Ecology and energetics of breeding Puffins (Fractercula artica) : variations in individual reproductive effort and success

Abstract

This study investigated reproductive effort and success of individual Atlantic Puffins (Fratercula arctica) on the Isle of May, Firth of Forth, Scotland. It placed particular emphasis on the role of body condition in breeding. An energetics approach was taken, where individual ’quality’ was considered in terms of foraging efficiency. The study also investigated whether breeding entailed costs for Puffins, in terms of individual survival and future reproductive potential, and whether such costs were mediated through body condition. Colour-ringed pairs of Puffins were followed through 3 successive breeding seasons and their reproductive performance and condition were monitored. Energy reserves carried by individuals (body condition indices) were estimated from live mass and body dimensions, using a carcass-derived equation to predict lean wet mass. Attempts were made at increasing the effort of rearing young, by playing chick begging calls and exchanging chicks between burrows, and decreasing effort, by supplementary feeding of young. Field energy expenditures were measured for a sample of parents during chick rearing using the doubly-labelled water technique, and these were compared with other potential measures of reproductive effort. The breeding success of individual parents was not related to body condition when a correlative approach was taken. Field metabolic rates (FMR’s) of 9 adults rearing young averaged 3.67 +/- 0.65 s.d cm' CO2 g'd ' or 874 +/- 151 kJd ' (c.3.5 times basal metabolic rate). Individual FMR’s were not related to other measures of reproductive effort used in the study. The above results were evaluated using graphical models, to demonstrate mechanisms by which the confounding effects of inter-individual differences in foraging efficiency on body condition could mask relationships between body condition, FMR and reproductive performance. The body condition of parents which experienced a decrea.se in rearing effort or an increase in effort did not differ significantly from that of controls at the end of the rearing period. This occurred even though parents whose young were fed substantially decreased the number of feeds they delivered to their young. Despite the apparent lack of an effect of the feeding treatment on condition, control pairs showed a lower return rate to the colony, lower breeding success, and produced young in ’poorer’ condition at peak mass than experimental parents whose young were fed for them in the previous year. The higher reproductive success of the experimental group suggested that Puffins on the Isle of May (control group) incurred inter-year reproductive costs when rearing young under natural conditions. These results were obtained in years when breeding conditions appeared to be relatively unfavourable for Puffins on the Isle of May, consistent with the view that reproductive costs may only be detectable in ’bad’ years. Mediation of such costs through body condition was not demonstrated, perhaps because the measure of condition used was unsuitable for Puffins; potential energy depots were discussed in relation to the life style of Puffins compared to other seabirds. Quantifying disturbance was not an original aim of the study but during fieldwork it became apparent that Puffins were sensitive to handling. The effects that handling and general disturbance had on the results of the .study were addressed but were unlikely to have influenced any of the conclusions presented.