Action: Provide supplementary food for songbirds to increase adult survival
- Seven studies from Europe and the USA found higher densities or larger populations in various songbird species in areas close to supplementary food. Six studies from Europe, Canada and Japan found that population trends or densities in some species were no different between fed and unfed areas. The American study found that populations appeared to follow food, with populations increasing after feeders were erected and decreasing after they were removed.
- Four studies from Canada, Europe, Japan and the USA found that birds had higher survival when supplied with supplementary food. However, in two studies this was only apparent in females or in one of two species studied. A controlled study in the USA found no evidence that birds were dependent on supplementary food: when food was removed, previously fed birds did not have lower survival than controls.
- A replicated, controlled study from the USA found that song sparrows Melospiza melodia had lower survival with feeding stations in their territories.
- Six studies from Europe and the USA found that birds supplied with supplementary food were in better physical condition or had larger fat supplies than unfed birds. However, in one replicated, controlled study this was only the case for females; in another two, only one of three species showed better condition, with one species in one study showing lower condition when fed; a final replicated and controlled study found that differences between treatments were only apparent in the breeding season.
- Two studies investigated the effect of feeding on behaviours: a randomised, replicated and controlled study in the USA found that male Carolina wrens Thryothorus ludovicianus spent more time singing when supplied with food; a replicated, controlled study in Sweden found no behavioural differences between wood nuthatches Sitta europaea supplied with food, and unfed birds.
- Thirteen studies from the UK, Canada and the USA investigated use of feeders. Four studies from the USA and the UK found high use of supplementary food by several species, with up to 21% of birds’ daily energy needs coming from feeders. However, another UK study found very low use of food, possibly because the feeder was not positioned close to natural food sources.
- One UK study found that use of feeders peaked in midwinter, although another found that the exact timing of peak use varied between species.
- Two replicated trials from the UK finding that the use of feeders increased with distance to houses and decreased with distance to cover, whilst a replicated Candadian study found that American goldfinches Carduelis tristis preferred using bird feeders in high positions. A large-scale replicated study in the UK found that preferences for feeder locations varied between species.
- Three studies from the UK argue that placing feeders over 1 km apart, and possibly 1.1–1.3 km apart will maximise their use whilst keeping the intervention practical.
Supporting evidence from individual studies
A replicated, controlled study in the winters of 1969-71 in three deciduous woods in southern Sweden (Källander 1981) found that great tit Parus major populations increased at two fed sites but declined at an unfed control site in a harsh winter, but increased at all sites in a milder winter. Adult survival over this period was highest in the site fed in both years (37-59% survival of 31 birds), intermediate in the control site (35-47% survival of 22) and lowest at the site fed only in 1969-70 (13-19% survival of seven). More yearlings remained in the two fed sites, compared with the control (14-22% of 85 birds remaining vs. 7-11% of 48). Blue tit P. caeruleus populations appeared unaffected by feeding at the same sites. Supplementary food consisted of hoppers filled with sunflower seeds provided from December (1969) or October (1970) until March.
A replicated and controlled study on Mandarte Island, British Colombia, Canada (Arcese & Smith 1988), found that song sparrows Melospiza melodia provided with supplementary food in 1985 were less likely to survive until the breeding season of 1986 (40-47% survival for fed adults, n = 15 vs. 66-79% for controls, n = 42). The authors note that 1985 was a year of peak song sparrow density and suggest lower adult survival could be due to increased costs of defending feeders from other song sparrows. This study also described the impact of feeding on reproduction, discussed in ‘Provide supplementary food to increase reproductive success’.
A controlled study in the winters of 1985-6 and 1986-7 in two small deciduous forest sites in Alberta, Canada (Desrochers et al. 1988), found that winter survival of black-capped chickadees Parus atricapillus was higher in the 2.6 km2 area provided with supplementary food, compared to the 1.9 km2control (unfed) area (1985-6: 88% of 163 birds in the fed area vs. 80% of 143 controls; 1986-7: 65% of 192 vs. 53% of 137). However, there were no significant differences in the proportion of chickadees acquiring local breeding territories, or in local breeding densities (1986: 54% of fed birds acquiring local breeding territories and 15.3 pairs/km2 vs. 66% and 17.2 pairs/km2 for controls; 1987: 66% and 16.1 pairs/km2 vs. 71% and 14.0 pairs/km2). Supplementary food consisted of multiple feeding stations, each with between two and four feeders, filled weekly with sunflower seeds.
