Monday, 24 March 2014

Young birds - how to determine their age

A young Striated Pardalote, less than one year old
On the last trip to study birds at Charcoal Tank Nature Reserve, NSW, over the weekend of 15-16th March, which was run by Mark Clayton as usual, we caught a variety of young birds. This was not surprising as it was only a month or so after the main breeding period for most birds in the area had ended, but we caught 26 species amongst the 119 birds caught altogether, and determining the age of birds can be tricky. So to help illustrate just how these young birds were identified, I have set out below some examples of the various methods that can be used to establish their age.

A front-on view of the same young pardalote
Immature Plumage -  The main, quickly seen features which were followed to determine this Striated Pardalote Pardalotus striatus as a young bird were the beginnings of only a few streaks on the crown and no white eyebrow behind the yellow flash above the eye. This bird will gradually moult out its first young-bird feathers and grow true adult-coloured feathers during its first year of life, with a fully streaked crwon and a wide white eyebrow.

A young Double-barred Finch
Feather development - To the trained eye, this Double-barred Finch Taeniopygia bichenovii is likely a young bird as it has narrower black bands across its breast than an adult would have. But that is a relative criterion, which is not of much use if there is not an adult to compare it with. More precisely, it can be aged as a young bird, less than one year old, by the incomplete covering of underwing feathers. In the second image, the flesh and line of the bones can be clearly seen because they are not fully covered like those in an adult bird. The most important feathers for a young  bird to grow are its body feathers for warmth, then flight feathers for mobility. The sooner they can leave the nest, the easier it is for them to escape from predators. So they gradually complete their body plumage in the less exposed parts, such as under their wings, as they continue to develop into full-grown birds. 

The partially bare underwing of the same young Double-barred Finch

A young White-plumed Honeyeater - in its first year
Pigmentation - this can be costly for a young bird to lay down, nothing comes without cost, even in nature. So young White-plumed Honeyeaters Lichenostomus penicillatus concentrate their nutritional resources in growing a basic bill, enough to serve the purpose of feeding, preening etc.. There is no need for deep pigmentation. That is added slowly during its first year of life, and by the following breeding season it will have a fully black shiny bill, a sign of an adult bird in breeding plumage.

This first-year White-plumed Honeyeater is probably a few months old and has begun to grow a dark bill,
 starting from the tip

Young Sacred Kingfisher - less than one year old
Buff-tips - many young birds have buff-tips to some of their feathers, such as this young Sacred Kingfisher Todiramphus sanctus, which has such feathers in its wing coverts and crown. Most of these will be moulted out during the first year, and the bird will be in adult breeding plumage in time for the next season. Some other species can retain a small number of these buff-tipped feathers into their second year while they are breeding, but if a Sacred Kingfisher has lots of such feathers between breeding seasons, then it can be confidently classified as a first-year bird. 

A back-view of the same kingfisher - this bird has a broken tail feather, snapped along the line of a fault-bar. This is a weak part of the bird's tail feather which is a result of less keratin being laid down in the feather during a period of less nutrition intake as the young bird's tail grew

A young rainbow Bee-eater
Ornamental feathers - or lack of.  Some species of birds grow elaborate ornamental feathers as part of their adult breeding plumage, like the Rainbow Bee-eater Merops ornatus which have extended central tail-feather streamers. The young birds however, do not grow these. To grow such feathers for no purpose in their first year, as they are only required for breeding display, would be an excessive waste of nutrition. 

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