5.7. Primary growth and temperature regulation

Primary length and development may be considered as indicators of a chick’s ability to fly and avoid predation. These parameters may also be used as indicators of plumage quality and insulation. It is probable that wild birds moult more rapidly than hand-reared birds, as a response to a more hazardous life in the wild (McCabe & Hawkins 1946). Handling of the chicks may not have disturbed the mode of moult (Holmberg 1992) in the present study. When wild and hand-reared grey partridge chicks were compared according to the primary length at the age of one week, it was obvious that the wild chicks had longer primary feathers than other chicks. However, chicks fed an insect-rich diet had longer primaries than chicks fed a low-insect or a fish diet (II).

There was a difference between diet groups in the speed and mode of the development of the primaries. Chicks fed an insect-rich diet had fully grown feathers and moulted earlier than other chicks. According to McCabe and Hawkins (1946), and Thompson and Taber (1948) the P1 primary is moulted at the age of 3.5 weeks. This was the case in the chicks fed an insect-rich diet, whereas the chicks fed a fish or a low-insect diet started to moult later than at the age of four weeks. In the chicks fed an insect-rich diet, moulting speed and stage at the age of seven weeks was in agreement with Thompson and Taber (1948) (Fig. 4).

Moulting always includes synthesis of epidermal proteins, mostly sulphur-rich keratin, which in turn includes a high amount of cysteine. The feather proteins include more cysteine than other tissues or food (Murphy & King 1982, Murphy et al. 1990). The process, in which a feather gets a hard form, is called keratinisation. Amino acids methionine and cysteine are known to positively affect plumage development (Murphy & King 1982, Bagliacca et al. 1985, Potts 1986, Murphy et al. 1990).

In the wild, birds usually get enough protein and proper amino acids from their usual nutrition, even during the moult (Ankney 1979, Murphy & King 1984). However, an effect of food quality on the feather development was reported in pheasant chicks (Woodard et al. 1977), where high protein content of food during the first five weeks of life facilitated the growth of tail feathers. Food protein content had an impact on body mass and feather growth in captive European quail Coturnix coturnix coturnix (Combreau & Guyomarc’h 1992). Insect food was essential for the survival and growth of sage grouse Centrocercus urophasianus chicks until the age of three weeks, and even after that it was important for optimal growth (Johnson & Boyce 1990). According to Hermes et al. (1984), the rock partridge chicks were not dependent on animal matter during the first weeks, if nutrients like vitamins and minerals were available. On the other hand, animal matter was an important source of phosphorous, B₁₂–vitamin (Savory 1974) and nitrogen (Dahlgren 1987). Although fish is rich in protein (Almqvist 1952) and methionine and cysteine (F.A.O. 1970), in comparison to many other foods, its negative rather than positive effect on the plumage growth of the grey partridge chicks revealed its inability to replace invertebrates in the diet of hand-reared chicks.

Seven-day-old wild chicks and chicks fed an insect-rich diet displayed superior temperature regulation in comparison to chicks fed a low-insect or a fish diet. The chicks that received an insect-rich diet exhibited a slower cooling rate, which was probably a reflection of their better insulation and higher body mass. It seems reasonable to assume that the insufficient amount of animal protein in the low-insect diet, and an inproper quality of the fish proteins, was reflected in the less-developed temperature regulation in chicks fed these diets. Food-restricted Japanese quail chicks may regulate their metabolic rate at a lower level. Their ability to increase heat production is maintained by shivering, which is used to compensate the decrease in diet-induced/growth related thermogenesis (Marjoniemi 2000).

Shivering and fluffed plumages were obvious in the seven-day-old chicks. According to Marjoniemi et al. (1999) shivering was detected in one-day-old domestic fowls, and two-day-old grey partridges and Japanese quails. Shivering was also obvious in the willow grouse (Aulie 1976, Aulie & Moen 1975) at the age of 2–3 days, or even earlier (Myhre et al. 1975). It was also assumed, that the thermogenic processes in muscles were fully developed at the age of five days, since the maximal thermogenesis did not increase after that time (Marjoniemi et al. 1995). Improved cold resistance later on would then result from increased body mass and improvement of the insulation (Jurkschat et al. 1989, Marjoniemi et al. 1995, Hohtola & Visser 1998). The development of insulation may be considered stepwise, an obvious shift from the age of 11 to the age of 15 days could be seen, (II, Marjoniemi et al. 1995). It must be kept in mind, that at this age chicks are still dependent on parental warming, at least at nighttime. In Finland, most of the grey partridge chicks hatch in the beginning of July (Moilanen 1981, Putaala & Hissa 1998), when ambient temperatures may occasionally drop below 0 °C.