A replicated, randomised, controlled study from December-January in 1985-1986 in 8 experimental and control (rotated) pairs of Carolina wrens Thryothorus ludovicianus in a woodland study site in Tennesse, USA (Strain & Mumme 1988), found that male wrens supplemented with food sang significantly more than unsupplemented males. Food supplementation, but not song playback, significantly increased both the song rate and the rate of song-type change (89.4 and 51.2 songs / hour; 1.3 and 0.7 song changes / hour for food supplemented and control males respectively). The authors point out that, because foraging and singing are mutually exclusive behaviours in Carolina wrens, the increase in vocal territorial behaviour associated with food supplementation may reflect a decrease in the time required for foraging. Wren pairs were allocated to food supplementation treatments randomly. Food supplementation consisted of 2 cans filled daily with 100 mealworms / territory. Daily observations began approximately 30 min be-fore sunrise and continued for 4 hours.
A replicated and controlled study in the breeding seasons of 1985-7 in grasslands on Öland, southern Sweden (Moreno 1989), found that female northern wheatears Oenanthe oenanthe, but not males, that were provided with supplementary food were significantly heavier than unfed controls (average of 26.9 g for 53 fed females and 24.4 g for 42 fed males vs. 24.3 g and 23.7 g for 48 and 32 unfed controls). However, there was no effect when females were feeding older chicks, (after they were able to regulate their body temperature. A few days after hatching, most food was delivered to chicks, not consumed by adults. Food consisted of 7 g of mealworms provided either during incubation, or for the entire breeding season.
A replicated controlled study in a deciduous wood in southern Sweden in winter 1980 and spring 1981 (Enoksson 1990) found that, although wood nuthatches Sitta europaea made up 28% of all visits to supplementary feeding stations, there was no difference in the time devoted to different behaviours between five fed pairs and nine control pairs. Supplementary food consisted of 60 kg of sunflower seeds supplied continuously from December 1980 until March 1981 at nine feeders spread evenly across the experimental area. The author suggests that the lack of differences could be due to variations in the intensity of foraging, rather than the time spent foraging.
A replicated, controlled study in ten deciduous forest plots in Ohio, USA, in the winter of 1988-9 (Grubb & Cimprich 1990) found that three species of songbirds grew longer replacement feathers when provided with sunflower seeds and suet in excess, compared to control birds which were not fed. However, most of these differences were not significant, with only tufted titmice Baeolophus bicolor (formerly Parus bicolor) consistently showing significant differences across ages and sexes. Replacement feathers appeared to grow faster (a proxy for nutritional condition) in titmice and white-breasted nuthatches Sitta carolinensis of all age classes and both sexes. Immature and male Carolina chickadees Parus carolinensis also showed more rapid growth when fed, the sample size for female chickadees was too small for comparison and there was no significant difference between fed and unfed adult chickadees. When corrected for body size, differences for male titmice and male nuthatches became non-significant. Sample sizes were: 13 male chickadees, five females, 15 adults and 28 immatures; six male titmice, 14 females, 12 adults and 17 immatures; 16 male nuthatches, ten females. The impact of feeding on downy woodpeckers Picoides pubescens is discussed in ‘Provide supplementary food to increase adult survival – Woodpeckers’.
Two studies in gardens in Cardiff, south Wales, in January-February 1988 and February-April 1989 (Cowie & Simons 1991) found that distance to both cover (a dense hedgerow) and housing had significant effects on rate of consumption of supplementary food by five songbird species. At a single site, as distance from cover increased, the proportion of food consumed decreased (32% consumed when the feeder was next to the hedge, 28% at 2.5 m away, 23% at 5 m and 17% at 7.5 m), with a greater impact on house sparrows Passer domesticus and blue tits Parus caeruleus than on greenfinches Carduelis chloris. Overall consumption increased with distance from housing in three other sites (34% of food consumed when three feeders were 10 m from housing, 25% at 7.5 m, 25% at 5 m, and 16% at 2.5 m), however this effect varied between species. Siskins C. spinus used all feeders equally; greenfinch use increased with distance from housing and house sparrows used feeders closest to the houses most frequently. Supplementary food consisted of 250 g of peanuts supplied each day.
