Ideas about the ontogenesis of the nominative subspecies of the Sandhill Crane are limited to two student publications (Boice, 1977; Reed, 1988). Our observations were carried out on 5.06-17.08.1991 in the vicinities of the Ust’-Chaun (= Rytkuchi) Village (68°54`N, 170°43`E) at the southeastern extremity of the Chaun Bay (North-Western Chukotka, Chaun District of Chukotka Autonomous Okrug, Russia) on the 7×9 km area between the southeastern shoreline of the Chaun Bay in the North-West and the Palyavaam River in the South (Winter, 2002, 2005).
At an elevation of 30-50 m from the nest, an 80-cm bar with a wide ribbon and a double-sided number was set vertically. The nestlings were marked with multi-coloured fabric strips on one or on both tibiae, weighed on a lever pharmacy scale and measured with a vernier caliper.32 measurements and descriptions of 16 chicks from 10 families were carried out from hatching to their 44th day of life. During the incubation period, on June 6-27, 27 crane pairs were visited 44 times, chronometring their behaviour for 32 hours. After hatching, on June 21 – August 17, 23 pairs were visited 94 times, and their behavior was chronometred for 48.4 hours.
Changes in the colour of featherless body parts during ontogenesis are described.
As well as the Eurasian, White-Naped and Demoiselle Cranes, the tip of the lower mandible in the Sandhill downy chicks is split at hatching and during the first two days; by the end of the 3rd day the distal part of the left and the right mandibles coalesce together forming a smooth dorsal edge. Regressions of three indicators of beak growth are later represented by the second-order polynomials.
The rudimentary claws on the wing (Phalanx digiti alulae – 1st toe) and the end of the wing (Phalanx digiti majoris – 2nd toe) remained almost unchanged for 3 weeks, and by the 44th day (24-fold increase in body weight!) they increased by 1.4-2.0 times.
The growth of the metatarsus, toes and their claws is represented by the second-order polynomials.
In the individual variability of the Sandhill Crane downy chicks, the unusual shape of their head is clearly visible. In 75% of the observations, it was similar to the gladiator’s head in a helmet: the short lustrous smooth down that forms a kind of slightly convex glasses around eyes, sharply contrasting with the considerably longer down on the vertex and occiput, forming a rounded crest (similar to that in the Jay, Garrulus glandarius) which begins with a ledge on the occiput.
Probably, the embryonic down (praepennae) in some of the Sandhill Crane chicks grows longer than the first 2-4 days of life than in the White-Naped Crane (Ilyashenko, 2005).
No areas of down contrasting in colour with the rest of the head and ‘face’ were found in the nestlings of other above-mentioned cranes.
The Sandhill Crane downy chicks have contrasting grayish brown spots on the light yellow, ‘golden’ background of the lower part of the frons over the beak base (upper edge of the eye-stripe) as well as crescent-shaped spots in front and above, behind and below the eyes.
Another unusual feature of the Sandhill Crane downy chicks is the presence of the egg «tooth» not only on the upper mandible (44.4%), but also on the lower mandible (55.6% of the chicks).
In two chicks from one nest aged 2 and 3 days, two rows (in one chick incomplete) of light grey embryonic fluff proximally and distally on the “heel” (intertarsal joint) were found. This fluff was completely worn within a week. This sort of the apteria has not been described in the monograph on chickpterilography of the 10 bird orders of the world fauna yet (Ilyashenko, 2015).
The visible on the abdomen outer part of the oval yolk sac (Saccus vitellinus), oriented by the long axis along the chick body, as well as the suture of the sphincter that closed it, are surrounded with the apterium of the yolk sac (we called it Apterium vitellinum;Winter, Gorlov, 2019), which is a diamond-shaped structure with rounded angles and is not yet known for the cranes.
In the first 3 days of life, the yolk sac degraded quickly, but its size depended not only on age, judging by its size variation in three-day-old chicks. Apparently, its size depended much on the food availability in the nest surroundings. Chicks picked berries on their own from 13-14 hours old and, probably, their abundance or scarcity can be explained by such an uneven loss of the initial size of the yolk sack. This observation contradicts the “golden rule” for the Demoiselle and Eurasian Crane downy chicks, in which in the first days of chicks life, the younger one was heavier and had a larger yolk sac (Winter et al., 1999; Winter, 2008; Winter, Gorlov, 2019).
During the two-hour observations of 3 crane families on the 14-18th days of the chick life, we did not record the food passing “from beak to beak”. Unlike the first week of life, the adult birds only pointed with their beaks to the food the chicks should peck.
Comparison of the behavior of 5 crane species near the clutches and chicks showed that the Sandhill Crane behaved abnormally, since the distance between the observer and the adults displaying their anxiety at the nest ranged from 15 to 40 m! At the same time, there were no sexual differences in the adults behavior near the nest. The half-squat pose with half-open and roof-shaped wings, raised above the back, with the body (neck-tail) at an angle of 30-35 degrees to the ground, is very similar in the Sandhill, Hooded and Eurasian cranes, while the Demoiselle Crane had different distraction poses.
Flying around the observer by adult birds at the height of 30-60 m near the nest or chicks is observed in all crane species that we know, but in the Sandhill Crane this action is obviously socialized. As the unprecedentedly high for cranes nesting density in 1991 was 0.74 nest per 1km2 and the average distance between the nests was1082.1 ± 62.6 m (Winter, 2002), the pair's anxiety near the nest had a clear social impact and probably decreased the number of the destroyed neighbours’ nests.
A disturbed pair of birds, with screams in flight, followed the observer until he reached the boundary of the neighbouring nesting area. Then the neighbouring pair also left its nest and flew around the observer. If he had not followed that pair before, the search for their nest was often unsuccessful.
Marking nests and nestlings has enabled estimation of the distance of the crane families from their nests in 56 observations from hatching to being capable of flight. The distance was significantly different (ß>0.95) only between the chicks from hatching to 10 day old and those of 11-20 day old. The maximum distance of the family from its nest was 1000-1100 m and was probably determined by the fact that by the time of hatching the non-breeding territorial pairs (making nests but having no clutches in them) and the pairs whose clutches died had already left their nesting sites.