Note: This program first aired on June 29, 2013.
The other morning I got a phone call. It was my niece, saying, “Sarah, Sarah, I found a baby bird on the lawn! I watched it for twenty minutes and it is still there, can I keep it for a day?” I told her to leave it where it was, and that I would be right over. When I arrived we found not one but two baby chipping sparrows, staying quite still in the grass, minding their own business, and two adult chipping sparrows, noisily and nervously flitting about in the trees above. The date was June 14, what many Americans celebrate as Flag Day; for me now it will always be Fledging Day.
Merriam Webster defines fledge, the verb, as “to acquire the feathers necessary for flight or independent activity, also to leave the nest after acquiring such feathers”. The word can also be used as a transitive verb, which simply means it has an object, as in “to rear until ready for flight” or, my favorite, “to cover with or as if with feathers or down”. The definitions make me feel that it is as much about getting the feathers as it is about leaving the nest, which may be, because you can’t leave the nest until you have the feathers.
The little chipping sparrows we found were in fact fledglings, young birds who, under the care of their parents, having grown sufficient feathers, made the leap from the nest into the big world beyond. These sparrows, like most small song birds, are what is called altricial at hatching, meaning they are tiny, without feathers and with eyes closed. They can’t keep themselves warm, and are dependent on their parents for everything. They look like little fetuses, which is essentially what they are. They will live in the nest for days to weeks as they grow, for example, those chipping sparrows were in the nest for about 10 days before they fledged. The rate at which these babies develop is quite amazing, it seems like you could literally watch them grow. For altricial birds, fledging means growing enough feathers to leave the nest, not necessarily having the ability of full flight.
On the other end of the baby bird spectrum (and it is a spectrum) are birds who are precocial at birth, meaning they have feathers, open eyes and are capable of movement when they hatch, and may or may not feed them selves from the start. Ducks and other shore and water birds exemplify this pattern. The babies may not be able to fly immediately, but often they can swim. For precocial birds, they have fledged when they can actually fly.
It is curious that there is such a difference in the stage of development of baby birds when they are born. This difference is a result of evolution. Nature has presented a problem to birds everywhere, namely “How do we keep our vulnerable young from getting eaten by some hungry predator?” (truly, the problem that faces all parents in the world). For birds, evolution has produced two quite elegant solutions, based on resource availability, and with pros and cons to each. Song birds migrate great distances to come to habitat with sufficient food resources and nesting areas. They have evolved to put their energy into simply getting here and getting down to the business of mating as soon as possible. Energetically that means that they put relatively little into their eggs, and the less you put into an egg, the less you are going to get out of it, hence the helpless altricial young. They are able to invest less bodily energy into egg production because once the young are born, they are able to collect plenty of food to feed them; the parents’ investment is on the outside of the egg shell. This is still a dangerous strategy, though, as finding a nest full of baby birds is like winning the lottery for a predator. It’s a concentrated source of nutrition, those tasty little niblets are all in one handy spot, the nest. Hence the speed with which these babies grow. Evolution has favored the fastest periods of in nest development, because the whole time they are in the nest, those baby birds are easy targets.
Precocial birds eliminate the possibility of having all of the young eaten in one predator attack, by dispersing them from the nest as soon as possible, mainly, as soon as they hatch. Individually they may be picked off, but they won’t all go together in a big gulp. The trade off for this in the physiological preparation of the parents. Their investment, particularly that of the mother, is on the front end of the nesting process; she must eat enough while developing the eggs to create a big fat juicy yolk laden egg. An egg like that contains enough nutrition to grow a baby bird that has feathers and run around as soon as it hatches. This strategy works really well, but obtaining that kind of nutrition on the front end is clearly not an option for many kinds of birds. The process of evolution has guided different bird species in different circumstances in different directions, and hatchling development is a perfect example of this. Happy Fledging Day everyone!
References:
Richard Sibly, et al, Energetics. Lifestyle and reproduction in birds PNAS, April 24, 2012 http://www.pnas.org/content/early/2012/05/17/1206512109.full.pdf
Mary Holland Naturally Curious 2010, Trafalgar Square Books—This fantastic book is also a popular blog and an email list serve: http://naturallycuriouswithmaryholland.wordpress.com/
Paul Ehrlich et al, The Birder’s Handbook: A field guide to the natural history of north American birds, 1988, Simon and Schuster. A classic, with detailed species accounts and a wide array of content essays.
One of the most comprehensive bird websites out there, from the Ornithology Lab at Cornell University http://www.birds.cornell.edu
The information about the evolutionary strategies of precocial vs. altricial young came from the Stanford University bird website: http://www.stanford.edu/group/stanfordbirds/text/essays/Precocial_and_Altricial.html They seem to have an active on campus birding community.