A replicated study from January-March in 1990 in one suburban garden garden in Ontario, Canada, using four bird tube feeders placed in different locations in Ontario, Canada (Dunn & Hussell 1991), found that American goldfinches Carduelis tristis preferred bird feeders that were placed in higher locations. The number of birds and the amount of food removed were significantly higher for the upper feeder (6.1 m) than the lower feeder (3.4 m) (average 75% birds seen and 67% food removed from high feeder). There was no preference for feeders placed in trees or in the open (both 2.4 m from the ground), although there was a much greater proportion of unexplained variation in the side-by-side experiment than in the height experiment. All tube feeders contained small black oil sunflower seeds. A flock of 15-30 goldfinches used the test feeders daily. Feeders were switched at the end of each trial (3 replicates each).
A controlled study in Wisconsin, USA, over five months in the winter of 1984-5 (Brittingham & Temple 1992) found that, following the removal of supplementary food, 49 black-capped chickadees Parus atricapillus did not have lower survival at a site where they had previously been provided with supplementary food, compared with 35 control chickadees, at a site where they had never been fed (84% monthly survival for previously-fed birds vs. 85% for controls). Food had been provided for 25 winters at the experimental site.
A small study in two deciduous forest sites in Wisconsin, USA, in the winters of 1983-5 (Brittingham & Temple 1992) found that approximately 83 black-capped chickadees Parus atricapillus used two feeders providing sunflower seeds each day throughout winter, and obtained approximately 21% of their daily energy requirements from them. Birds with home territories nearer the feeders used it more than more distant birds, and more bird used the feeders (and fed at a higher rate) in the evening (an average of 39 chickadees using the feeders within two hours of sunset vs. 17 within two hours of sunrise and 36 at midday). Feeders were used the most in autumn and least in spring, with temperature not affecting feeder use. Feeders were monitored for 15 days over the two winters.
A randomised, replicated and controlled paired study in parkland and mixed woodland in southern Scotland in spring and summer 1990 (Johnston 1993) found that great tit Parus major females with artificially enlarged broods lost significantly less weight whilst provisioning young when nestlings were provided with supplementary food, compared to females with control (enlarged but not fed) broods (average of 2.2 g lost between day ten of incubation and day 13 of provisioning for experimental females, n = 8 vs. 2.9 g for control females, n = 7). There were no data on survival of adults. Broods were enlarged by the addition of three nestlings (added after hatching). Supplementary food consisted of an average of 2.2 g of minced meat and nutritional supplements fed twice daily to half the experimental brood on days six through 12 after hatching. This represents most of a nestling’s daily energetic requirements. This study also investigated the impact on the nestling growth, discussed in ‘Provide supplementary food to increase reproductive success’.
A replicated and controlled study in a mixed forest in the central Netherlands in 1987 (Verhulst 1994) found that female pied flycatchera Ficedula hypoleuca from pairs provided with supplementary food had significantly higher survival rates than those from control pairs (survival in subsequent years of 58% for fed females, n = 12 vs. 27% for controls, n = 60). There was no difference in male survival (survival in subsequent years of 55% for fed males, n = 11 vs. 33% for controls, n = 51) or in adult weights when chicks were seven days old (average of 12.1 g for fed males, n = 11 vs. 12.2 g for control males, n = 13; 12.5 g for fed females, n = 12 vs. 12.5 g for controls, n = 14). Supplementary food consisted of mealworms provided in excess beginning two days after chicks hatched. This study also examines the impact of feeding on reproductive success, discussed in ‘Provide supplementary food to increase reproductive success’.
A replicated and controlled trial in a dune and scrubland system in Florida, USA, in 1993 (Schoech 1996), found that breeding Florida scrub jays Aphelocoma caerulescens from territories provided with supplementary food had significantly higher body lipid levels (i.e. were in better physical condition) than adults from control (unfed) territories (fed males approximately 6% body fat, n = 9 vs. 3% for controls, n = 18; fed females approximately 4.5%, n = 9 vs. 1.5% for controls, n = 17). There were no differences in non-breeding individuals. Feeding consisted of providing dried dog food, peanuts and mealworms were provided twice daily at feeding stations in the middle of the territories from late January until females finished laying. Food was provided ‘in excess’. This study also reported on the effects on reproduction, discussed in ‘Provide supplementary food to increase reproductive success’.
