Measuring range use

There are a number of ways in which range use can be measured on a commercial farm and the method used depends on access, resources and the question being addressed. Perhaps the most popular method employed is to count or estimate the number of birds on the range (or a subsection of it) at a point in time (either once or repeated and an average taken). To account for factors such as age and climate, counts may be repeated throughout the flock cycle and various weather conditions. This method of assessing range use is usually presented as a percentage of all birds in the flock so for clarification will be referred to as ‘percentage range use’ throughout this review. Farmers themselves may sometimes make these estimates instead of researchers.

Estimated percentage range use does not give information on the time spent outside by individual birds. A given overall percentage range use could reflect occasional usage by all or most birds, or frequent usage by a small minority. This information is important in establishing how many birds receive the direct benefits associated with ranging and whether birds inside the house benefit indirectly through reduced internal stocking densities. To obtain this information, individual birds must be tracked. This is technologically difficult and expensive although some work has been done using radio frequency identification (RFID) tags to assess pophole use by a subset of birds (Richards et al., Reference RICHARDS, WILKINS, KNOWLES, BOOTH, TOSCANO, NICOL and BROWN2011; Gebhardt-Henrich et al., Reference GEBHARDT-HENRICH, TOSCANO and FROEHLICH2014). Development of more readily accessible and improved methods is needed in this area as the current technology can produce skewed data if birds sit in the popholes causing repeated counts on the RFID recording system (Richards et al., Reference RICHARDS, WILKINS, KNOWLES, BOOTH, TOSCANO, NICOL and BROWN2011). Evidence using RFID tags suggests that although the percentage range use is often below 40%, a higher proportion of birds use the range at least once during the day.

Consumers may assume that most birds will use the outdoor area each day, and for a significant proportion of time. However, to provide consumers with reassurance we must first understand how long individual birds spend on the range under ‘ideal’ baseline conditions i.e. when both the internal house environment and external range are managed according to current best practice. Knowing whether individual hens choose to range for 1 h/day or 6 h/day under these ideal conditions, would permit a better interpretation of scan samples of percentage range use. Another valuable measure is the distribution of birds in relation to the house. For example, Hegelund et al. (Reference HEGELUND, SORENSEN, KJAER and KRISTENSEN2005) counted birds at close, middle and remote distances from the house.

Prevalence of range use

Range use has been measured in multiple studies, with varying scope, flock sizes and consequent results. Table 1 indicates that often fewer than half of the birds in commercial flocks use the range at any one time, sometimes less than 10%. Taken together with the figures for individually tagged birds it appears that a higher percentage access the open range at least once a day. There is a need for future research studying ranging bout lengths to test this.

Table 1

Summary of the literature where figures for range use have been reported.

Factors influencing range use

The likelihood of hens ranging on a given farm is influenced by both motivation and physical ability. Motivation is influenced by both internal and external causal factors (for a discussion of the concept of motivational causation see Jensen and Toates, Reference JENSEN and TOATES1993). For example, the tendency to use the range might be increased by an increased desire to forage, provoked by a combination of falling glucose levels (internal factor) and the sight of dry, friable dirt on the range (external factor). However, even highly motivated hens may be physically blocked from accessing the range if house design is inappropriate or if birds are in poor health. The high prevalence of keel bone fractures in laying hens (including those in free-range systems) is relevant in this regard (Wilkins et al., Reference WILKINS, BROWN, ZIMMERMAN, LEEB and NICOL2004; Reference WILKINS, MCKINSTRY, AVERY, KNOWLES, BROWN, TARLTON and NICOL2011). Birds with keel fractures have reduced mobility, finding it more difficult to jump down onto the ground (Nasr et al., Reference NASR, MURRELL, WILKINS and NICOL2012) and use the popholes less (Richards et al., Reference RICHARDS, WILKINS, KNOWLES, BOOTH, TOSCANO, NICOL and BROWN2012).

