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(a) Visual field of a European starling, Sturnus vulgaris. Light grey indicates where one eye can see, dark grey indicates where both eyes can see (i.e. the binocular field) and black indicates where neither eye can see (i.e. the blind area). Reproduced from Martin (1986). (b) Body orientations when birds are in parallel and antiparallel orientation. (c) Division of the visual field of European starlings based on Martin (1986) and Dolan and Fern andez-Juricic (2010). We divided the visual field into five categories: (A) the resting binocular field; (B) front periphery, the region in front of the most converged position of the fovea and the binocular field; (C) foveal area, the sector between the most diverged and converged position of the fovea; (D) rear periphery, the sector behind the most diverged position of the fovea and the blind area; and (E) the blind area. 

(a) Visual field of a European starling, Sturnus vulgaris. Light grey indicates where one eye can see, dark grey indicates where both eyes can see (i.e. the binocular field) and black indicates where neither eye can see (i.e. the blind area). Reproduced from Martin (1986). (b) Body orientations when birds are in parallel and antiparallel orientation. (c) Division of the visual field of European starlings based on Martin (1986) and Dolan and Fern andez-Juricic (2010). We divided the visual field into five categories: (A) the resting binocular field; (B) front periphery, the region in front of the most converged position of the fovea and the binocular field; (C) foveal area, the sector between the most diverged and converged position of the fovea; (D) rear periphery, the sector behind the most diverged position of the fovea and the blind area; and (E) the blind area. 

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Copying others can be used to enhance foraging and mating opportunities, but can be costly due to the need to monitor the actions of others, which can take time away from foraging and antipredator vigilance. However, little is known about the way animals monitor conspecifics. We investigated the mechanism that European starlings, Sturnus vulgaris,...

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... these head movements. We included all head movements that followed the above criteria, even very small ones, meaning that we did not have a 'cutoff' amplitude. C.H. also recorded the distance between the birds (in cm), and whether their bodies were oriented in parallel (i.e. in the same direction) or antiparallel (in opposite directions) (Fig. 1b) relative to each other. Five people coded other aspects of the videos. After S.R.B. engaged in self-training, and once she and the other four individuals were 95% consistent with each other in identifying the end of head movements within two frames (at 30 frames/s) of each other, they identified the end of all the head ...
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... that can move their eyes, such as European starlings (Martin, 1986;Tyrrell et al., 2015), have a dynamic visual field: the size of the binocular field, the lateral field and the blind area changes depending on whether the eyes are at rest or are converged or diverged (Fig. S1). We used this sensory information to divide the focal visual field into five categories, while assuming that the eyes were at rest (Fig. 1c): resting binocular field, resting blind area, foveal area, front periphery and rear periphery. We assigned the resting binocular and resting blind area based on the measurements of the binocular ...
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... 1986;Tyrrell et al., 2015), have a dynamic visual field: the size of the binocular field, the lateral field and the blind area changes depending on whether the eyes are at rest or are converged or diverged (Fig. S1). We used this sensory information to divide the focal visual field into five categories, while assuming that the eyes were at rest (Fig. 1c): resting binocular field, resting blind area, foveal area, front periphery and rear periphery. We assigned the resting binocular and resting blind area based on the measurements of the binocular field and blind areas with the eyes at rest (Fig. 1a). This means that in the area labelled 'resting blind area', starlings can actually ...
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... to divide the focal visual field into five categories, while assuming that the eyes were at rest (Fig. 1c): resting binocular field, resting blind area, foveal area, front periphery and rear periphery. We assigned the resting binocular and resting blind area based on the measurements of the binocular field and blind areas with the eyes at rest (Fig. 1a). This means that in the area labelled 'resting blind area', starlings can actually gather some visual information when their eyes are diverged (Fig. S1). Although we made the assumption that the eyes were at rest, the fovea is a small spot on the retina and using just that projection would correspond to a point rather than a region in ...
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... foveal area, front periphery and rear periphery. We assigned the resting binocular and resting blind area based on the measurements of the binocular field and blind areas with the eyes at rest (Fig. 1a). This means that in the area labelled 'resting blind area', starlings can actually gather some visual information when their eyes are diverged (Fig. S1). Although we made the assumption that the eyes were at rest, the fovea is a small spot on the retina and using just that projection would correspond to a point rather than a region in the visual field. However, starlings move their eyes (and therefore their fovea) often ( Tyrrell et al., 2015), so we defined the foveal area as the ...
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... that projection would correspond to a point rather than a region in the visual field. However, starlings move their eyes (and therefore their fovea) often ( Tyrrell et al., 2015), so we defined the foveal area as the sector of the visual field subtended by the fovea between the maximally converged and maximally diverged posi- tions of the eyes (Fig. S1). Finally, we assigned the regions in front of and behind the foveal area as the front and rear periphery, respectively. We also used these angles to calculate the magnitude of each of the focal's head movements by subtracting the final position from the initial angle for each head ...
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... latency for the focal to make a head movement after the nonfocal's head movement was significantly influenced by the event preceding the focal head movement (nonfocal head move- ment or random frame) (Table 1, Fig. 2a). More specifically, the average latency was significantly shorter than that predicted by chance (i.e. ...
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... found that the behavioural response of the focal differed in latency and magnitude depending on the sector of visual field that the focal had used to orient towards the nonfocal, after controlling for the effects of nonfocal distance and sex (Tables 1, 3, Fig. 4c,d). Latency was the shortest (i.e. ...
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... when focals directed their resting blind area towards the nonfocals, they made a greater head movement than when any other region of the visual field was oriented towards the nonfocal (Table 3, Fig. 4d). The category of visual field had a medium effect size on both la- tency and magnitude (Table 1, 3). ...

