Despite a long-term (1989-2013) reduction in size of fish brought to Atlantic Puffin Fratercula arctica chicks and a parallel increase in numbers of fish in the food loads, there was no evidence of changes in chick growth rates at Hornøya, NE Norway. Recent declines in chick survival and cessation in population growth suggest, however, that environmental conditions have changed to an extent that breeding success is now compromised.

Adult seabirds generally feed their chicks an energy-rich, high quality diet, but quantities and quality can vary tremendously in relation to the availability of suitable prey, often resulting in large differences in growth rates of the chicks (Visser 2002). Atlantic Puffins Fratercula arctica (hereafter 'Puffins') feed their chicks on a wide diversity of fish that they carry back to the nest in the beak. Although > 50 fish species plus some invertebrates have been recorded worldwide (Harris & Wanless 2011), at any one colony prey often consists of one or two energy-rich species such as sandeels Ammodytes sp., Capelin Mallotus villosus or clupeids Clupeidae sp. (e.g. Barrett 2002; Baillie & Jones 2004; Harris & Wanless 2011). Numbers of fish in loads delivered to chicks in Norway vary from one large fish to tens of small larvae or self-feeding post-larvae (Figure 1; Barrett et al. 1987). Because the mass and energy content of fish increases approximately with length³, a diet consisting of large numbers of small fish in a load is considered a poor alternative to a few large fish and is often associated with reduced chick growth, extended fledging periods and ultimately breeding failure (Anker-Nilssen 1987; Martin 1989; Barrett & Rikardsen 1992; Barrett 2002; Harris & Wanless 2011).

Long-term changes in food load characteristics are not uncommon (e.g. Martin 1989; Baillie & Jones 2004; Harris & Wanless 2011; Miles et al. 2015), and a 21year (1980-2000) study of Puffins at Hornøya, NE Norway (70°23'N 31°09'E) documented a gradual change in chick diet and signs of a deterioration in feeding conditions through increases in numbers of small fish in beak loads (Barrett 2002). A supplementary feeding experiment in 2003 failed, however, to reveal any difference in mass growth rate between extra-fed chicks and control chicks (Dahl et al. 2005). Assuming 2003 was representative of the time period, this suggested that the overall amount and quality of food delivered by their parents over the two decades remained sufficient to maintain normal growth presumably through compensatory behaviour by the adults (Barrett 2002; Eilertsen et al. 2008). Until the time of the experiment (2003), this was corroborated by a relatively high rate of chick survival (> 80%) and a steady increase in numbers of occupied Puffin burrows in the colony. Since then conditions seem to have deteriorated further with an evident decline in chick survival (see below) and an apparent culmination of the population increase around 2000 (The National Seabird Monitoring Programme, www.seapop.no).

Using a further 13 years of data, this study addresses the effect of a long-term change in diet (species composition, fish size and number) on the growth and survival rates of Puffin chicks on Hornøya, and tests the association between increases in numbers of fish/load and reduced chick growth (Harris & Wanless 2011).

I thank the Norwegian Coastal Administration for permission to use the lighthouse on Hornøya as a base for fieldwork and Håkon Dahlen, Karin Eilertsen and Magne Asheim for help with food observations and collections. I am also grateful to Tycho Anker-Nilssen, and two referees for comments on earlier drafts of the manuscript. The study was financed by the Norwegian Research Council, The Norwegian Directorate for Nature Management (now Norwegian Environment Agency), the University of Tromsø and the Norwegian SEAPOP programme (www.seapop.no).

Anker-Nilssen, T. & Aarvak, T. 2006. Tidsseriestudier av sjøfugler i Røst kommune, Nordland. Resultater med fokus på 2004 and 2005. NINA Rapport 133, Trondheim.

Baillie, S. M. & Jones, I. L. 2004. Response of Atlantic Puffins to a Decline in Capelin Abundance at the Gannet Islands, Labrador. Waterbirds 27: 102-111. [Crossref]

Barrett, R.T. 2002. Atlantic puffin Fratercula arctica and common guillemot Uria aalge chick diet and growth as indicators of fish stocks in the Barents Sea. Marine Ecology Progress Series 230: 275-287. [Crossref]

Barrett, R. T., Anker-Nilssen, T., Rikardsen, F., Valde, K., Røv, N. & Vader, W. 1987. The food, growth and fledging success of Norwegian Puffin chicks Fratercula arctica in 1980-1983. Ornis Scandinavica 18: 73-83. [Crossref]

Barrett, R.T., Fieler, R., Anker-Nilssen, T. & Rikardsen, F. 1985. Measurements and weight changes of Norwegian adult Puffins Fratercula arctica and Kittiwakes Rissa tridactyla during the breeding season. Ringing and Migration 6: 102-112. [Crossref]

Barrett, R. T. & Rikardsen, F. 1992. Chick Growth, Fledging Periods and Adult Mass Loss of Atlantic Puffins Fratercula arctica during Years of Prolonged Food Stress. Colonial Waterbirds 15: 24-32. [Crossref]

Dahl, H. K., Barrett, R. T. & Ims, R. A. 2005. Effects of supplementary feeding on provisioning and growth rates of Atlantic Puffin Fratercula arctica chicks in North Norway. Atlantic Seabirds 7: 133-143.

