Peregrine falcon chicks change rapidly as they grow from hatch to fledge in approximately 40 days.
Here are illustrations of their weekly development using photos of peregrines at the University of Pittsburgh's Cathedral of Learning and the Gulf Tower in Downtown Pittsburgh.
At Hatching: 35-40 grams, feeble, damp, pink, sparse down, eyes closed except when begging, open eye is slit-like.
At 5 days: Weight has doubled since hatch day, sits up, open eye is round. No second down yet.
At 6 days: Second down begins on wings (humeral and alar tracts, dorsal surface of wing).
At 7 Days (1 week): Second down begins on abdomen and legs (femoral and crural tracts), chicks form a huddle in the nest scrape, can sit up but still wobbly. Sleep in a pile.
At 8 Days: Second down begins on spinal tract. Primary (wing) feathers begin.
At 10 Days: Second down complete. Rectrices (tail) feathers begin.
At 12 Days: Ear is distinct. After Day 13 can age the chick based on length of rectrices which emerge 2 mm/ day.
At 14 Days (2 weeks): Second down is long and fluffy. Pin feathers begin to emerge at wing tips and tail (might not be visible on camera). Chicks move off nest scrape and walk around on tarsi, sleep and eat. They sit like white Buddhas.
At 15 Days: Sits upright, alert. All primaries emerged from sheaths.
At 20 Days: Heavy down. Contour feathers visible on wing edges. Gets a "face."
At 21 Days (3 weeks): (Camera is zoomed out because chicks are very active.) Feathers now define the face, feather tips quite noticeable on wings and tail. Chicks very active on gravel surface. Often sleep individually instead of in a pile.
At 25 days: Begins to stand and walk on feet, still rests on tarsi. Body contour and back feathers visible.
At 27 days: Regularly walks on feet rather than tarsi.
At 28 Days (4 weeks): Body feathers give the chicks a speckly look that camouflages them on the nest. They beg loudly. Wings look longer and fuller as wing feathers grow. Chicks open wings and run across gravel surface. Chicks have not left the nest surface yet. In the week ahead their feathers will grow rapidly, pushing out the down which they will pick off as they preen.
At 30 days: Half down, half feathers. Face is pronounced.
At 35 Days (5 weeks): Chicks are brown and cream colored with some down patches on wings, on top of head and on "pantaloon" legs. The youngest chick is obvious because it has more down. (All primary and secondary feathers still blood-rooted and growing.) Chicks now perch and flap wings for exercise. Chicks "ledge walk" off camera and might not return to the nest at night. Parents perch out of reach above the nest. Chicks beg loudly and snatch incoming food from parents. In the coming week they will fly from ledges that cannot be seen on camera.
At 38-45 Days: Fully feathered except for bits of fluff on top of head and under wings. Peregrine chicks make their first flight, approximately Day 40. The oldest males fledge first, then the females and youngest. Youngest chick is alone on the nest as the others walk away. After they fly they are permanently off camera. Fledglings now perch on other ledges and other buildings.
The young stay with their parents for part of the summer while they learn to hunt. Before the end of summer they leave home forever.
(photos from the National Aviary falconcams at Univ of Pittsburgh and the Gulf Tower and by Kimberly Thomas)