Action: Provide supplementary food for raptors to increase reproductive success
- A single small before-and-after study in Italy found evidence for a small increase in local kite Milvus spp. populations following the installation of a feeding station.
- Four European studies found that kestrels Falco spp. and Eurasian sparrowhawks Accipiter nisus laid earlier when supplied with supplementary food than control birds. One study found that the earlier feeding began, the earlier average laying date was.
- Three studies from the USA and Europe found evidence for higher chick survival or condition when parents were supplied with food, whilst three from Europe found fed birds were more likely to lay or laid larger clutches and another found that fed male hen harriers Circus cyaneus bred with more females than control birds.
- Four studies from across the world found no evidence that feeding increased breeding frequency, clutch size, laying date, eggs size or hatching or fledging success. A study from Mauritius found uncertain effects of feeding on Mauritius kestrel Falco punctatus reproduction.
- There was some evidence that the impact of feeding was lower in years with peak numbers of prey species.
Supporting evidence from individual studies
A replicated and controlled study in mixed conifer forests in southern Scotland between 1971 and 1979 (Newton & Marquiss 1981), found that 13 pairs of Eurasian sparrowhawks Accipiter nisus provided with supplementary food in the pre-laying and laying period had larger clutch sizes, were less likely not to lay and laid earlier than 22 control (unfed) pairs (5.1 eggs/clutch for fed birds, 0% of pairs building nests but not laying, average laying date of 12th May vs. 4.0 eggs/clutch, 27% and an average laying date of 17th May for controls). Food was not provided after clutch completion and fed pairs did not have higher hatching or fledging success (69% hatching success and 62% fledging success for fed birds vs. 54% and 48% for 100 controls). Food provided consisted of either half a pigeon carcass (species not given) or two quail Coturnix coturnix carcasses every 1-4 days, was started on the 23rd-30th April and continued until late May at the latest.
A replicated controlled trial near wetlands in the northern Netherlands in 1978-80 (Dijkstra et al. 1982) found that European kestrel Falco tinnunculus pairs that were provided with supplementary food initiated clutches earlier and laid larger clutches than control (unfed) pairs in two out of three years (six fed pairs started laying on 6-17th April, average of 5.0-5.7 eggs/clutch vs. 8-10th May and 4.5-4.6 eggs/clutch for 30 controls). In 1980, the differences were far smaller (23rd April and 6 eggs/clutch for three fed pairs vs. 25th April and 5.6 eggs/clutch for 18 controls), possibly due to it being a peak vole year. Significance levels were not provided. Supplementary feeding consisted of 100-120 g of mouse meat every daily (approximately twice the daily needs of captive kestrels), provided from late January (1978) or early March (1979 and 1980) until the start of incubation in late April or early May.
A replicated controlled trial in subtropical savanna in northeast South Africa in 1989-90 (Simmons 1993) found that Wahlberg’s eagle Aquila wahlbergi pairs that were provided with supplementary food did not breed more often, have significantly larger clutches or eggs and did not lay earlier than control (unfed) pairs (75% of eight fed pairs breeding vs. 76% of 74 control pairs; average of 1 egg/clutch for both six fed and 56 unfed pairs; average egg volume of 76.7 ml for five fed pairs vs. 76.1 ml for five controls). Feeding consisted of approximately 200 g of meat provided daily from when pairs arrived in a territory until seven days after the first egg was laid (between three and six weeks of feeding).
A randomised, replicated controlled study of kestrels Falco tinnunculus in Cuenca, central Spain, in 1985-92 (Aparicio 1994) found that food-supplemented breeding pairs laid earlier and had larger clutches than control pairs that had receive no food supplementation, and pairs fed from earlier (28th February, at least 50 days before laying vs. 17th April, 17 days before laying) laid earlier than late-fed pairs (average laying date of 30th April for nine early-fed pairs and May 9th for seven late-fed vs. May 15th for 15 controls; average of 5.1 eggs/clutch for fed pairs vs. 4.3 for controls). The onset of laying was not affected, but the average laying date was earlier. Supplementation affected clutch size independently of laying date. In control pairs, there was a seasonal decline in clutch size, but this decline was not seen with supplemented pairs.
