Action: Provide supplementary food for gulls, terns and skuas to increase reproductive success
- Four studies of three experiments from Europe and Alaska found that providing supplementary food increased fledging success or chick survival in two gull species, although a study from the UK found that this was only true for one island, with abnormally low breeding success. A second island with higher success was not affected by feeding. Two of the experiments fed parent birds and one fed the chicks directly.
- One study from the Antarctic found no effect of feeding parent skuas on productivity.
- One study from Alaska found increased chick growth when parents were fed; one study from the Antarctic found no increase in chick growth.
Supporting evidence from individual studies
A replicated, controlled experiment in summer 1989 on Flat Holm and Skomer islands, south Wales (Hiom et al. 1991), found that lesser black-backed gulls Larus fuscus provided with supplementary food laid significantly larger clutches and eggs on Flat Holm but not on Skomer (2.7 eggs/clutch for 21 fed clutches vs. 2.4 eggs/clutch for 34 unfed clutches on Flat Holm; 2.9 for 23 fed clutches on Skomer vs. 2.7 for 42 unfed). Average clutch size of unfed gulls on Flat Holm was also lower in 1989 than in previous years, whilst unfed clutches on Skomer were similar to the national average. This suggests that the population on Flat Holm was to some extent food limited in 1989, although laying date did not differ between fed and unfed clutches. Birds were provided with 200 g of fish each day.
A randomised, replicated and controlled study at a mixed gull colony on the island of Terschelling, the Netherlands, in April-July 1992 (Bukacinski et al. 1998) found that pairs of lesser black-backed gulls Larus fuscus whose chicks were provided with supplementary food until fledging had significantly higher fledging success than control pairs (1.9 fledglings/nest and 87% of nests fledging at least one chick for 12 fed nests vs. 0.9 fledglings/nest and 56% success for 14 control nests). Fledging success of pairs whose chicks were fed until seven days old was intermediate (1.3 fledglings/nest and 67% success, 12 nests). In addition, significantly fewer chicks were predated in fed nests (0.5 chicks/nest for fully fed nests; 0.8 chicks/nest for partially fed and 1.3 chicks/nest for control nests). There were no significant differences in clutch size, egg size or hatching success between groups. Food was provided at an average of 46 g/day increasing to 76 g/day after a week and 150 g after three weeks, continuing until approximately 40 days old.
A replicated, controlled study during the breeding seasons of 1996-7 in the northern Gulf of Alaska, USA (Gill & Hatch 2002) using the same data as (4), found that black-legged kittiwake Rissa tridactyla pairs that were provided with supplementary food had significantly higher fledging success in both years than pairs fed until laying or hatching, or than control (unfed) pairs (81-85% fledging success and 1.0-1.3 chicks/pair for 43 fed pairs vs. 51-53% and 0.3-0.6 chicks/pair for 128 controls). Fed pairs also had larger clutch sizes and higher hatching success in 1997 but not 1996 (1996: 1.9 eggs/clutch and 76% hatching success for 73 and 49 clutches from fed pairs vs. 1.9 eggs/clutch and 65% for 59 and 83 clutches for controls; 1997: 1.8 eggs/clutch and 74% hatching success, 76 and 50 clutches from fed pairs vs. 1.6 eggs/clutch and 50%, for 59 and 85 control clutches). There was no effect of feeding on laying success (92-97% success for 157 fed pairs vs. 91-94% for 128 controls). Supplementary food consisted of 163 g/day of small pieces of herring, provided two or three times daily beginning in May, three weeks before first laying and ending in mid-August.
A replicated, controlled study during the breeding seasons of 1996-7 in the northern Gulf of Alaska, USA (Gill et al. 2002) using the same data as Gill & Hatch 2002, found that black-legged kittiwake Rissa tridactyla pairs that were provided with supplementary food had earlier laying and hatching dates, faster growing chicks and higher chick survival than control pairs (for 157 fed pairs, first eggs laid on 3-4th June, hatched 30th June-1st July, 79-82% chick survival to 40 days vs. first eggs laid on 7th June, hatched 4th July, 51-53% chick survival to 40 days for 128 control pairs). However, there were no significant differences in egg volume, incubation period or chick fledging weight between treatments. Supplementary food consisted of 163 g/day of small pieces of herring, provided two or three times daily beginning in May, three weeks before first laying and ending in mid-August.
A randomised replicated and controlled trial on King George Island, Antarctic Peninsula, in the boreal summer of 2000-1 (Ritz et al. 2005) (as part of the same study as Ritz 2006) found that south polar skua Catharacta maccormicki (also known as Catharacta maccormicki) pairs provided with supplementary food did not raise significantly more chicks than control (unfed) pairs (average of 1.3 chicks/pair for 27 fed pairs vs. 1.5 chicks/pair for 27 controls). Supplementary food consisted of 25 g of fish provided to adults every other day when chicks were 6-35 days old and 100 g of fish when 35-55 days old. This corresponds to approximately 20% of a chick’s daily energy needs.
A randomised, replicated and controlled trial on King George Island, Antarctic Peninsula, in the boreal summer of 2000-1 (Ritz 2006) (as part of the same study as Ritz et al. 2005) found that male south polar skuas Catharacta maccormicki (also known as Stercorarius maccormicki) from 27 pairs provided with supplementary food were present at nests more often than males from 27 control (unfed) pairs (males present for 83% of observations when chicks were 7-50 days old for fed pairs vs. 74% for controls, a total of 955 observations). There was no difference in female attendance (81% attendance for fed pairs vs. 80% for controls, 955 observations). Chicks from fed pairs were not significantly larger than chicks from control (unfed) pairs, although wing growth was slightly faster in fed chicks (there were no changes in mass, head size or tarsus growth rates). Supplementary food consisted of 25 g of fish provided to adults every other day when chicks were 6-35 days old and 100 g of fish when 35-55 days old. This corresponds to approximately 20% of a chick’s daily energy needs. This study also investigated the impact of feeding on adult condition, discussed in ‘Provide supplementary food to increase adult survival’.
- Hiom L., Bolton M. & Monaghan P. (1991) Experimental evidence for food limitation of egg production in gulls. Ornis Scandinavica, 22, 94-97
- Bukacinski D., Bukacinska M. & Spaans A.L. (1998) Experimental evidence for the relationship between food supply, parental effort and chick survival in the lesser black-backed gull Larus fuscus. Ibis, 140, 422-430
- Gill V.A. & Hatch S.A. (2002) Components of productivity in black-legged kittiwakes Rissa tridactyla: response to supplemental feeding. Journal of Avian Biology, 33, 113-126
- Gill V.A., Hatch S.A. & Lanctot R.B. (2002) Sensitivity of breeding parameters to food supply in black-legged kittiwakes Rissa tridactyla. Ibis, 144, 268-283
- Ritz M.S., Hahn S. & Peter H.U. (2005) Factors affecting chick growth in the South Polar Skua (Catharacta maccormicki): food supply, weather and hatching date. Polar Biology, 29, 53-60
- Ritz M.S. (2006) Sex-specific mass loss in chick-rearing South Polar Skuas Stercorarius maccormicki- stress induced or adaptive? Ibis, 149, 156-165