Durant, J. M., Anker-Nilssen, T. & Stenseth, N. C. 2003. Trophic interactions under climate fluctuations: the Atlantic puffin as an example. Proceedings of the Royal Society of London, Series B 270: 1461-1466. [Crossref]

Eilertsen, K., Barrett, R. T. & Pedersen, T. 2008. Diet, growth and early survival of Atlantic Puffin (Fratercula arctica) chicks in North Norway. Waterbirds 31: 107-114. [Crossref]

Essington, T. E. & Punt, A. E. 2011. Editorial. Implementing Ecosystem-Based Fisheries Management: Advances, Challenges and Emerging Tools. Fish and Fisheries Special Issue: Implementing Ecosystem-Based Fisheries Management 12: 123-124. [Crossref]

Gaston, A. J. 1985. Development of the Young in the Atlantic Alcidae. In: Nettleship D. N. & Birkhead, T. R. (eds.) The Atlantic Alcidae: 319-354. Academic Press, London.

Gladics, A. J., Suryan, R. M., Parrish, J. K., Horton, C.A., Daly, E.A. & Peterson, W.T. 2015. Environmental drivers and reproductive consequences of variation in the diet of a marine predator. Journal of Marine Systems 146: 72-81. [Crossref]

Harris, M. P. & Wanless, S. 2011. The Puffin. Poyser, London.

Irons, D. B., Anker-Nilssen, T., Gaston, A. J., Byrd, G. V., Falk, K., Gilchrist, G., Hario, M., Hjernquist, M., Krasnov, Y.V., Mosbech, A., Olsen, B., Petersen, A., Reid, J. B., Robertson, G. J., Strøm, H., & Wohl, K. D. 2008. Fluctuations in circumpolar seabird populations linked to climate oscillations. Global Change Biology 14: 1455-1463. [Crossref]

Johnsen, I., Erikstad, K. E. & Sæther, B. E. 1994. Regulation of parental investment in a long-lived seabird, the Puffin Fractercula arctica: an experiment. Oikos 71: 273-278. [Crossref]

Lyons, D. E. & Roby, D. D. 2011. Validating growth and development of a seabird as an indicator of food availability: captive-reared Caspian tern chicks fed ad libitum and restricted diets. Journal of Field Ornithology 82: 88-100. [Crossref]

Martin, A. R. 1989. The diet of Atlantic Puffin Fratercula arctica and Northern Gannet Sula bassana chicks at a Shetland colony during a period of changing prey availability. Bird Study 36: 170-180. [Crossref]

Miles, W. T. S., Mavor, R., Riddiford, N. J., Harvey, P. V., Riddington, R., Shaw, D. N., Parnaby, D. & Reid, J. M. 2015. Decline in an Atlantic Puffin Population: Evaluation of Magnitude and Mechanisms. PLoS ONE 10(7): e0131527. [Crossref]

Morrison, K.W., Hipfner, J.-M., Gjerdrum, C. & Green, D. J. 2009. Wing length and mass at fledging predict local juvenile survival and age at first return in tufted puffins. Condor 111: 433-441. [Crossref]

Øyan, H. S. & Anker-Nilssen, T. 1996. Allocation of growth in food-stressed Atlantic puffin chicks. Auk 113: 830-841. [Crossref]

Rodway, M. S. & Montevecchi, W. A. 1996. Sampling methods for assessing the diets of Atlantic puffin chicks. Marine Ecology Progress Series 133: 41-55. [Crossref]

Skern-Mauritzen, M., Ottersen, G., Handegaard, N.O., Huse, G., Dingsør, G. E., Stenseth, N. C. & Kjesbu, O. S. 2015. Ecosystem approaches are rarely included in tactical fisheries management. Fish and Fisheries: DOI 10.1111/faf12111. [Crossref]

Visser, G. H. 2002. Chick Growth and Development in Seabirds. In: Schreiber, E. A. & Burger, J. (eds.) Biology of Marine Birds: 439-465. CRC Press, Boca Raton. [Crossref]

Wernham, C. V. & Bryant, D. M. 1998. An experimental study of reduced parental effort and future reproductive success in the puffin, Fratercula arctica. Journal of Animal Ecology 67: 25-40. [Crossref]