As Saunders explains in his study? abstract:
It is important to know the age of nestling birds for many ecological and behavioral studies. Various methods have been developed for individual species; most are based on measurements of growth in wings, tarsi or heads/bills, or observations of changes in size, plumage and behavior over time. However, techniques for aging nestlings have not been established for most avian species. This paper sets out two methods to age nestling Carnaby? cockatoo, Calyptorhynchus latirostris, an endangered species endemic to southwestern Australia.
The researchers point out that accurate nestling aging is essential for many ecological studies. The data could be used in investigating population dynamics, life histories, behavior, longevity, conservation planning and management. It could also help in scheduling the visits of breeding areas so that the disturbance for the populations is minimized without compromising the results.
One way that the researchers used to determine the age of the chick based on changes in the physical appearance of nestlings over the 10 to 11 week nestling period. The other relies on measurements of a nestling’s folded wing length and its comparison with growth curves from measurements of nestlings of known age. In their paper the Australian team also examines the timing and length of the egg-laying season. Their research is published in the journal Nature Conservation.
The scientists found out that observing the changes in a nestling’s size and feathers is less accurate than measuring the folded wing length. Its main disadvantage turned out to be the lack of distinguishable physical changes once the birds become about nine-weeks-old. However, “with experience it may be useful for gaining an approximation of the commencement and end of the breeding season without having to handle nestlings to take measurements,” the team said.
Their research on the egg-laying dates concluded that the most effective approach for examining nestlings is to conduct two visits per breeding season. Curiously, their findings showed that in wetter autumns the egg-laying begins earlier.
The team also suggests that their methods could be adopted for aging the currently under-researched closely related Baudin’s cockatoo until more species-specific technique is found.
Figure 1, from the study:
A) Week 1 (days 1?, with day 1 being hatching day): On hatching, Carnaby? cockatoo nestlings are covered in pale yellow down. They are blind, can sit unaided and have a prominent egg tooth. Note the size of the nestling in relation to the width of the hatched egg which is about 34.5 mm (Saunders and Smith 1981) B) Week 2: The nestling? eyes remain closed, it is still covered with pale yellow down with small developing dark pin feathers, the egg tooth is still present and, if touched the nestling will beg immediately. The scale in the foreground is numbered in cm C) Week 3: The nestling? eyes begin to open, pin feathers burst through the skin on all feather tracts, giving the nestling a greyish appearance because of the feather sheaths under the down. The egg tooth starts to disappear D) Week 4: Eyes are completely open, grey stripes become more prominent on the upper bill, down feathers are lost progressively as black feathers burst from their sheaths. The tail feathers begin to emerge and the cheek patch begins to appear E) Week 5: The cheek patch is now clearly visible and sexing based on colour and shape of the cheek patch is possible from this age (Saunders 1979b), most down feathers are gone and black feathers with scalloping are prominent. The remnant of the egg tooth is no longer visible F) Week 6: Tail feathers are a 2? cm long, down feathers continue to disappear, with body feathers almost full size and primary feathers extend almost to the tail. The small size of the cheek patch with darker suffusion and the non-circular shape indicates the nestling pictured is a male G) Week 7: Very few down feathers, white tail band starts to emerge, bill end sharpens and crest becomes more prominent. The dusky shading and non-circular shape of the cheek patch indicate the nestling illustrated is a male H) Week 8: White bands in tail feathers are 3? cm long, body feathers have a black sheen and are the same size as those of an adult, primary feathers are longer than the tail and some down feathers may be still be present. The size, clarity and more rounded shape of the cheek patch indicate the nestling illustrated is a female I) Week 9: White bands in tail are 5? cm long, down feathers no longer present, nestling now resembles a small adult. It may be aggressive when handled or when an observer checks its nest hollow. The dirty colour of the cheek patch indicates the nestling illustrated is a male J) Week 10: The size of the white bands in the tail feathers and the length of the primary feathers are close to those of adults. The nestling resembles an adult. It is capable of flight and if disturbed may fledge. The clarity of the cheek patch indicates the nestling illustrated is a female.