Action: Can supplementary feeding increase predation or parasitism?
- A replicated, controlled study in the USA found that providing seeds in predictable areas did not increase predation on seven species of songbird.
- A replicated and controlled trial in Spain found higher levels of potentially dangerous gut microflora when fed on livestock carrion, compared to those fed on wild rabbits. A replicated study in Spain found higher levels of predation on artificial nests close to carcasses provided for vultures.
Supplementary feeding could potentially have deleterious effects on both the target population, for example through spreading disease or drawing birds to feeding areas where they can easily be predated, or non-target species, for example by bringing large numbers of predators into the area.
Supporting evidence from individual studies
A replicated, controlled study at two pairs of mixed habitat sites in Kansas, USA, in the winters 2000-1 and 2001-2 (Rogers & Heath-Coss 2003) found that predation rates on seven species of songbird by Cooper’s hawk Accipiter cooperi and sharp-shinned hawk A. striatus were no higher in an area supplied with supplementary food, compared with control (unfed) sites (two attacks in fed sites vs. no attacks in control sites). 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 songbird body condition are discussed in ‘Provide supplementary food to increase adult survival’.
A replicated, controlled trial in 2004 in Castile and Leon and Madrid, Spain (Blanco et al. 2006), found that red kites Milvus milvus that fed on carrion from stabled livestock had higher levels of potentially harmful gut microflora, compared to birds fed largely on wild rabbits (potentially dangerous Salmonella serotypes found in 25% of 80 faecal samples from colonies fed with livestock vs. 3% of 33 samples from colonies fed on wild rabbits). More generally, gut flora were more similar between the two colonies supplied with livestock, than either were with wild-fed birds. Livestock consisted mainly of domestic pigs Sus scrofa, but also cows, sheep, poultry and domestic rabbits.
A replicated study in dry scrubland on Fuerteventura, Canary Islands, Spain, in spring 1996 Cortés-Avizanda et al. 2009), found that artificial nests were more likely to be predated when they were within 200 m of a carcase (both naturally-occurring and supplied to a vulture ‘restaurant’), compared to more distant nests. A total of 312 nests were laid in 12 lines. Nest predation occurred in 67% of lines, with a maximum of 92% of nests on a line being predated. The restaurant was supplied with approximately 200 kg/week of goat and pig carcasses whilst naturally occurring carcasses consisted of one goat and one yellow-legged gull Larus michaellis. Nests imitated either those of lesser short-toed larks Calandrella refescens or cream-coloured coursers Cursorius cursor and contained two Japanese quail Cortunix japonica eggs.
- Rogers C.M. & Heath-Coss R. (2003) Effect of experimentally altered food abundance on fat reserves of wintering birds. Journal of Animal Ecology, 5, 822-830
- Blanco G., Lemus J.A. & Grande J. (2006) Faecal bacteria associated with different diets of wintering red kites: influence of livestock carcass dumps in microflora alteration and pathogen acquisition. Journal of Applied Ecology, 43, 990-998
- Cortés-Avizanda A., Carrete M., Serrano D. & Donázar J.A. (2009) Carcasses increase the probability of predation of ground-nesting birds: a caveat regarding the conservation value of vulture restaurants. Animal Conservation, 12, 85-88