Motivation to range has a diurnal pattern with peaks in the morning and early evening (Bubier and Bradshaw, Reference BUBIER and BRADSHAW1998; Mahboub et al., Reference MAHBOUB, MÜLLER and VON BORELL2004; Hegelund et al., Reference HEGELUND, SORENSEN, KJAER and KRISTENSEN2005; Richards et al., Reference RICHARDS, WILKINS, KNOWLES, BOOTH, TOSCANO, NICOL and BROWN2011; Nagle and Glatz, Reference NAGLE and GLATZ2012). By opening the popholes late or closing them early (due to bad weather or the early training period before 21 weeks) producers may restrict ranging by preventing access during prime ranging periods.

Most studies of ranging behaviour have used brown hybrids (Table 1). Mahboub et al. (Reference MAHBOUB, MÜLLER and VON BORELL2004) looked at differences in the use of an outdoor area between white and brown hybrids, finding that brown hybrids spent more time outside but white hybrids moved more frequently to this area. This suggests that there may be differences in ranging behaviour between genetic strains although this work was on small groups of 50 birds so cannot be easily generalised to commercial conditions.

Internal stocking density, flock size and house size

There is some evidence that internal house stocking density affects range use in commercial free-range hens. Gilani et al. (Reference GILANI, KNOWLES and NICOL2014) conducted a study on the effect of various factors on percentage range use in 33 flocks and produced predictive models of ranging behaviour. Percentage range use was significantly higher with reduced flock size and stocking density (lay period only). Stocking density is a function of both flock size and the area available. As these three factors are closely linked it can be difficult to differentiate which is causing any effects on percentage range use. Houses with different stocking densities differ in either flock size or house size, making it hard to test the effect of stocking density alone. Each of these three factors should be considered when discussing the effect of one. It should be noted that as birds range, the stocking density in the house will decrease.

Stocking density will not necessarily have the same effect at different flock sizes. Small flocks kept in small houses may achieve relatively even distributions of hens due to the confining effects of walls although some clustering around resources may occur (Collins et al., Reference COLLINS, ASHER, PFEIFFER, BROWNE and NICOL2011; Lentfer et al., Reference LENTFER, GEBHARDT-HENRICH, FRÖHLICH and VON BORELL2013). However, in larger houses with larger flocks (at the same stocking density) birds can move greater distances from each other, causing increased clustering and uneven stocking densities around the house. Clustering in larger pens (as discussed in Appleby, Reference APPLEBY2004) results in reduced freedom of movement for hens in certain areas of the house compared with others.

The effect of stocking density on hen movement was investigated by Carmichael et al. (Reference CARMICHAEL, WALKER and HUGHES1999) in an early multi-tier system. Birds spent less time moving and more time standing at 19.0 birds/m² compared with 9.9 birds/m², suggesting that movement becomes more difficult with increased crowding. Appleby et al. (Reference APPLEBY, HUGHES and HOGARTH1989) found that locomotion decreased linearly with increased stocking density from 3.4 to 10.7 birds/m². High stocking densities could therefore limit the ability of birds to move to the popholes and get onto the range area. Whether this effect was caused by physical blocking or social factors is not so clear. Grigor et al. (Reference GRIGOR, HUGHES and APPLEBY1995c) found that hens were slower to enter a test pen when passing an unfamiliar bird and the latency to enter the test pen increased with the number of unfamiliar birds. High stocking densities may force more unfamiliar birds into close proximity, limiting movement. However, in commercial sized flocks hens are unable to discriminate between individuals and do not form a traditional hierarchy (Pagel and Dawkins, Reference PAGEL and DAWKINS1997; D'Eath and Keeling, Reference D'EATH and KEELING2003). Instead hens respond to physical attributes of conspecifics to detect potential aggressors (D'Eath and Keeling, Reference D'EATH and KEELING2003). Perhaps the movement of commercial hens could therefore be limited by being forced into close proximity with such ‘aggressor hens’ although this is speculation and work is yet to be done in this area.