A small controlled before-and-after trial in two mixed woodlands on Honshu, Japan in the spring of 1996 (Nakamura & Kubota 1998) found that survival of varied tits Parus varius was significantly higher at a 67 ha site supplied with supplementary food, compared to a 46 ha control (unfed) site 3 km away (100% survival over the 51 day study period for 17 tits in the fed area vs. 70% survival for 20 tits from the control area). Many birds changed breeding partners in the control site, whereas pair bonds were maintained in the fed site. Food consisted of 500 g sunflower seeds provided two or three times a week from seven feeders, each at least 150 m from each other. Feeders were visited on average 30 times/hr by three individuals.
A small controlled before-and-after trial (Kubota & Nakamura 2000) in two mixed woodlands on Honshu, Japan (using the same study sites as in Nakamura & Kubota 1998), found that neither the local population density nor the home range size of varied tits Parus varius increased over a six-week period when supplementary food was provided, compared to a control area where no food was provided. In September 1995 there were 23 tits in the 67 ha experimental site and 17 in the 36 ha control (unfed) site. By the 12th December (the end of the feeding period), there were 19 tits in the experimental area and 17 in the control area. Food consisted of 500 g sunflower seeds provided two or three times a week from eight feeders, one in each tit territory in the experimental area and all tits in the area used feeders at an average of 5.2 visits/hr.
A randomised, replicated and controlled study in coniferous woods along a road in Maine, USA (Wilson 2001) found that significantly more black-capped chickadees Parus carolinensis (also known as Poecile carolinensis) were recorded during censuses at four sites fed continuously from late October 1995 to mid-March 1996, compared to at four unfed control sites (average of 5.5 birds/census for fed sites vs. 0.1 birds/census for controls, 18 censuses at each). Sites provided with food from October until January (early-fed sites) had significantly higher chickadee numbers than those fed from January until March (late-fed sites), with both being lower than continuously-fed sites and higher than controls (average of 1.8 birds/census and a maximum of approximately 5 birds/census for early fed vs. average of 0.1 birds/census and maximum of 3.0 birds/census for late-fed, 18 censuses at each). Chickadee numbers declined at early-fed sites when feeders were removed and increased at late-fed when feeders were established. No such patterns were seen at control or continuously-fed sites. Birds took longer to discover feeders at late-fed sites, compared to those supplied from October (all feeders at both continuously- and early-fed sites discovered within 15 days, a maximum of 33 days before the last feeder was discovered in late-fed sites). Feeding consisted of two feeders at each site refilled every week with black oil sunflower seeds.
A replicated, controlled study in 54 woodlots and riparian corridors in an agricultural landscape in Ohio, USA, in the winters of 1995-9 (Doherty & Grubb 2002) found 315 Carolina chickadees Parus carolinensis (also known as Poecile carolinensis) had significantly higher survival rates in riparian sites provided with supplementary food, compared to those in unfed control sites (50% survival for fed areas vs. 43% for controls). This effect was not found in 346 white-breasted nuthatches Sitta carolinensis or 529 tufted titmice Baelophus bicolor. Chickadees in large plots supplied with food also had higher survival than those in unfed large plots. There was no such difference in smaller plots and no significant differences in the other species studied. The impact of feeding on downy woodpeckers Picoides pubescens is discussed in ‘Provide supplementary food to increase adult survival – Woodpeckers’. Supplementary food consisted of sunflower seeds and suet provided in excess throughout winter.
A replicated, controlled study from May-June in 1992-1998 in one experimental (3 km²) and four unmanaged arable farms in Leicestershire, England (Stoate 2002) found that the abundance of nationally declining songbird species and species of conservation concern significantly increased through time in the site where supplementary food was provided. Although there was no overall difference in bird abundance, species richness or diversity between the experimental and control sites, numbers of nationally declining species rose by 102% (except for Eurasian skylark Alauda arvensis and yellowhammer Emberiza citrinella). Nationally stable species rose (non-significantly) by 47% (with 8 species exhibiting net increases, especially greenfinch Carduelis chloris 68%, and 4 species exhibiting net decreases). The author concludes that supplementing food (grain provided through winter across the farm), as part of an integrated management package, provides the greatest benefits to species of conservation concern but does not affect species diversity at the farm scale.
