EFFECTS OF DIET QUALITY ON THE NESTLING GROWTH OF A WILD INSECTIVOROUS PASSERINE, THE HOUSE MARTIN DELICHON-URBICA

1. Many studies have shown environmental effects on the growth and body size of wild birds. Most investigations have centred on the role of nestling nutrition in these effects. However, few of these studies have investigated the role of diet quality in the growth of wild birds. In this study I investigate the effect of altering nestling diet quality on the growth of a hirundine, the house martin. 2. The quality of the diet fed to nestling house martins was reduced through direct fat feeding. This resulted in a diet of insects and fat which satisfied energy demand while providing fewer nutrients than the usual wholly insect diet. Effects of fat feeding were monitored by recording parental visits, by collecting the faecal Output of broods and by measuring nestling growth. 3. In both years, fat feeding reduced parental feeding visits and fat-free faecal output of broods while producing little alteration in estimated total energy intake. Fat-free faecal mass was positively correlated with an index of parental visits. Mass of fat fed explained most variation in the faecal output of broods. 4. Field metabolic rate of 14-day-old nestlings (mean 54.8 kJ day-1), measured using the doubly labelled water technique, did not differ between treatments after correction for nestling diet artefacts. 5. Fat-fed broods grew more slowly and were smaller at day 20 and at fledging than control broods even though fledging later. Wing growth was correlated with fat-free faecal output. 6. It was concluded that diet quality differences resulting from fat feeding had caused the difference in nestling growth and that this diet-quality effect may be present in wild bird populations. However, in house martins natural variation in food abundance is probably more important in determining growth than variation in diet quality.