Cassin's Kingbird Madrona Marsh Torrance Southern California -171
"The first step in determining what species of young bird you see is to be sure it is, in fact, a juvenile bird. Some adult birds look similar to fledglings, but these characteristics can help you be certain that you are seeing juvenile birds.
Bill Size: Juvenile birds often have bills that seem proportionally too large for their head. This is because their heads have not fully grown and their feathers are not yet covering the edges of the bill, making it seem larger.
Feather Length: Young birds have not grown full flight feathers, and the feathers on their wings and tail will be noticeably stubbier than on adult birds. The feathers on their breast, flanks, and abdomen may also seem fluffier and less organized than those of adult birds, not only because they're not fully grown, but also because young birds do not preen as effectively as adults. Some feathers may be so underdeveloped that the birds even show bare skin, especially on the face.
Color: Many juvenile birds have drab, dull plumage similar to that of an adult female. As they molt, their feathers may have additional speckles, buff edges, or other disorganized colors that help camouflage them until they can fly skillfully.
Eye Size: Very young fledglings and nestlings have bulging eyes that seem too big for their head. This should not be strongly noticeable in birds that are ready to leave the nest, but if the birds were displaced very early their eye size can indicate their youth.
Behavior: Young birds often try to draw the attention of their parents. They may beg for food or flutter their wings, especially when near adults. They may also be uncertain in flight or may visit feeders but seem unsure of how to eat what is offered. It should be noted, however, that many juvenile behaviors, including fluttering and begging, are similar to the courtship of adult birds. Behavior alone is not a sure indication of whether or not a bird is juvenile, but it can be a valuable clue."
thespruce.com
Cassin's Kingbird Madrona Marsh Torrance Southern California -171
"The first step in determining what species of young bird you see is to be sure it is, in fact, a juvenile bird. Some adult birds look similar to fledglings, but these characteristics can help you be certain that you are seeing juvenile birds.
Bill Size: Juvenile birds often have bills that seem proportionally too large for their head. This is because their heads have not fully grown and their feathers are not yet covering the edges of the bill, making it seem larger.
Feather Length: Young birds have not grown full flight feathers, and the feathers on their wings and tail will be noticeably stubbier than on adult birds. The feathers on their breast, flanks, and abdomen may also seem fluffier and less organized than those of adult birds, not only because they're not fully grown, but also because young birds do not preen as effectively as adults. Some feathers may be so underdeveloped that the birds even show bare skin, especially on the face.
Color: Many juvenile birds have drab, dull plumage similar to that of an adult female. As they molt, their feathers may have additional speckles, buff edges, or other disorganized colors that help camouflage them until they can fly skillfully.
Eye Size: Very young fledglings and nestlings have bulging eyes that seem too big for their head. This should not be strongly noticeable in birds that are ready to leave the nest, but if the birds were displaced very early their eye size can indicate their youth.
Behavior: Young birds often try to draw the attention of their parents. They may beg for food or flutter their wings, especially when near adults. They may also be uncertain in flight or may visit feeders but seem unsure of how to eat what is offered. It should be noted, however, that many juvenile behaviors, including fluttering and begging, are similar to the courtship of adult birds. Behavior alone is not a sure indication of whether or not a bird is juvenile, but it can be a valuable clue."
thespruce.com