Another analysis (Doherty & Grubb 2002) of the same data as Doherty & Grubb 2002 also examined nutritional condition, judged by the size of feather growth bars, and found that 37 Carolina chickadees Parus carolinensis (also known as Poecile carolinensis) living in large woodlots were in significantly better nutritional condition (judged by the size of feather growth bars) when provided with supplementary food, compared with unfed controls. There were no significant differences in smaller woodlots. White-breasted nuthatches Sitta carolinensis had lower growth rates when fed, compared with controls and there was no impact of feeding on 48 tufted titmice Baelophus bicolor. The impact of feeding on downy woodpeckers Picoides pubescens is discussed in ‘Provide supplementary food to increase adult survival – Woodpeckers’.
A replicated, controlled study at two pairs of mixed habitat sites in Kansas, USA, in the winters 2000-1 and 2001-2 (Rogers & Heath-Cross 2003) found that seven songbird species had higher levels of visible subcutaneous fat in areas supplied with supplementary food, compared with those in control (unfed) areas. Most differences were significant in both years and both pairs of sites, but differences were always small. Body mass relative to size showed a similar trend, but the differences were not significant. Supplementary feeding ran from early December until early March and consisted of four sunflower seed feeders and a 6 x 3 m area beneath them sprinkled with mixed seed. The species studied were dark-eyed junco Junco hyemalis, Harris’s sparrow Zonotrichia querula, song sparrow Melospiza melodia, American tree sparrow Spizella arborea, northern cardinal Cardinalis cardinalis, black-capped chickadee Parus atricapillus (also known as Poecile atricapillus) and tufted titmouse Parus bicolor. The effects of feeding on predation rates are discussed in ‘Can supplementary feeding increase predation or parasitism?’.
A replicated study in mountain forests in Tennessee, USA, in 1999-2001 (Podolsky et al. 2004) found that 92% of 24 breeding pairs of ovenbirds Seiurus aurocapilla and 79% of 38 wood thrush Hylocichla mustelina pairs fed on live mealworm Tenebrio monitor larvae from feeding stations consisting of moss placed over overhead projector film (clear plastic film) and placed on the ground near nests (6-12 m away from wood thrush nests, 3-6 m from ovenbird nests). Mealworms could burrow into the moss to avoid desiccation but could not escape because of the film. Previous work showed that birds avoided artificial feeders such as bowls and baskets, but removed 70-100% of mealworms within four hours from moss. Food was provided daily and nests were monitored for six (ovenbird) or eight (wood thrush) days.
A study at a farmland site in northwest England between January 2003 and February 2004 (Raine 2004) found that twite Carduelis flavirostris used a supplementary feeding station (established in spring 2002) frequently outside the breeding season, with up to 250 birds being seen at once. However, twite used the station far less during the breeding season, when they relied more on wild seeds. Birds from another feeding station (see Raine 2004) and other breeding colonies up to 20 km away used the feeding station, as well as individuals from a nearby colony of 20-30 birds. Supplementary food consisted of nyjer Guizotia abyssinca spread in a thick 2 m x 5 cm line on a 2 m x 2 m patch of bare earth and replenished every week.
A study at a farmland site in northwest England between January 2003 and February 2004 (Raine 2004) used an identical feeding procedure to Raine 2004 at a site 12.6 km away and found that twite Carduelis flavirostris used the supplementary feeding station frequently outside the breeding season, with up to 150 birds being seen at once. However, twite used the station far less during the breeding season, when they relied more on wild seeds. A large number of birds from near the feeding station in (23) and colonies up to 20 km away used the feeding station, as well as birds from the two nearby colonies (each approximately 1.5 km away and 20-30 birds).
A study at a farmland site in northwest England between January 2003 and February 2004 (Raine 2004) used an identical feeding procedure to Raine 2004 to establish a feeding station approximately 1 km from a colony of six pairs of twite Carduelis flavirostris. This station was only used occasionally and only by one or two birds at a time. The author suggests that the lack of use could have been due to the small size of the colony and the fact that it was not positioned close to natural feeding areas for twite.
