Post-Hatching Development of Posture and Behavior in the Barn Owl (Tyto alba): From a General Behavioral Pattern of Vertebrates to the Typical Owl Behavior

Abstract

Hatching captive barn owl chicks underwent four developmental phases. In the first 10 days (phase 1), behavior consisted of lateral movements that gradually changed to forward progression and peaked a few days before and after eye-opening. This behavior resembled a general developmental pattern that characterizes other vertebrates. Chicks also underwent a postural change, from head bent underneath the ventrum to resting on the rear end of the trunk with the head lifted in the air. Then, once their eyes were opened, chicks became more stationary and preoccupied with visual exploration, manifested in lateral head movements and peering episodes (phase 2, until day 20). The latter behavior, which is also typical of mature owls when not on the wing, characterized the chicks' behavior after post-hatching day 20 (phase 3), when their flight and contour feathers grew, along with shedding the down plumage and standing tall. Development culminated in active flight, first observed by days 50-60 (phase 4). Altogether, during post-hatching development, barn owl chicks gradually shifted from egg posture to the adult upright body posture. At the same time, their behavior consisted of speciation from the general developmental pattern to the typical stationary barn owl behavior, and, finally, flight was incorporated.

Keywords: fledglings; hatchlings; mobility gradient; motion parallax; peering; pivoting; warm up.

Figure 1

The three types of pivoting. Closed circle (●) represents the location of the imaginary vertical axis around which the chick pivots. In pivoting with a forward step of the contralateral leg (C), the chick rotated around the ipsilateral leg. In pivoting with a backward step of the ipsilateral leg (I), the chick rotated around the contralateral leg. In both these types of pivoting, the leg that was the imaginary axis of rotation rotated sideways while remaining in contact with the substrate without stepping. In pivoting with both forward stepping of the contralateral leg (C) and backward stepping of the ipsilateral leg (I), the chick rotated around the center of the pelvis.

Figure 2

(a) Lateral head movement based on angular lateral shift of the head. (b) Peering movements in which the head is transported sideways in parallel to its longitudinal axis, in this illustration first to the left (1), and then to the right (2). This enables the owl to take snapshots of a target (mouse in this illustration) from two directions, therefore providing by triangulation an accurate assessment of the distance to the mouse. While in peering (b) vision could be fixated on the target, a lateral head movement (a) allows it to scan the area.

Figure 3

Postures adopted during post-hatching days 1–10, before eye opening. Starting from an “egg posture” (Day 1), the head is extended forward (Day 2), and then lays first on its side (Day 3), and then stretches forward, allowing lateral head movements (Day 4). The rapidly growing trunk starts to adopt a diagonal posture (Day 6) so that the chick rests on the rear end of the trunk (Day 7). The head then becomes elevated from the substrate (Day 9), with the relatively large trunk still resting on its rear end (Day 10).

Figure 4

Representative postures of chicks immediately after hatching (top) and on post-hatching days, 4, and 10.

Figure 5

Representative postures in post-hatching days 11–20. When active, the head was up in the air and could make large amplitude movements (top left), although it could retain the “egg posture” when resting (top right). The chicks spent extended periods in visual exploration (bottom, right), and could rotate the head backwards almost 180° (bottom left).

Figure 6

Day 25 (top): Down feathers cover the trunk and the chicks sit on their tarsal–metatarsal segments. The frontal axis of the head is relatively narrow. Day 30 (center): chicks stand tall for the first time and their wingspans are long, with developing flight feathers. Day 40 (bottom): Flight and contour feathers cover the body. The head frontal (side-to-side) axis is considerably longer than on day 25.

Figure 7

On day 50 (top), the owl on the perch (right) swoops down from the perch to the floor (left). On day 60 (bottom), one owl is flying up from the floor to a perch (left) and another owl is flying up from the perch (right).

Figure 8

(a–d) In each inset, each set of color circles represent an individual chick, and the dashed red line depicts the medians. As shown, there was no consistency, and one chick that was very active one day could hardly move the next day.

Figure 9

Lateral head movements (a) and episodes of peering (b). The red dashed line depicts the median values for each day and the dots depict individual chicks.

Figure 10

The two basic types of lateral movements. The dashed line represents the median and the dots depict the performance of individual chicks.

Figure 11

Performance of the two types of pivoting over the 20 post-hatching days.

Figure 12

The cumulative amplitude of pivoting episodes as measured in units of 45° (left) and the cumulative distance of forward progression as measured by the owl’s body length (right) are depicted in post-hatching days 0–20 for all individuals. Median values are depicted by a dashed red line.

Figure 13

Shifting from pivoting to forward walk.

Figure 14

Warm-up sequence in an owl chick (top), a rat pup (center), and an adult hedgehog (bottom), each featuring a different morphology yet displaying the same behavioral sequence. The owl chick and the rat are shown from the top, whereas the hedgehog is shown in the bottom view via a transparent glass. On the left of each image, they are immobile initially, with the hedgehog notably arched to hide its head, legs, and ventrum. They then perform lateral head movements (side-to-side in the owl and hedgehog, but only to one side in the rat pup). The trunk is subsequently recruited to the lateral movements, and when steps are also incorporated, they pivot and then switch from pivoting to forward progression.