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... Many studies have examined the importance of neighbours for foraging behaviour. Some studies have found that birds foraging in a head-down position can monitor the activities of neighbours within a limited range, and make behavioural decisions based on the presence and activities of nearby neighbours only (Butler, Hosinski, Lucas, & Fern andez-Juricic, 2016; Fern andez-Juricic, Beauchamp, & Bastain, 2007;Lima & Zollner, 1996). For example, multiple neighbours fleeing could give a strong signal of danger and instigate a focal individual to cease foraging, to take a vigilant posture or flee (Lima, 1995). ...
... At any given instant, an individual goose could only monitor its neighbours within its field of view. Butler et al. (2016) showed that European starlings, Sturnus vulgaris, monitor their neighbours with low acuity vision that includes about half of the 360º field of view. Even if this were true for geese, then the number of neighbours under surveillance in this study should be approximately half of the numbers actually present, which would not change our main results. ...
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The idea that animals gain a higher food intake rate when foraging in larger groups has been reported in many studies. However, some studies suggested that the number of neighbours under surveillance may play a more important role in affecting food intake rate than flock size. In addition, the effect of the number of neighbours should also depend on the position of an individual within a flock. To test these hypotheses, we examined the effects of flock size, number of neighbours and position within a flock on foraging time, foraging efficiency and intake rate of wintering greater white-fronted geese, Anser albifrons. We observed the foraging behaviours of 490 individual geese from 71 flocks during December 2016–March 2017 at Shengjin Lake, Anhui, China. We found that mean foraging time, foraging efficiency and intake rate were not influenced by flock size, whereas, at the individual scale these variables were significantly affected by the number of neighbours and position within a flock. Moreover, the effect of the number of neighbours on the foraging parameters did not differ between central and peripheral individuals, despite central individuals always having greater foraging time, foraging efficiency and intake rate regardless of the number of neighbours. Both the decreased foraging time and decreased foraging efficiency indicated a potential increase in cryptic competition from neighbours. More neighbours contributed to a decreased intake rate. Our study highlights the possible effects of cryptic competition among neighbours on their foraging behaviour. We hypothesize that an increase in intraspecific competition between neighbours in areas of shrinking wetland habitats may contribute to population declines of wintering geese and other wildfowl.
... As group size increases, animals can adjust their behavior decisions based more upon social information acquisition to compensate for the reduction of personal anti-predator vigilance, which would enable them to exploit food discoveries by other group members or even avoid more dominant individuals (Danchin et al. 2004, Dall et al. 2005, Magrath et al. 2015. Existing studies examining the relative importance of anti-predator vigilance and social vigilance were mainly conducted in mammals where head orientation has been used as a surrogate of target of vigilance (Beauchamp 2017b, Allan andHill 2018), and some small bird flocks in experimental conditions where social vigilance was speculated from head movements (Butler et al. 2016). However, when animals form large aggregations, it is difficult to determine the targets of vigilance. ...
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... As a result, birds move their heads rapidly to align their centres of acute vision with objects of interest (Dawkins, 2002;Moore, Tyrrell, Pita, Bininda-Emonds, & Fern andez-Juricic, 2017). None the less, changes in the head orientation of birds can be indicative of visual exploration and visual fixation behaviours (Butler, Hosinski, Lucas, & Fern andez-Juricic, 2016;Dawkins, 1995;Fern andez-Juricic et al., 2011), and a few avian studies have used qualitative scoring of head orientation to assess the response to conspecific alarm signals. On hearing an alarm call indicating a predatory threat overhead, domestic hens, Gallus gallus domesticus, rotated their heads, probably to make use of their lateral vision (Evans, Evans, & Marler, 1993). ...
... To associate changes in head orientation with more specific changes in gaze direction, it is necessary to know the position of the centres of acute vision (e.g. foveae) in the retina to project them into the visual space (Butler et al., 2016). This is important because of large interspecific variation in the position of the centres of acute vision in birds (Moore et al., 2012), even between closely related species (Moore, Pita, Tyrrell, & Fern andez-Juricic, 2015). ...
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... In several cases, low-cost vigilance is devoted mainly to social contexts (e.g., Coolen & Giraldeau 2003, Monclús & Rödel 2008 while high-cost vigilance is devoted mainly to antipredator contexts (e.g., Coolen & Giraldeau 2003, Monclús & Rödel 2008, Favreau et al. 2013. Some studies also consider gaze orientation (acute vision) to fine-tune the distinction between lower-cost social vigilance (Butler et al. 2016) and higher-cost antipredator vigilance (Butler & Fernández-Juricic 2018). Even short open-eyed periods during sleep allow visual alertness in incubating birds (Javůrková et al. 2011). ...
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... To reduce stress, each bird participated in trials every other day, at most. Animals that participated in this study were also part of another experiment where we assessed their scanning behavior when perching in pairs [19], but we do not believe that this experiment affected their behavior in the current study. All animal capture, care, and use procedures were approved by the Purdue Institutional Animal Care and Use Committee (IACUC protocol 1306000876). ...
... Given these technological limitations, we chose to use head movements as a proxy of fixation behavior. Head movements are actually a commonly used proxy of fixation and gaze shifts in birds [5,10,19,[31][32][33]. ...
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... It has also been suggested that by synchronizing their vigilance bouts, individuals could reduce the risk of being left behind during an attack, which could be dangerous if predators preferentially target laggards (Sirot and Touzalin, 2009). Vigilance synchronization has been documented in several species of birds and mammals, suggesting that monitoring neighbors is common in prey species (Fernández-Juricic et al., 2004;Pays et al., 2007aPays et al., ,b, 2009Pays et al., , 2012Beauchamp, 2009;Ge et al., 2011;Michelena and Deneubourg, 2011;Öst and Tierala, 2011;Butler et al., 2016;Podgórski et al., 2016). ...
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