A review of an integrated conservation programme for the endangered Mauritius kestrel Falco punctatus from 1973-1994 in montane forest habitat and a captive breeding centre in Black River, Mauritius (Jones et al. 1996) found that the provision of supplementary food had uncertain effects on breeding productivity. Fed birds produced more eggs, but did not necessarily fledge more chicks and, in some cases, fledged fewer. The authors speculate that Mauritius kestrels are less able to tolerate fur and feathers than other kestrels because young kestrels are fed primarily on Day geckos Phelsuma spp and roughage causes digestive prroblems. Captive-bred birds and hacked birds were fed mice and day-old chicks.
A randomised, replicated and controlled trial in mixed conifer forests and scrub in New Mexico, USA, in 1992-3 (Ward & Kennedy 1996), found that northern goshawk Accipiter gentilis nestlings from territories supplied with supplementary food had significantly higher survival than those from control (unfed) territories in 1993 but not 1992 and only when chick, rather than nest was the unit of analysis (1993: 90% survival for ten fed chicks vs. 37% survival for eight controls; 80% survival for five fed nests vs. 40% survival for five controls; 1992: 80% survival for 15 fed chicks vs. 100% survival for 16 controls). The authors suggest that this is due to increased attendance by females, as most nestling losses were due to predation and there were no significant differences in nestling size between treatments (average of 590-682 g for fed chicks and 541-676 g for controls). Supplementary food consisted of dead Japanese quail Cortunix japonica provided every other day starting the day after hatching and continuing until most control birds left the area. This study also examined differences in adult goshawk weights whilst provisioning, discussed in ‘Provide supplementary food to increase adult survival’.
A randomised, replicated, controlled and paired trial in southeast Alaska, USA, in 1994-5 (Gende & Willson 1997), found that bald eagle Halieetus leucocephalus pairs provided with supplementary food did not raise significantly more chicks than control (unfed) pairs and fed chicks were not significantly heavier (average of 2 chicks/nest fledged from 18 fed nests, average weight of 4.0 – 4.4 kg/chick vs. 2 chicks/nest and 4.1 – 4.2 kg/chick for 18 control nests). The authors note that nest failures and brood reductions were rare following hatching in both fed and control pairs, with most losses being during incubation (22 of 60 nests failed before hatching, three nests failed after hatching). Supplementary food consisted of a pink salmon Oncorhynchus gorbuscha provided daily from the date of hatching until hatchlings were seven weeks old, the amount of supplemental food carried to the nests was estimated to provide approximately 50% of the energy requirements of the nestlings.
A randomised, replicated and controlled trial in mixed conifer forests in Utah, USA, in 1996-7 (8), found that northern goshawks Accipiter gentilis chicks from territories provided with supplementary food (Japanese quail Coturnix japonica provided from close to hatching to chick independence) were significantly heavier (although not larger) than those from control (unfed) territories (average of 778 g for 29 fed chicks vs. 723 g for 22 controls). Nestling survival was significantly higher in fed nests in 1997 (100% survival of 19 fed chicks throughout the study vs. 56% survival of 18 controls) but not 1996 (87% survival of 15 fed chicks throughout the study vs. 89% survival of 18 controls). The authors suggest this difference is due to variations in natural food supply as predation was not a primary mortality factor (although females did stay closer to nests in fed territories, compared to controls). This study also examined differences in adult female weights whilst provisioning, discussed in ‘Provide supplementary food to increase reproductive success’.
A randomised, replicated and controlled study in heathland on Orkney Mainland, Scotland, in 1999-2000 (Amar & Redpath 2002) found that male hen harriers Circus cyaneus provided with supplementary food (chicken Gallus domesticus chicks and quarter pieces of European rabbit Oryctolagus cuniculus or brown hare Lepus europaeus) bred with significantly more females than control (unfed) males (100% of 11 fed males mated and 36% mated with more than one female vs. 80% of nine unfed males mated, 11% mated with more than one female). There was no effect of feeding on clutch size or hatching success (average of approximately 5.1 eggs/clutch for 13 fed clutches vs. 4.7 eggs/clutch for four unfed clutches), but productivity still increased. Hooded crows Corvus cornix were also removed from all territories, discussed in ‘Control avian predators on islands’.
