1988, published online on 20 June 2018 ahead of print | doi:10.1126/sciadv.aar1988
Also cited:
Susan M. Smith (1967). Seasonal changes in the survival of the Black-capped Chickadee, The Condor, 69(4):344-359 | doi:10.2307/1366198
Kimberley A. Sullivan (1989). Predation and Starvation: Age-Specific Mortality in Juvenile Juncos (Junco phaenotus), Journal of Animal Ecology, 58(1):275-286 | doi:10.2307/5000
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