Internal stocking density is usually relatively tightly specified by law (e.g. EU max 9 birds/m²; Council Directive 1999/74/EC), making commercial studies of its effects difficult. Flock size however, is more variable and evidence suggests that flock size affects ranging behaviour. Bestman and Wagenaar (Reference BESTMAN and WAGENAAR2003) studied 63 Dutch organic flocks and found flock size was a highly significant factor, with smaller flocks having higher percentage range use (all under optimal ranging conditions). Hegelund et al. (Reference HEGELUND, SORENSEN, KJAER and KRISTENSEN2005), in agreement, reported a tendency for lower percentage range use with increasing flock size. The lack of statistical significance may be because few flocks over 3000 birds were included in the study. Whay et al. (Reference WHAY, MAIN, GREEN, HEAVEN, HOWELL, MORGAN, PEARSON and WEBSTER2007) examined larger free-range flocks (3000-16000) and found that bigger flocks had lower percentage range use. Appleby and Hughes (Reference APPLEBY and HUGHES1991) made unpublished observations of reduced percentage range use in flocks exceeding 1000 birds. Conversely, in a study using individually tagged birds to assess ranging, no association was found between the percentages of birds registered outside and flock size (Gebhardt-Henrich et al., Reference GEBHARDT-HENRICH, TOSCANO and FROEHLICH2014). Despite this, hens from small (2000-2500 birds) and medium (5000-6000) sized flocks visited the outside area more frequently and for longer than hens in large flocks (9000+). Additionally, foraging behaviour on the range was more frequent and extensive in small and medium flocks. This suggests that flock size may affect behaviour on the range as well as whether hens leave the house. Currently, no legal maximum flock size for free-range units exists in most parts of the world, although the Lion Code (BEIC, 2013) and RSPCA Assured (RSPCA, 2013) standard in the UK both set a limit of 16,000 birds (in colonies of 4000 maximum).

House area, shape and size may also affect range use by increasing the distance needed to travel to reach a pophole. The interior contents of the house will further affect this and will be discussed in detail later in this review.

Pophole number, size and design

Popholes usually provide the sole access point to the range, and ensuring suitable design and management of these openings is important to encourage good range use. Minimum pophole availability in the EU as set by council directive 1999/74/EC (1999) is 2 m per 1000 hens (35cm high x 40cm wide). Gilani et al. (Reference GILANI, KNOWLES and NICOL2014) measured pophole availability (cm/bird) for 33 commercial flocks, and found that percentage range use significantly increased with availability (average of 0.5 cm/bird, range of 0.1-1.9 cm/bird). Sherwin et al. (Reference SHERWIN, NASR, GALE, PETEK, STAFFORD, TURP and COLES2013) found that that percentage range use increased with the number of popholes available per bird, although actual figures were not provided. Hens have been seen to perch for periods of time in the popholes (Richards et al., Reference RICHARDS, WILKINS, KNOWLES, BOOTH, TOSCANO, NICOL and BROWN2011) consequently reducing the available pophole space and range access for others.

Conversely, a study looking at the effect of pophole dimensions on range use (Harlander-Matauschek et al., Reference HARLANDER-MATAUSCHEK, FELSENSTEIN, NIEBUHR and TROXLER2006) found no effect of pophole width (range of 0.2-1.2cm/bird). This study looked at relatively small groups of hens (256 birds) compared with commercial flocks. Range use is often greater in smaller flocks and this may have overridden the importance of pophole dimensions. Additionally, as the experiment began when birds were 32 weeks, previous range experience may have affected the results. The importance of pophole availability may be greater in commercial flocks where more factors limit pophole access and this may explain the significant effect seen in the Gilani et al. (Reference GILANI, KNOWLES and NICOL2014) study.

Many commercial houses have popholes on one side of the building only. This effectively increases the average distance needed to travel to reach a pophole. Evidence suggests that hens do not access all areas of the house but instead remain in certain horizontal or vertical localities (Freire et al., Reference FREIRE, WILKINS, SHORT and NICOL2003; Nakarmi et al., Reference NAKARMI, TANG and and XIN2014). If popholes are only available in one area of the house, access to the range may be limited to a relatively small proportion of the flock that happen to stay in that area.

