bioenergetics of molt in the chaffinch (fringilla coelebs)

BIOENERGETICS OF MOLT IN THE CHAFFINCH

(FRINGILLA COELEBS)

VICTOR R. DOLNIK 1 AND VALERY M. GAVRILOV 2

•Zoological Institute, Soviet Academy of Sciences, 199164, Leningrad, USSR and 2Cathedra of Vertebrate Zoology, Biological Faculty, Moscow State University, 117234, Moscow, USSR

ABSTP,•½T.--The annual molt of adult Chaffinches begins in the third week of June and is completed by the end of September. In first brood juveniles, molt begins in early July, in second brood juveniles in early August, and in both molt is completed about the same time as in adults. Metabolized energy, measured as the difference between food intake and excreta, increased during the first part of molt and then decreased to a level following molt completion of 1.5 kcal bird -1 day -• lower than before it began. Metabolized energy increased with a drop in ambient temperature at the same rate in all stages of molt (0.44 kcal øC • day-1). Total productive energy (metabolized minus existence energies) used for molt under natural conditions totaled 240 kcal bird i in adults and 147 kcal bird • in juveniles. Net productive energy at a constant temperature of 26øC was 140 kcal bird •, while the amount used at a constant temperature of 7øC was 95% greater. Catabolism of resting, pestabsorptive birds at night, calculated from oxygen consumption, was 1.1 kcal bird -1 day i higher at thermoneutrality during molt than before molt began. This represents a minimum estimate of the net energy cost of plumage replacement. Below thermoneutrality, the cost of molt in adults changed from 0.384 kcal øC • day -1 before molt to 0.596 kcal at mid-molt to 0.332 kcal after molt. In juveniles the rate of increase at mid-molt was 0.504 kcal øC i day 1. The caloric equivalent of weight loss at night was lower in molting than in nonmolting birds and lower at high than at low ambient temperatures, due largely to the greater amounts of water loss involved. Protein catabolism, as shown by the amount of nitrogen in the urine, increased during molt to the extent of 8.33 g. The nightly respiratory quotient was 0.88. Received 7 June 1977, accepted 20 August 1978.

THERE are many reasons for assuming that molting is a physiological stress. It occurs at a time of the year when other energy demanding processes are normally minimal: reproduction is generally completed, temperatures are high and photoperiods long, and food is abundant and readily found. Body fat stores are at a low level at this time, nocturnal locomotor activity ceases in migratory species, and caged birds die more frequently than at other seasons (Boeme 1952). Direct measurements show that the energy cost of molt is high, being 150 to 330 kcal in several species of sparrow-sized birds (Dolnik 1965, Blackmore 1969).

Our aims here are to report (1) measurements of the energy requirements for molting at several temperatures, and (2) changes in body composition and protein balance during the molt. The observations and experiments are based on adult and juvenile Chaffinches (Fringilla c. coelebs) and were made between 1966 and 1970. The species has only an autumnal molt.

We have shown elsewhere (Dolnik and Gavrilov, in press) that adult Chaffinches begin to lose feathers about the third week in June, the rate of feather loss increasing to a maximum during late July and early August and ceasing about the third week in September. Regeneration of new feathers begins late in June, reaches a peak during the second week of August, and is completed by the end of September. Regeneration of feathers begins in early July for first brood and in early August for second brood juveniles, reaches a peak in both broods during early September, and is completed about the same time as in adults.

METHODS

Adult and juvenile Chaffinches were obtained from the nesting population at the Kurishe Nehrung, USSR, on the Baltic Sea. The birds were placed in large aviaries and were maintained in excellent

253 The Auk 96: 253-264. April 1979

254 DOLNIK AND GAVRILOV [Auk, Vol. 96

condition on special food mash and water ad libitum. The ambient temperature and photoperiod in the aviaries were natural for this lattitude (55 ø N). The food mash contained 12% digestible proteins and 1.2 g of cystine and 1.7 g of cysteine per 100 g total proteins. The caloric equivalent of the food was 4.7 kcal g-• dry weight.

One group of 35 adults and 50 juveniles was maintained as a control, being examined only for the progress of molt and general body condition. A second group of 30 adults and 30 juveniles was used for measurements of metabolic rate. A third group of 60 adults and 60 juveniles was kept in the aviary, like the controls, until needed for carcass analysis.

