Behavioral observation

The behaviors were recorded with a surveillance system (4 mm infrared network cameras, DS-2CD3210D-15, hardware recorder, DS-7816N-E2, Hangzhou Hik-vision Digital Technology Co., Ltd., Hangzhou, China) located at the opposite side of the cages which was 0.5m away from the target cages with full view of the cages. All hens from each cage were observed for behavioral collection and leg tags with 6 different colors (white, blue, red, yellow, orange, and green) were used for individual identification. The behavioral observation was started on the fourth day after 3 days acclimation to the new cage environment. Aggressive behavioral data were collected from 14 to 18 weeks of age for two periods of the observation time (08:30 to 10:30 h and 13:00 to 15:00 h) in each observing day (2 days/week), and each group was observed for 40 hours. The aggressive behavior parameters included pecking, displacing, chasing, and threatening and are listed in Table 1 and they were recorded with continuous recording method [18,19]. Clutton-Brock index (CBI) of fighting success was used to determine the SRO of an individual among a cage [20]. All aggressive behaviors of each hen over two observational periods were pooled and recorded as the total number of its win (W) or loss (L).

Behavioral categories and definitions

Behavioral definitions from Appleby [18]; Shimmura et al [19].

According to the results of CBI of each individual, the social orders of the individual laying hens in each group were determined according to the methods of Shimmura et al [19]. Briefly, the 1st and 2nd higher CBI index hens were regarded as the high SRO hens (HSR), and the 3rd and 4th higher CBI index hens were as the medium SRO hens (MSR) and the 5th and 6th CBI index hens were as the low SRO hens (LSR). Therefore, the experiment was a design of 2×3 factors (2 levels of resource allocations and 3 levels of SRO).

The study mainly observed the perching, dustbathing and also observed general behaviors (listed in Tables 2 ​toto ​4)4) were sampled from 14 to 18 weeks of age and were recorded continuously for two periods (08:30 to 10:30 h and 13:00 to 15:00 h) in each observing day (2 days/week). For perching, dust-bathing behaviors and general behaviors, continuous recording and one-zero sampling were used for the focal hens on the perches and at the dust-bath. The state behaviors were represented in percentages (converting the behavior data into a percentage of the total observation time.), for the event behaviors, each incident behavior was recorded as one time, which was expressed by frequencies (the total number of occurrences per minute) [21]. The perching behaviors and the dustbathing behaviors were recorded separately and divided into event behaviors and state behaviors. Perching event behaviors were included preening, staring, exploring and comforting behavior, perching state behaviors were included standing, lying and walking behavior. Dustbathing event behaviors were included bill raking, pecking, vertical wing shaking, side scratching, vigorous body shaking and body movement behavior, dustbathing state behaviors were included head forward and head under wing behavior. The general behaviors were also divided into two categories. One was general state behaviors which included feeding, standing, lying, walking, perching and dustbathing, and the other was general event behaviors which included drinking, preening, staring, pecking, comforting, head shaking feather pecking and escaping behavior. All behavioral observation was conducted by the two experienced technicians.

Definitions of perching behavior

Brender et al [38]; Casey-Trott and Widowski [39].

Definitions of dustbathing behavior

Brender, et al [38]; Casey-Trott and Widowski [39].

Definitions of general behavior

Brender et al [38]; Casey-Trott and Widowski [39].