What is the difference between a fledgling and a juvenile? (1 Viewer)

My ornithology book defines the fledgling period as, "the time interval between hatching and flight from the nest; may be the same as the nestling period."

When does a fledgling become a juvenile, and what marks this transition?

Mmmm.... I've always understood a fledgling to be a bird just out of the nest and not yet able to fly (in passerines).

There's some explanation regarding this and the confusion between immature and juvenile terms in the Opus Dictionary, which you might like to look at too.

Yes, "fledgling" definitely refers to a young bird out of the nest (to fledge is to leave the nest), but beyond that I suspect that these terms are not strictly defined, in the sense of clear demarcations that mark the end of these periods. I generally use the term fledgling to mean out of the nest, may or may not be capable of flight, but still largely dependent on parents for food. In this stage you could also correctly call it a juvenile, although the juvenile term might be extended through late summer/early fall even after the bird has reached independence... although you could instead use the term "hatch year" at this point.

My experience is that different books define these terms somewhat differently.

Niels

If I remember correctly, juvenile refers to a plumage and not really an "age". It is the first "real" plumage a bird has after leaving the nest. Most small passerines will quickly start moulting this plumage and thus aren't really juveniles for very long. After the moult they can be referred to as hatch-year birds, but not juveniles. Other birds may have different moult strategies. However, it seems not everyone agrees with this definition...

A fledgling for me is a bird that has just left the nest but is still dependent on it's parents. I don't believe it is very defined.

Thanks, all. Very interesting.

I would use fledgling only for birds that seem to have just left the nest. A small passerine that looks unkept like someone right out of bed before first coffee, hair uncombed, you know the look. Maybe vestiges of yellow on beak/gape base? Fluttering feebly, begging food from adults that land nearby. Equivalent of preschool/kindergarten child.

For juveniles I would imagine a flying gull with completely brown feathers, or duck past the fluffy duckling phase, or a magpie that already looks like adult (not as something that fell out of nest this morning) but with short tail; equivalent of elementary school child/pre-teen.

The most literal meaning of "fledge", going back to the origins of the word (from the same root as modern German "flug"), is "become ready to fly". A related meaning, almost as old, is "to grow feathers" as in its variant spelling "fletch". A bird is "fledged" when it has flight feathers; since feather growth is a gradual process, a bird may not be "fully fledged" for a little while. Take chickens, ducks and geese, examples which will have been quite familiar to 16th and 17th century Englishmen. The hatchlings are covered in fluff; when they start gaining real feathers they become fledglings. Note that the feathering process does not necessarily correlate with the timing of leaving the nest: these three types of birds all can be found outside the nest while still hatchlings.
"Juvenile" is a general term for youngster; some birders use it to refer to a particular plumage but don't always agree which one; others invent terms like "juvenal" to try to avoid ambiguous terms. "Fledgling", however, is pretty clear: it's a bird growing (or just grown) its first real feathers, before any feather molt. If it's still awkward and babyish, it's a fledgling. Whether that ends as soon as all the feathers are in, or whether there has to be a molt before a fledgling becomes something else, I couldn't say.