A replicated, randomised, controlled study from February-March in 1999 in 2 mixed experimental and control lesser kestrel Falco naumanni colonies, 2 full experimental and 3 full control colonies within an agricultural landscape in La Mancha, Spain (Aparicio & Bonal 2002) found that lesser kestrels provided with food initiated clutch laying earlier than non-supplemented nests or colonies. In mixed colonies, fed pairs laid earlier than unfed ones by 5.8 days on average. There was no significant difference in mean laying date between unfed pairs of mixed colonies and pairs in all-unfed colonies, or between fed pairs of mixed colonies and pairs in all-fed colonies. Additionally, laying date was significantly earlier in all-fed than in all-unfed colonies by 5.7 days on average. The authors suggest, therefore, that laying date is restricted by food availability and is not confounded by individual quality. Extra food consisted of day-old cockerel chicks (35–40 g) placed within each experimental nest.
A replicated before-and-after study in 37 Spanish imperial eagle Aquila adalberti territories in Castilla y León, Castilla-La Mancha, Madrid and Extremadura provinces, Spain (Gonzalez et al. 2006) found that the fledging rate of eagles was significantly higher when territories were supplied with supplementary food, compared to before feeding (average fledging rate of 1.56 young/pair with feeding vs. 0.72 young/pair without, 37 clutches investigated). Siblicide was also lower in fed nests (6% of 50 chicks lost to siblicide in fed nests vs. 45% of 86 in unfed nests) and overall productivity increased in 89% of pairs and declined in just 8%. The increase in fledging rate was even higher in pairs with at least one subadult parent (1.57 young/pair in fed territories vs. 0.54 young/pair in unfed territories) and occurred in both high and low-quality habitats. Food was supplied to territories with more than one chick (and only after the second chick hatched) and consisted of one European rabbit Oryctolagus cuniculus provided every two to three days in territories with three chicks or every four days for two chicks.
A small before-and-after study in southern Italy (Gustin et al. 2009) investigated the effect of a feeding station, active between February 2004 and September 2007, and found that no red kites Milvus milvus bred between 2004 and 2006, but three pairs bred in 2007. Three pairs of black kites Milvus migrans bred in each of 2005-7, before this it had been only an irregular breeder. The station consisted of a 40 x 40 m enclosure surrounded by a 1.8 m high fence. An average of 50 sheep carcasses were provided each year. Between one and 23 red kites and five and 53 black kites were seen every year alongside a large number of non-target corvids. This study also describes the impact of the station on Egyptian vultures Beophron percnopterus, described in ‘Provide supplementary food to increase reproductive success’.
- Newton I. & Marquiss M. (1981) Effect of Additional Food on Laying Dates and Clutch Sizes of Sparrowhawks. Ornis Scandinavica, 12, 224-229
- Dijkstra C., Vuursteen L., Daan S. & Masman D. (1982) Clutch size and laying date in the kestrel Falco tinnunculus: effect of supplementary food. Ibis, 124, 210-213
- Simmons R.E. (1993) Effects of supplementary food on density-reduced breeding in an African eagle: adaptive restraint or ecological constraint? Ibis, 135, 394-402
- Aparicio J.M. (1994) The seasonal decline in clutch size: an experiment with supplementary food in the kestrel, Falco tinnunculus. Oikos, 71, 451-458
- Jones C.G., Heck W. & Lewis R.E. (1996) The restoration of the Mauritius kestrel Falco punctatus population. Ibis, 137, S173-S180
- Ward J.M. & Kennedy P.L. (1996) Effects of Supplemental Food on Size and Survival of Juvenile Northern Goshawks. The Auk, 113, 200-208
- Gende S.M. & Willson M.F. (1997) Supplemental feeding experiments of nesting bald eagles in southeastern Alaska. Journal of Field Ornithology, 68, 590-601
- Dewey S.R. & Kennedy P.L. (2001) Effects of supplemental food on parental-care strategies and juvenile survival of northern goshawks. The Auk, 118, 352-365
- Amar A. & Redpath S.M. (2002) Determining the cause of hen harrier decline on the Orkney Islands: an experimental test of two hypothesis. Animal Conservation, 5, 21-28
- Aparicio J.M. & Bonal R. (2002) Effects of food supplementation and habitat selection on timing of lesser kestrel breeding. Ecology, 83, 873-877
- González L.M., Margalida A., Sánchez R. & Oria J. (2006) Supplementary feeding as an effective tool for improving breeding success in the Spanish imperial eagle (Aquila adalberti). Biological Conservation, 129, 477-486
- Gustin M., Giacoia V. & Bellini F. (2009) Establishment of a feeding station near the Laterza LIPU Reserve to provide additional food for three declining necrophagous raptor species in Apulia, Italy. Conservation Evidence, 6, 66-70