5.1. Body mass reflected the diet and changes in it

Body mass is generally used as an indicator of the nutritional status of birds (Brittas & Marcström 1982, Barton & Houston 1993, but see van der Meer & Piersma 1994). Bone marrow fat is considered an accurate index of the nutritional status in the barn owl Tyto alba (Thouzeau et al. 1997), but this method demands killing the birds.

Because of low locomotive activity and ad libitum feeding with high-energy commercial food, hand-reared grey partridges (Putaala & Hissa 1995), pheasants (Robertson et al. 1991, 1993) and spruce grouse (Pendergast & Boag 1971a) are heavier than their wild counterparts. Results obtained from hand-reared and wild capercaillies (I) were not in agreement with these results. This finding may be a result of the small sample size, or high variation in the data resulting from the wide age-class. Capercaillies grow slowly and reach their adult mass in the second year (Lindén 1984a).

Grey partridge chicks that received insect-rich diet were heavier than other chicks, and low-insect diet produced the lightest chicks. This result was consistent with the observations by Dahlgren (1987). Wild chicks had similar body mass to chicks fed an insect-rich or a fish diet. Chicks that received insect-rich diet were still heavier than chicks fed fish at the age of eight weeks. However, the body mass of chicks fed low-insect and fish diet increased rapidly after receiving commercial food. According to Dahlgren (1987) the effects of an insufficient diet during growth may last until the first breeding season. Both Moss et al. (1993) and Marjoniemi et al. (1995) found that chicks fed commercial food grew fast. Heavier wild chicks are assumed to fly earlier than the lighter ones, thus facilitating their survival (Potts 1986).

Grey partridges fed natural food (III, V) lost body mass soon after the change in diet, but began to regain mass in about one week (III). Mass loss and the period of very low body mass coincides with the period of high mortality rate among released birds. In 1996 fifteen of twenty released female grey partridges died during the first week (unpublished data). Birds fed natural food remained lighter in weight than other chicks. The birds were still growing in both studies and probably did not get energy enough from the natural food for growth. The mass loss later during the feeding trial may have been a response to the experimental conditions and the stress experienced.

Tannin was not seen to lower body mass, in contrast to the results of Elkin and Rogler (1991), Voltura and Wunder (1994), Helsper et al. (1996), and Hewitt et al. (1997). As a matter of fact, tannin fed birds were heavier than other birds during the feeding trial. Birds may have met their protein requirements in spite of the added tannin because of ad libitum feeding (Helsper et al. 1996), or the pelleting of food may have decreased the effects of tannin, as assumed by Elkin and Rogler (1991). The excess protein from commercial food may have been used for binding tannins. It is also possible that tannins partly denaturate proteins, and this could further make them easier to digest (Butler 1989). In prairie voles Microtus ochrogaster quebracho tannin was reported to be lethal at all levels of protein, because of inhibited feeding (Lindroth & Batzli 1984).