| Effects of supplementary feeding on the body condition and breeding success of released pheasants | ||
|---|---|---|
| Prev | Chapter 5. Results and discussion | Next |
In the experimental trials described in Articles IV and VI, the density of territorial males was higher when supplementary food was provided. The presence of feed hoppers affected the location of male territories. A higher proportion of the males were territorial when supplementary grain was provided (IV), although this difference was not detected in VI. The presence of feeders are also known to increase territory density in male red-winged blackbirds (Agelaius phoeniceus) (Ewald & Rohwer 1982). There was no difference in harem acquisition by cocks, or in the number of hens under supplemented and un-supplemented conditions (IV). Consequently, harem size was lower when supplementary grain was provided. The results suggest that male quality was more important than territory quality or availability of nesting cover, in agreement with previous studies on mate choice of hen pheasants (Ridley 1983, Ridley & Hill 1987, Göransson et al. 1990, Grahn et al. 1993a,b, Robertson 1996). Conversely, in VI, both territorial male and hen density were increased by supplementary feeding, implying that supplementary feeding influenced the behaviour of hens. To account for the differences in hen density between treatments, feeding either reduced post-winter dispersal of hens and/or attracted hens from surrounding unfed areas. Dispersal distances of hens once they are in established breeding territories are short (up to 250 m) (Robertson 1986, Woodburn & Robertson 2000), but during seasonal shifts pheasants can move up to, though rarely more than, between 1.6 and 3km (Gates & Hale 1974, Dumke & Pils 1979). Dispersal distances of pheasants are influenced by the quality of their immediate environment, which if adequate, will result in only minimal movements Leopold et al. 1938, Wilson et al. 1992). It appears that food availability and/or supplementary feeding may influence dispersal of hens from their winter flocks.
Supplementary feeding may help preserve energy reserves and increase predator avoidance by decreasing foraging activities (Johnson & Gaines 1990). Studies of non-game species have demonstrated a positive effect of supplementary feeding on winter survival in birds including willow tit (Parus montanus) (Orell 1989, Lahti 1997), great tit (Parus major) (Van Balen 1980), and black capped chickadees (Parus atricapillus) (Brittingham & Temple 1988). In Britain, variation in the annual survival rates of many declining farmland passerine species (Siriwardena et al. 1998a, Siriwardena & Robinson 2002) is considered to be the main factor driving the downward trend in populations which have taken over the last 25 years (Siriwardena et al. 1998b, Chamberlain et al. 2000). Reduced availability of grains and weed seeds on arable farmland in winter is generally agreed to be an important factor influencing over winter survival (Campbell et al. 1997, Wilson et al. 1999, Robinson & Sutherland 2002), though experiments investigating the effects of winter food supplementation for songbirds on farmland have only been recently initiated (Hart et al. 2002, Perkins & Anderson 2002).
In game species, Townsend et al. (1999) demonstrated a positive effect of supplemental feeding on winter survival in bobwhite quail, and Gabbert et al. (1999) reported higher survival of pheasants that used corn food plots. The consensus in these studies was that survival was improved through nutritional benefits provided by supplementary feeding during times of severe winter stress, typically extremely low ambient temperatures and reduced availability of food due to snow cover. Conversely, Valkeajärvi and Ijäs (1994) found that spring feeding had a negative effect on black grouse (Tetrao tetrix) survival due to increased susceptibility to raptor predation at feed hoppers. There was no difference in spring to summer survival of hens between fed [31%, (IV) and 37% (VI)] and unfed plots [24% (IV), 48% (VI)]. Hen pheasants are particularly vulnerable to red fox predation during the breeding season (Hessler et al. 1970, Robertson 1986, 1988, Brittas et al. 1992, Schmitz & Clark 1999, Riley & Schulz 2001). Indeed, in IV, 89% of all deaths were attributed to predation by foxes. Putaala (1997) postulated that when physiologically and behaviourally mal-adapted released birds are forced to forage for natural foods to maintain a positive energy and nutrient balance, then less time is spent on other activities such as predator avoidance. It is perhaps not surprising that supplementary feeding did not have either positive or negative impacts on hen survival. This is because the main environmental factors responsible for improved survival in the winter studies cited above were not present in IV and VI, i.e. extremely low temperatures and reduced food availability due to snow cover. In addition, pheasants were not predisposed to goshawk predation at feed hoppers, which has been cited as a problem with spring supplementary feeding black grouse in Finland (Valkeajärvi & Ijäs 1994), as the current British goshawk population is relatively low (Tapper 1999).