A randomised, replicated and controlled study at three farmland sites in England in the winters of 1999-2000 until 2001-2 (Robinson et al. 2004) found that farmland birds showed mixed responses to supplementary food. Chaffinches Fringella coelebs, linnet Carduelis cannabina and yellowhammer Emberiza citrinella all showed significant short-term increases on at least one plot provided with food (chaffinch densities increased by 80-200% on three of six fed plots; yellowhammer densities increased by 230-400% on four of six; data for linnets not provided). There were no corresponding short-term changes on nearby control plots. Eurasian skylarks Alauda arvensis did not show any consistent response to food at any of the sites and there was no longer term impact of feeding on bird densities. Supplementary food consisted of 36 kg/ha of mixed grains broadcast over fields. The authors suggest that the lack of effect of feeding at some sites may be due to a very low natural seed density in the soil, meaning that even with supplementary food, the level of food was too low to attract birds.
The results from two replicated studies found that contextual variables affecting the use of supplementary food by a range of farmland songbirds were not consistent across species or regions (Siriwardena & Stevens 2004). The ‘BirdAid’ programme (run between October and March in the winters of 2000-1 until 2002-3 across the UK) found that all three target species (tree sparrows Passer montanus, yellowhammers Emberiza citrinella and corn buntings Miliaria calandra) used supplementary food, consisting of 25 kg of seeds supplied each week. Tree sparrows and yellowhammers tended to use feeding stations more if they were closer to cover and in mixed landscapes, the opposite was true for corn buntings. The Winter Food for Birds project, run from October 2002 to March 2003 at ten replicates of seven sites across eastern England, found that six of eight target species used supplementary food, consisting of 5 kg each of millet and sunflower seeds supplied each week, sufficiently often for analysis. At the local and landscape scale, only human habitats and woodlands had uniform effects, increasing and decreasing the use for three and four species respectively. All other habitats had different impacts on different species.
A replicated, controlled study covering November-July in 2002-2004 in 10 sites each containing 7 feeding stations (placed at the centre of a 2 x 2 km tetrad) separated at set distances from each other (100 m, 500 m, 1 km, 2 km, 5 km and 10 km) in East Anglia, UK (Anon 2005) found that supplementary provision of seeds increased local seed-eating bird abundance, especially species of conservation concern. Yellowhammer Emberiza citrinella and chaffinches Fringilla coelebs used the feeding stations most extensively (93–100% of all stations). Although genuine population trends were difficult to infer from the experimental setup, the authors argue that food provisioning increased the local abundance of several otherwise declining species (yellowhammers, reed buntings Emberiza schoeniclus, house sparrows Passer domesticus and chaffinches) over two winters. Colour-ring re-sighting and radio-tracking revealed that target granivores move small distances between food resources (500m – 1km) and the authors suggest placing food resources (over-winter stubbles and wild bird cover crops) at a minimum of 1km apart in order to be cost effective in reaching the most number of populations. Supplementary seed (10 kg of equally distributed sunflower hearts and millet) was replenished weekly. Avian use of the feeding stations was monitored twice weekly (20 min observation sessions)
A replicated study analysing annual survey data from 458 garden bird feeders in the UK from 1970-2000 (Chamberlain et al. 2005) found that, of 41 species analysed, 21 increased in occurrence at garden feeders over time. Robins Erithacus rubecula, blackbirds Turdus merula, blue tits Parus caeruleus and greenfinches Carduelis chloris occurred at all sites, whilst dunnocks Prunella modularis, song thrushes T. philomelos, great tits P. major, starlings Sturnus vulgaris, house sparrows Passer domesticus and chaffinch Fringilla coelebs occurred at more than 95% of sites. Species were more likely to occur at feeders in years when they had high a country-wide population estimate and peak use tended to occur some time in midwinter.
The Winter Food for Birds project (see Siriwardena & Stevens 2004) was continued in the winter of 2003-4 and the data from three winters are discussed in Siriwardena et al. 2006. For four songbird species (blue tit Parus caeruleus, chaffinch Fringilla coelebs, great tit P. major and robin Erithacus rubecula), feeding stations were used more frequently and by more birds if they were more than 500 m from other stations, compared with stations less than 500 m from neighbours. The same pattern was seen (but not significant) in blackbirds Turdus merula and house sparrows Passer domesticus. Yellowhammers Emberiza citrinella and reed buntings E. schoeniclus, however, used clustered sites more. There was no significant impact of distance of feeder use by greenfinches Carduelis chloris, goldfinches C. carduelis or dunnocks Prunella modularis. Local populations of all species divided themselves between multiple stations if they were closer than 500 m apart, but used only single stations if they were more widely spaced. The authors use this information to recommend that supplementary food resources provided for conservation purposes should be stations are placed at least 1 km apart to maximise cost-effectiveness (i.e. to ensure the maximum number of birds have access to supplementary food).