The height of a pophole from the ground is important because elevated popholes may prove physically difficult for hens to negotiate. Studies on the ability of hens to jump between and up to perches suggest that vertical jumps over 50 cm start to present difficulties (Scott et al., Reference SCOTT, LAMBE and HITCHCOCK1997). Pophole elevation is not typically regulated by law although RSPCA Assured (2013) regulations suggest ramps if the pophole is above ground level. As yet, no studies have looked at the specific effect of pophole elevation on range use. The height of the opening itself is also relevant as hens show some aversion to entering spaces with vertical heights below 46 cm (Dawkins, Reference DAWKINS1985).

General system design

Free-range housing design varies greatly. As previously mentioned, hens tend to restrict themselves to certain areas within the house potentially resulting in limited pophole access for some individuals. This effect is emphasised if the interior contents of the shed are arranged in such a way that birds are physically blocked from accessing certain areas of the shed (e.g. by tiers of nestboxes or multi-tier rows with the litter underneath fenced off).

It is important to encourage birds to use the areas of the house that adjoin the popholes. Popholes are often accessed from litter areas and so anything that discourages litter usage will also discourage range use. Poor litter quality may put off birds from performing certain behaviours (Odén et al., Reference ODÉN, KEELING and ALGERS2002), as will the use of electric wires crossing the litter area (used in some countries to reduce floor eggs) or along the walls of the litter area (commonly used in the UK during early lay). The ‘relative attractiveness’ of the house may also play a role. If the house is much more attractive to hens than the range (e.g. through readily available resources, a quality foraging/dustbathing substrate and security) they may be less likely to go outside (Keeling et al., Reference KEELING, HUGHES and DUN1988). This should be addressed by improving the range rather than reducing the attractiveness of the house.

Feeders and drinkers are usually placed on the slatted area or the first two tiers of a multi-tier system. Hens feed with great frequency during daylight hours (Nicol et al., Reference NICOL, CAPLEN, EDGAR and BROWNE2009) and typically do not stray far from these important resources. If food and water is only provided indoors, hens are less likely to range at any distance. Hens will run to the troughs prior to feeding (Bubier and Bradshaw, Reference BUBIER and BRADSHAW1998). Highly regular feeding schedules may therefore prevent birds staying on the range for very long or ranging far from the house as they would be disadvantaged for feeding (Bubier and Bradshaw, Reference BUBIER and BRADSHAW1998). Indeed, in a small study of four farms the farm with ad libitum feeding had the highest proportion of the flock outdoors (Bubier and Bradshaw, Reference BUBIER and BRADSHAW1998). This result was likely affected by the smaller size of this flock.

Climatic conditions

Range use is largely influenced by weather conditions (Keeling et al., Reference KEELING, HUGHES and DUN1988; Nicol et al., Reference NICOL, POTZSCH, LEWIS and GREEN2003; Hegelund et al., Reference HEGELUND, SORENSEN, KJAER and KRISTENSEN2005; Richards et al., Reference RICHARDS, WILKINS, KNOWLES, BOOTH, TOSCANO, NICOL and BROWN2011). Nicol et al. (Reference NICOL, POTZSCH, LEWIS and GREEN2003) collected both farmer and researcher observations, and reported a higher percentage of birds ranged when the weather was ‘calm and dull’ than when it was ‘wet’, ‘cold’ or ‘sunny’. Hegelund et al. (Reference HEGELUND, SORENSEN, KJAER and KRISTENSEN2005) studied 37 organic flocks over a four year period and found significant effects of temperature, wind, precipitation and season on percentage range use. Usage decreased in higher winds and precipitation and increased up to 17°C ambient temperature. Richards et al. (Reference RICHARDS, WILKINS, KNOWLES, BOOTH, TOSCANO, NICOL and BROWN2011) tracked individual popholes and found reductions in use during high winds and rainfall. An increase in pophole use was found with both temperature and hours of sunshine, although the effect of sunshine was more noticeable at lower temperatures and the authors hypothesise that this was due to the warming effect of sun.