Energy availability can affect clutch size and time of breeding in birds (Yom-Tov & Hilborn 1981) and there is good evidence for earlier laying in a range of bird species when provided supplementary food (Boutin 1990). In experimental feeding trials supplementary feeding has been shown to induce earlier nesting in Florida scrub jays (Aphelocoma coerulescens) (Schoech 1996) and alpine accentors (Prunella collaris) (Nakamura 1995). It has also been demonstrated experimentally that restricted diets can delay the onset of egg laying in pheasants (Breitenbach et al. 1963, Gates & Woehler 1968, Barrett & Bailey 1972). These authors found that egg laying did not commence until birds had built up large fat reserves. In contrast, the mean start date of incubation by hen pheasants was not significantly influenced by supplementary feeding (IV), even though fat reserves were greater in birds provided supplementary grain (III & V). However, the time taken to re-nest in the event of nest loss was shorter in the supplementary fed group (IV). There were also (although not significantly), more re-nesting attempts in the fed group (IV). All birds had access to supplementary grain in winter, and it appears that this provided sufficient energy reserves for the control group to successfully lay a first clutch. The results in IV suggest that under unsupplemented conditions hens may not have sufficient energy reserves to re-nest successfully after they have depleted their fat reserves during their first nesting attempt.
Radio-telemetry revealed that there were no differences in clutch size, hatchability or nesting success between supplementary fed and unfed groups with only 18.0% of nests hatching in the un-supplemented group and 24.6% in the supplemented group (IV). This is low compared to other estimates of nesting success of pheasants in Britain: 50.7% and 38.7% (Robertson 1991), 52% (Bence 2001) and was due primarily to high levels of nest predation by foxes and corvids which are important nest predators of pheasants (Robertson 1991, Bence 2001, IV). Fox and corvid predation accounted for most nest losses (38% and 30%, respectively IV). The proportion of failed nests did not differ between treatments, and similarly, there were no differences in rates of nest desertion (fed 13.6%, unfed 17.2%, IV). Consequently, due to the high levels of nest predation, recruitment of chicks to the autumn population was very low (IV). Due to the high levels of mortality and nest loss encountered in IV, it is possible that any subtle effects of supplementary feeding on nesting biology may have been ‘masked’ and therefore difficult to measure due to the relatively higher importance of the effect of predation. For example, it is surprising that nest survival was not improved by supplementary feeding, as one would expect that hens in the unsupplemented group would spend more time off the nest feeding and hence leave the eggs exposed for longer periods, rendering them more prone to corvid predation. This is an area of research currently being investigated by the author. Indeed, Hoodless et al. (2001) found that hen pheasants provided supplementary grain in spring spent less time foraging than unsupplemented group of hens.
The results in VI showed that the provision of supplementary grain led to increases in the density of young pheasants in the fed plots. Almost twice as many juveniles were observed under fed conditions compared to when feeding ceased at the end of the shooting season [fed: 11 young/km2, unfed: 6 young/km2, (VI)]. However, it is not clear from the data in VI exactly which mechanism was responsible for the higher levels of recruitment when supplementary grain was provided. It is likely to be due in part to the higher density of hens in spring under fed conditions and a combination of a difference in the proportion of hens with broods, and brood size (VI). The higher densities in fed plots provide a strong case for reduced emigration from and perhaps immigration into fed plots. Although the differences in the proportion of hens with broods and brood size between fed and unfed conditions were not significant, (VI), it is suggestive that improved body condition of hens in fed plots (III & V) may have been important too. Poor body condition in red grouse (Lagopus lagopus scoticus) can reduce the ability of the hen to brood chicks successfully resulting in smaller brood sizes (Hudson 1992, Newborn 2001). The strong re-nesting capabilities of ring-necked pheasants are important in determining overall breeding success (Johnsgard 1999). The increase in the proportion of hens with broods [67% when supplemented compared with 52% when unsupplemented (VI)] though not significant, perhaps provides further evidence of an improved ability to re-nest when provided with supplementary grain (IV).
Although the results show that feeding led to increased densities of young pheasants in fed plots, supplementary feeding alone is not sufficient to increase productivity to comparable levels of wild birds. In Britain, wild populations managed for sustainable shooting typically have a young:hen ratio of around 3:1, mean brood sizes of 3.8 and 4, resulting in between 80 and 100 young/km2 (Boatman 2000, Sage 2000).
The sites used for study (VI) were not ideal for pheasant reproduction in the wild. They were chosen as they were representative of pheasant management on sites where pheasants are released. Therefore, the observed differences are perhaps not as large as they might have been had the work been conducted on sites more conducive to pheasant reproduction. These sites do exist in Britain but they are in the minority. The work in VI attempted to test the response of released pheasants in the real situation. There would be scientific value in conducting further research on sites where deficiencies in habitat and predation control were not such limiting factors.