A replicated, controlled study from November-March in 2004-2007 in 10 experimental and 10 control tetrads (composed of four 1 km2 sites) of arable farmland in East Anglia, UK (Anon 2007) found that provision of seeds during winter significantly increased body mass and breeding population sizes of seed-eating species. Supplementary food was most used in early to mid winter for generalist species and late winter for specialist species (such as chaffinch Fringilla coelobs and yellowhammer Emberiza citrinella). Radio tracking and mark-recapture techniques revealed that resource patches (such as wild bird cover crops and over-winter stubbles) should be separated by 1.1 – 1.3 km to be both cost and conservation effective for priority species (like yellowhammers). The authors suggest that year-round resource delivery could be achieved by placing breeding habitat 2.7 – 3.6 km from winter food patches. They caution that specific inter-patch distances may vary according to species and habitat but should be based on species of conservation concern. Experimental sites contained one central feeding station provided ad libitum with seed (10 kg of equally distributed millet, rape, wheat and sunflower seeds; replenished twice a week) and were fenced (50 cm in height) using 50 mm mesh wire and bamboo canes.
A series of randomised, replicated trials at two sites in England in the winters of 2000-1 and 2001-2 (Siriwardena et al. 2007) found that five songbird species took supplementary food when provided and preferentially took wheat over oats and oats over barley. Tree sparrows Passer montanus and reed buntings Emberiza schoeniclus also fed on maize, preferring it to all cereals except wheat, whilst house sparrows P. domesticus preferred maize to all cereals. Corn buntings E. calandra and yellowhammers E. citrinella preferred all cereals to maize. Tree sparrows selected both cereals and oily seeds (sunflower seeds, oilseed rape etc) but avoided rye grass seed. All species preferred cereals to sunflower seeds and none showed any distinction between wheat and a ‘weed seed mix’. At one site, food was provided in tubular feeders, in the other it was heaped on the ground. Survival rates of birds were not monitored.
A replicated study using some of the same data as Siriwardena & Stevens 2004 and combining them with data from control areas between 2000 and 2003 did not find robust evidence for supplementary winter feeding increasing breeding abundances of farmland songbirds (Siriwardena et al. 2007). There were no effects of the Bird Aid programme on target species, although sites used more frequently had increased populations of yellowhammers Emberiza citrinella and corn buntings Miliaria calandra, but decreased populations of tree sparrows Passer montanus. Four of five insect-eating/generalist species declined faster in Winter Food for Birds (WFFB) experimental areas than in controls. There was no such effect for six seed-eating species. Declines in dunnocks Prunella modularis, robins Erithacus rubecula and yellowhammers Emberiza citrinella were lower in WFFB sites provided with more food and centrally-placed WFFB sites, compared to those provided with less food or those around the periphery of WFFB clusters.
Another study, using the same data as Siriwardena et al. 2006 and additional data from a second landscape-scale experiment, investigated how use of supplementary food by farmland songbirds varied over the winter months (Siriwardena et al. 2008). Supplementary food-use peaked in or before January for five generalists and ‘human-associated granivores (blackbirds Turdus merula, goldfinches Carduelis carduelis, greenfinches C. chloris, house sparrows Passer domesticus and robins Erithacus rubecula), whilst yellowhammers Emberiza citrinella, reed buntings E. schoeniclus, chaffinches Fringilla coelebs and dunnocks Prunella modularis all used supplementary food most in February or later. Use by great tits Parus major and blue tits P. caeruleus declined overwinter. The authors suggest food use reflects demand and the first group use food most when temperatures are lowest and daylight hours shortest, whilst the second group (which are heavily dependent on farmland seed) use food most when ambient food sources are at their most scarce and that the third group’s pattern of food use reflects the available pool of individuals as mortality occurs through the winter.. They caution that these results are likely to be dependent on the mix of farming types across the landscape, with eastern England being dominated by arable fields.
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