In the precocial chicks in the present study, the development of homeothermy was characterised by the early attainment of shivering thermogenesis in leg muscles and the use of behavioural thermoregulation (I, II). Chicks of the three galliform species studied, were endothermic at the age of 1–2 days or earlier, as soon as they were able to increase heat production in response to a decrease in ambient temperature. This timing of endothermy does not differ from that observed in earlier studies in Galliformes (e.g. Aulie & Moen 1975, Matthew 1983, Gdowska et al. 1993, Dietz & Kampen 1994). The metabolic rate was age-dependent, and the biphasic development of mass-specific resting metabolic rate, observed in precocial species (e.g. Visser 1998), was clearly visible in the grey partridges and the Japanese quails (I, II), the maximum occuring approximately at days 5 and 2–7, respectively. In Japanese quail, the earlier study by Freeman (1967) dated the maximum mass-specific RMR to the age of 5 days. In the domestic fowl, the maximum can be dated as occurring between days 1–2 and 7. The mass-specific metabolic rates (W·kg-1) in chicks of two domestic fowl strains (Pilch and Isabrown/Warren) calculated from the data of Visser (1991), show that the maximum occurs approximately at 6 days of age, which is again in line with this present study. After the breakpoint, the decrease in the mass-specific metabolic rate most likely results from the decrease in mass of the metabolic active tissue/BW ratio (Dietz et al. 1995, Weathers & Siegel 1995) and from the growth rate (Klaassen & Bech 1992). The resting metabolism showed a positive correlation with the growth rate, possibly partially due to the high proportion of postprandial heat production during the highest growth phase.
The altricial species in the present study, the domestic pigeon, was endothermic at 6 d of age when a thermogenic response to the decline in ambient temperature was already apparent. The regulatory thermogenesis was enabled by shivering principally in the breast muscles. The appearance of endothermy is consistent with the development of brain temperature regulation in which the most rapid increase occurs during the first 5–6 post-hatching days (Arad 1989). In line with this Arad’s work as well, is the study by Ginglinger and Kayser (1929), where the pigeon nestling’s ability to maintain a body temperature at mild cold (~30°C) was observed to improve markedly between the post-hatching days 3–6. In addition, Koskimies and Lahti (1964) found that the cooling rate in the pigeon also decreases between days 0–2. However, since the pigeon lacks regulatory thermogenesis at that age, as was observed in this present study, this fall most probably is attributable to an increase of body mass and a decrease of conductance. The metabolic scope in the above three studies increased after the age of 6 days but in the present study the increase in mass-specific resting metabolic rate with age was not clearly visible. The data of the pioneer studies by Ginglinger and Kayser (1929) and Riddle et al. (1932) indicate that the metabolic rate reaches its maximum at 9–11 days of age and thereafter decreases. Thus it seems obvious that the development of the resting metabolic rate in the pigeon shows a similar biphasic pattern as in precocial species and is similar to that in another large-sized altricial species, in white-necked ravens (Mishaga & Whitford 1983). This development in the pigeon differs from small-sized altricials, where the mass-specific RMR generally increases from a low hatchling level more or less linearly to the high adult level (e.g. Dawson & Evans 1960, Shichun et al. 1979, Clark 1982, Olson 1992).
The ability to withstand cold ambient temperatures decreases significantly if insulation is lost. Wet plumage in small chicks leads to an increase in the cooling rate even though heat production is increased (Hissa et al. 1983). In precocial chicks, the ability to withstand cool ambient temperatures is attained (Aulie and Moen 1975) and subsequently locomotory activity is increased (Boggs et al. 1977) just when the hatching down dries. Even in such a good thermoregulator as the eider duckling, which is capable of increasing regulatory heat production as a late embryo, wet hatchling down offers very poor insulation (Steen & Gabrielsen 1988). A wet eider hatchling is unable to compensate adequately in cold for the increased heat loss but soon becomes exhausted. Steen et al. (1989) observed that only the drying of the down in eider chicks enables thermoregulation. In aquatic precocial chicks, hatching down, once dried, is no longer easily wetted. In contrast to that, the plumage of the grey partridge chicks of the present study remained vulnerable to wetting for weeks (I). At the age of three weeks, the water impermeability of the plumage was still improving although the insulation of the dry plumage was not appreciably getting any better. This indicates that especially during rainy and cool days, the chicks’ ability to thermoregulate is poor in the wild in comparison with the laboratory conditions, and the chicks are absolutely dependent on their parents for warming.
Due to the limited insulation, the role of behavioural thermoregulation in resisting cooling was enhanced during the first post-hatching weeks (I). Chicks were found to be dependent on the external heat source, as in experiment to study behavioural responses in thermal gradient. With the attainment of thermal independence, movement activity also increased (I) as in chicks of the willow ptarmigan (Boggs et al. 1977, Pedersen & Steen 1979). The utilisation of insulation was also found to be dependent on the nutritional state, as was seen in the Japanese quail chicks (IV). In chicks exposed to short-term fasting, the most crucial decline in conductance occurred during cold exposure after which heat production started to increase, while in control quails increased insulation and metabolism simultaneously to defend their body temperature in cold. This is a novel finding, which suggests that in a normal nutritional state, chicks do not have to utilise their full insulative capacity. It is also possible that postprandial heat production increases circulation in surface tissue layers and thus exposes chicks to greater heat loss (Misson 1982). The decreased conductance due to fasting has been observed previously, e.g. in domestic fowl chicks (Visser 1991). The decreased conductance in each individual chick and the further decrease due to mutual behavioural thermoregulation in the form of huddling, as seen in fasted Japanese quails (IV), result in energy sparing which obviously has a great value for survival during food deficiency.
In all the species studied in the present study, both ambient temperature and age had a major effect on body temperature (I, II, Fig. 1). The body temperature measured from the colon increased during the post-hatching development. In Galliformes, the most rapid change was observed simultaneously with the achievement of maximum mass-specific metabolic rate. In the pigeon, the development of body temperature in thermoneutrality made a quantum leap between post-hatching days 2–4. The study by Arad (1989) revealed that both in the development of brain and body temperature, the major change occurs within the first 5–6 post-hatching days. In domestic fowl chicks in the present study, the major increase occurred during the first 10 days and thereafter body temperature increased only slightly but steadily (II). This is consistent with the results obtained with bantam hen chicks published by Myhre (1978). Although a diurnal rhythm in body temperature was poorly visible in newly-hatched domestic fowl chicks (Fig. 1), it clearly exists in their movement activity (Höchel et al. 1999) and heart rate (Moriya et al. 1999). In muscovy ducklings (Cairina moschata), a diurnal rhythmicity in Tb is already apparent in one-day-old chicks (Nichelmann et al. 1999). Because internal rhythms usually develop with increasing amplitude, a small initial amplitude in Tb may not be apparent.