Procedures of molt observations, registration, and calculation of the plumage regeneration index are described in a separate paper (Dolnik and Gavrilov, in press). The index of plumage regeneration represents the rate of new feather synthesis, not weight of molted feathers, at a given date.

Before the start of molt, one group of 15 adults and 15 juveniles (Group A) was transferred to individual cages of the type described by Martin (1967). Of these, 5 adults and 5 juveniles were exposed to natural photoperiods (17.3 h per day on 20 June to 11.6 h per day on 1 October) and temperatures (mean daily temperatures of 22øC in July to 14øC in late September). The remaining 20 birds were divided between two cabinets under simulated natural photoperiods where measurements of metabolism were made at one of two constant temperatures, 7øC (the lowest natural temperature during the molt period) and 26øC (the highest). A second group of 15 adults and 15 juveniles (Group B) was used for measurements of metabolism at several temperatures, from 5 ø to 30øC. Experimental birds from each group were caged individually and supplied with water and measured amounts of the food mash, and the gross energy intake and excreta voided were determined for an interval of 3-4 days. The birds of both groups were then returned to the aviary and other individuals from each group were placed in the cages. Successive alternation of birds between aviary and cages was desirable to allow maintenance of flight muscle condition, to permit sand and water bathing, and for intake of gastroliths.

The caloric values of the food and excrement were determined after drying at 96øC. The energy metabolized was the difference in the caloric value of the gross energy intake and the excreta. There were only minor variations in body weight during these experiments. Existence metabolism was measured at each experimental temperature before the molt began and after it was completed, with the birds maintaining constant weight within -+0.3 g per 3 days. It was assumed that the energy cost of locomotor activity was the same before, during, and after molt, and that variations of metabolizable energy above a stable existence energy level, therefore, reflect only the costs (productive energy) of molt, unconfounded by variation in locomotor costs.

To determine the metabolic rate of resting birds at night, the rate of oxygen consumption was measured at night by the method described by Kendeigh et al. (1977). The birds were at rest, perched inside plexiglass boxes, in total darkness, and in a postabsorptive state. Measurements were made at several air temperatures (see Fig. 3). The birds were weighed before and after each experiment and rates of oxygen consumption calculated according to data on the respiratory quotient. The respiratory quotient was determined by the open-circuit Haldane method under the same conditions in the middle of molt and after the molt was completed. The caloric value of the weight loss was calculated from the energy equivalent of the oxygen consumed and nightly weight loss during the same time intervals.

Nighttime nitrogen losses were determined in the following manner. Five to 7 h before sunset, birds were placed in small cages with a fine screen-wire floor. After sunset, a glass plate was placed under each cage. In the morning the urinary wastes were extracted from the feces by aspiration into capillary tubes, dried at 96øC, and weighed. The nitrogen content in the urine was determined by the Kjeldahl method. Urinary energy was calculated, considering that 1 g of urinary nitrogen is equivalent to 26.6 kcal of catabolized proteins (King and Farner 1961).

The day before a carcass analysis was scheduled, all birds in the same stage of molt (first, middle, or latter part) were transferred to a separate aviary. Samples for carcass analysis were killed at sunset and at sunrise. The birds were weighed and the feathers cut off. The feathers were weighed, dried at 96øC for 96 h, and weighed again. The birds without feathers were weighed and dissected. The flight muscles, liver, skin, brain, digestive tract, and carcass were weighed separately, dried at 96øC for 96 h, and weighed again. They were then ground in a mill, and the fat was extracted for 24 h with petroleum ether in a Soxhlet apparatus. After extraction the samples were dried and weighed. The water, fat, and dry nonfat components were determined for each bird. The protein content of the dry nonfat material was determined by the Kjeldahl method (1 g N = 6.25 g of protein). Fresh samples of liver and muscle tissue were preserved in a trichloracetic acid-ethanol solution for colorimetric estimation of their glycogen contents by the method of Seifert et al. (1950).