The effect of weather and the management practices best suited to dealing with it depend on the country and its climate. Provision of shade, shelter, wind breaks, outdoor water and ground drainage are all management practices that can reduce the effect of adverse weather on ranging behaviour. Hens from different climates will be affected differently by weather. Hens that have acclimatised to higher temperatures have improved heat tolerance compared with non-acclimatised birds (Hutchinson and Sykes, Reference HUTCHINSON and SYKES1953). In the case of range use, it would be reasonable to assume that particularly hot days will discourage hens in warm climates from ranging less than those in colder climates.

Range design

High outdoor stocking densities may affect bird health by increasing exposure to intestinal worms (Sherwin et al., Reference SHERWIN, NASR, GALE, PETEK, STAFFORD, TURP and COLES2013). Sick birds may have a reduced ability to use the range. A higher outdoor stocking density has been shown to increase percentage range use (Sherwin et al., Reference SHERWIN, NASR, GALE, PETEK, STAFFORD, TURP and COLES2013), perhaps because the increased presence and proximity of other birds on the range increases their sense of security. Hens on the range may therefore act as a form of enrichment, further encouraging more birds out. Keeling et al. (Reference KEELING, HUGHES and DUN1988) found increased numbers of birds further out on the range with increased percentage range use, suggesting that the presence of ranging birds affects distribution. Intended stocking densities on the range will not necessarily reflect the actual stocking densities experienced by the birds. For example, 2500 birds/ hectare on the range assumes that the birds are evenly spread and all ranging together. Instead, as for indoor stocking densities, we see uneven distributions resulting in higher densities in certain areas - near the popholes/house is common. This is particularly apparent on the range as birds using the outdoor area at a given time can be so variable. Spacing behaviour in small groups is influenced by forces that bring individuals closer such as protection from predators and forces that keep them apart such as reducing competition for food (Keeling, Reference KEELING1995). However, the spatial needs of commercial hens kept in large groups have not been assessed and remains unclear.

Various studies have provided resources to try and encourage good ranging, and include opportunities to forage or dustbath may encourage birds out as they are highly motivated to perform these behaviours (Weeks and Nicol, Reference WEEKS and NICOL2006). Poorly maintained ranges, with puddles of water, mud or compacted earth will prevent hens from using the ground to perform these behaviours.

Nicol et al. (Reference NICOL, POTZSCH, LEWIS and GREEN2003) found that outdoor use was positively correlated with the presence of trees and hedges on the range. Shelter, whether natural or artificial, provides protection from the elements and can reduce bird fearfulness by increasing their perception of safety from predators (Collias, Reference COLLIAS1987). Experimental work has demonstrated that hens are more likely to emerge into an open area if familiar objects are present (Grigor et al., Reference GRIGOR, HUGHES and APPLEBY1995b). As chickens are prey animals, providing shelter may increase the range use if they feel safer. In both experimental and on-farm trials carried out in Australia, shelterbelts and artificial shelter increased percentage range use. In fact, 17 times more hens used the shelterbelt areas than control areas (Nagle and Glatz, Reference NAGLE and GLATZ2012). This effect may have been exacerbated by the hot climate and need for shade. Zeltner and Hirt (Reference ZELTNER and HIRT2008) found that flocks with structures installed on the range had significantly greater percentage range use than controls. Similarly the presence of artificial cover has been found to increase percentage range use (Hegelund et al., Reference HEGELUND, SORENSEN, KJAER and KRISTENSEN2005). A study using small flocks of 256 birds found that percentage range use averaged 31% for areas negligible cover but increased to 38% for those with vegetation cover (Harlander-Matauschek et al., Reference HARLANDER-MATAUSCHEK, FELSENSTEIN, NIEBUHR and TROXLER2006).