April 1979] Chaffinch Molt Energetics 255

26 I

?• 24

• 22 4

.• 20

12

I

I 0 0 5 I0 15 20 25 30øC Ambient Temperature

Fig. 1. Effects of temperature and stage of molt on metabolized energy in adults. 1, first part of molt (20 June-25 July, 55 days); 2, mid-molt (25 July-1 September, 38 days); 3, latter part of molt (1 September-1 October, 30 days); 4, before molt; 5, after molt. Lines fitted by eye; vertical bars indicate -+ one SE.

RESULTS

Metabolized energy.--Chaffinches increased metabolized energy demands as the molt began (Fig. 1). Requirements were high in the first part of the molt (1) but then decreased in the latter part of the molt (3) and following its completion (5) they were lower than before the molt began (4).

The regressions of metabolized energy on temperature are parallel or nearly so at all stages of molt (average: 0.44 kcal øC -• day •). This agrees with our finding (Kendeigh et al. 1977) that the slopes of regression lines for existence metabolism on temperature are the same during summer and winter in spite of the heavier plumage in the winter.

The difference in existence metabolism between premolting (4) and postmolting (5) birds at temperatures below 20øC is about 1.5 kcal day •. The better insulation provided by the new plumage conserves 150-300 kcal per bird (1.5 kcal x 100-200 days) during autumn, winter, and early spring when ambient temperatures are low.

Productive energy.--Productive energy, calculated as the difference between metabolized energy and existence metabolism at each temperature, was measured for individuals in Group B. The differences between metabolized energy during the first part of molt (1) and the premolting period (4) were 5.0 kcal bird • day -• and were


the same at all temperatures. This, then, is the net productive energy during the first part of the molt. During the middle part of the molt, productive energy was 2.75 kcal bird -• day 1 (2 vs 4). Some productive energy was still being used by adults in September, as regression line 3 in Fig. 1 is higher than line 5. In adult Chaffinches molting under natural temperatures, no productive energy was used before 17 June (Fig. 2). Productive energy in these adults reached a peak during the second week of July and diminished apparently to zero by 25 August. In juveniles a fairly uniform rate of productive energy use was maintained from early July until early September (Fig. 2). These curves are only approximations, as existence metabolism decreases below the premolt level as the plumage approaches complete development. This productive energy included both the net energy of molt and the additional heat production for compensation of heat loss through increased conduc- tion of the plumage.

The total energy cost of the molt under natural temperature conditions is at least 240 kcal per bird in adults and 147 kcal in juveniles. Dry weight of the new plumage in adults is 1,400 mg and 850 mg in juveniles. The ratio of total energy cost to the weight of the plumage is similar in both age classes, 172 kcal g-1.

In birds molting at a constant temperature of 26øC (thermoneutrality), the total energy cost of molt in adult birds was 140 kcal; in birds molting at 7øC, 273 kcal. The value of metabolizable energy at 26øC is within the Chaffinch's thermoneutral zone (Fig. 3) and therefore is the net cost of feather synthesis uncomplicated by a thermoregulatory increment. The additional cost at the lower temperature necessary for body temperature control amounts to 95% of the net energy of molt [(273 - 140)/ 140]. Under fluctuating ambient temperatures, the increased cost above thermoneutrality is 71% of the net energy of molt [{240 - 140)/140]. If the energy value of the new plumage is 5.5 kcal g-1 (King and Farrier 1961), then only 3.2% of the total productive energy is stored in the feathers (5.5/172), or only 5.5% of the net productive energy (5.5 x 1.4/140). In the House Sparrow (Passer domesticus), net energy of molt is 185 kcal per 1.7 g feathers (Blackmore 1969) or 218 kcal per 2.0 g feathers (Dolnik and Gavrilov 1975), reasonably close to our estimates for the Chaf- finch.

Catabolism at night.--The metabolic rate of resting birds in the zone of thermoneutrality before molt began (standard metabolism) was 0.333 kcal h -1 (Fig. 3). With the start of molt it rose 0.045 kcal to 0.378 kcal h -1, declining to the premolt and postmolt rates in the latter part of molt. These data show intensification of basal metabolic processes in the first part of molt, when synthesis of new feathers was intensive (see also dry weight of plumage, Fig. 6), and a return to standard level one month before molt was completed. The lower critical temperature of the zone of thermoneutrality remained at 20 ø- 2 iøC until the latter part of molt, when it rose to 23.5øC.