Rearing and experience of outdoor environments

Hens are initially fearful of novel environments such as the outdoor range, becoming more confident with repeated exposure and increased familiarity (Jones, Reference JONES1977; Grigor et al., Reference GRIGOR, HUGHES and APPLEBY1995b). Early life experience of the range increases the birds readiness to emerge into an outdoor environment experimentally (Grigor et al., Reference GRIGOR, HUGHES and APPLEBY1995a). However, Gilani et al. (Reference GILANI, KNOWLES and NICOL2014) found no effect of rearing with or without range access on subsequent percentage range use commercially at 35 weeks. It is possible that rearing did lead to an effect, but these may have been overcome by subsequent experience by this age.

Pullets reared without access to perches or similar raised structures will be less physically able than birds reared with perches, with effects on both their spatial and navigational abilities (Gunnarsson et al., Reference GUNNARSSON, YNGVESSON, KEELING and FORKMAN2000). This will affect their ability to navigate the house and exit through popholes if doing so presents a physical challenge.

It is a reasonably common practice on laying farms to keep birds enclosed on the slatted area for a period of time when they are first housed. This experience makes floor litter less familiar, potentially increasing fear responses, and reducing likelihood to access litter-based popholes. Conversely, new research on the effect of this practice indicated that welfare was not adversely affected, and some parameters even suggested improved welfare in those held off the litter for two weeks (Alm et al., Reference ALM, WALL, HOLM, WICHMAN, PALME and TAUSON2015). As this study looked at hens in a non-free-range system, the effect of this practice on subsequent ranging remains unclear.

Factors influencing bird distribution on the range

Overall range-use is not the only issue where commercial norms fail to meet consumer expectations. The distribution of birds on the range is rarely even, with the majority of hens staying close to the house (Hirt et al., Reference HIRT, HÖRDEGEN and ZELTNER2000; Zeltner and Hirt, Reference ZELTNER and HIRT2003; Hegelund et al., Reference HEGELUND, SORENSEN, KJAER and KRISTENSEN2005; Rault et al., Reference RAULT, VAN DE WOUW and HEMSWORTH2013, Steenfeldt and Nielsen, Reference STEENFELDT and NIELSEN2015). This leads to overuse of pasture near the house (Maurer et al., Reference MAURER, HERTZBERG, HECKENDORN, HÖRDEGEN and KOLLER2013), reducing potential foraging opportunities and potentially increasing concentrations of parasites in certain areas. Additionally, accumulation of phosphorus and nitrogen in the soil occurs (Maurer et al., Reference MAURER, HERTZBERG, HECKENDORN, HÖRDEGEN and KOLLER2013), posing a hazard to the environment. Very high stocking densities near the popholes may limit the movement of birds in the area, reducing their ability to exit the shed.

When a simple roofed box with sand was placed in the furthest quarter of the range, a higher percentage of birds were found in this area (Zeltner and Hirt, Reference ZELTNER and HIRT2003). Flocks with structures on the range had a significantly greater percentage of birds using the middle area of the range compared with controls (Zeltner and Hirt, Reference ZELTNER and HIRT2008). Similarly, the addition of vertical structures encouraged hens to range further (Rault et al., Reference RAULT, VAN DE WOUW and HEMSWORTH2013). Gilani et al. (Reference GILANI, KNOWLES and NICOL2014) found a strongly significant effect of cover on the range in increasing the proportion of birds ranging away from the house. These studies show that structuring the range can affect the distribution.

Although distribution on the range is skewed to areas near the house, hens have been observed ranging over 50m from the house (Zeltner and Hirt, Reference ZELTNER and HIRT2003). Small ranges, or those of a shape that doesn't allow birds to move outwards further than a few metres may therefore restrict freedom of movement on the range for some hens (Zeltner and Hirt, Reference ZELTNER and HIRT2008). Cooper and Hodges (Reference COOPER and HODGES2010) reported that trees have positive effects on ranging up to 50 m but no further, likely due to other limiting effects such as the desire to stay close to the resources, such as feed, provided in the house. When different structures were provided outside, hens were mostly found near those that provided shelter and shade (Zeltner and Hirt, Reference ZELTNER and HIRT2008). The literature indicates that by providing shelter, either natural or artificial, we can both encourage more birds out onto the range and encourage them to travel further.