The resting metabolic rate at 0øC, as estimated from a projection of the regressions of Fig. 3, was 0.67 kcal h -1 before molt. With the beginning of molt, it rose to 0.83 kcal h -1, and at mid-molt it reached 0.89 kcal h l, where it remained until the end of molt. After the molt was completed, the resting metabolism at 0øC was 0.62 kcal h •, below the level before molt.

Changes in the level of resting metabolism at 0øC doubtlessly result from the loss and renewal of the plumage insulation. Thermal conductivity, calculated from the temperature coefficients of the regressions (Fig. 3), was 0.016 kcal øC 1 day-1 before molt, 0.025 kcal at the mid and latter parts of molt, and 0.014 kcal after molt.

April 1979] Chaffinch Molt Energetics 2 5 7

4

3

June July Aug Sept

Fig. 2. The amount of productive energy used during molt at natural temperatures in adults (above) and juveniles (below). Vertical bars indicate _+ one SE.

When expressed on a daily basis, the temperature coefficients for catabolism at night (0.384 kcal øC -• day -• before molt, 0.596 kcal at mid-molt, 0.332 kcal after molt) are lower before and after molt than for metabolized energy (0.437 kcal øC -• day -1) but higher in molting birds. In the total metabolized energy of caged birds, heat increments of feeding and activity apparently compensate for or obscure the changes noted in resting, fasting birds.

The rate of resting metabolism in juvenile birds in the zone of thermoneutrality at mid-molt was about the same as that for adults, but at 0øC it was 0.06 kcal h -• less. Thermal conductivity was 0.021 kcal øC -• h -• or 0.504 kcal øC -• day-L

The caloric equivalent of weight loss at night varies, but is around 2 kcal g-• in the zone of thermoneutrality and between 4 and 5 kcal g-• at 0øC during molt. Before and after molt, the caloric equivalent at thermoneutrality is about 3 kcal g-• and at 0øC between 5.5 and 6.0 kcal g-•. Variations in values of the caloric equiv- alents probably depend on the composition of the materials catabolized and espe- cially on the amount of water loss (Dolnik 1968). Changes in body weight loss from 0.33 g kcal -• before and after molt to 0.50 g kcal -• during molt show that water loss increased during molt by 0.17 g kcal -•. If this water is lost through evaporation, it dissipates 0.17 g x 0.56 kcal g-• (the coefficient of evaporation) = 0.095 kcal for every kcal of metabolism, that is, 9.5% of the heat produced at thermoneutrality.

We measured the cloacal temperature in molting birds by microthermistor during nighttime. Cloacal temperatures were 40.6 ø _+ 0.3 ø (SE) in premolting and post-


0.8 Before Molt 0.8•Start of Molt • •,.

0.4 0.4 •, 4 •

• 0.8 0.8 • • _

• o.• o.• • •

•0.2 0,2 2

o o , ,

• 0.8 After Molt 08 Mid-Molt • • • juveniles •

0.4 4 0.4 4 •

0 I • • • xlO 0 •' 0 •0 20 50 • C •0 20 50 • C

Ambient Temperature Fig. 3. Metabolic rate of resting, postabsorptive birds at night (dark circles) and the caloric equiv-

alence of the nightly weight loss (open circles) at various temperatures and at different sta•es of molt. All panels except the lower right present information from adults. Regression lines fitted by eye through points lower than 20øC to converge at 39øC. Vertical lines show • one SE.

molting birds. It was 0.8 ø + 0.2 ø higher in the first part of molt, in June. During the latter part of molt, in September, it was 0.5 ø + 0.2 ø lower than 40.6 ø. The increases in both water loss and body temperature during the first part of molt show the positive balance between heat production and heat loss at this time.

The amount of nitrogen excreted in the urine at night, representing protein catabolism, increased to a maximum during the first week of August and then decreased (Fig. 4). Changes from week to week in protein catabolism were identical to changes in the rate of feather regeneration, as shown in another paper (Dolnik and Gavrilov, in press), and to changes in the standard metabolic rate. During molt, a total of 8.33 g of protein was catabolized above the level in nonmolting birds.

The mean nightly respiratory quotient in molting Chaffinches was 0.88 + 0.01 (SE), but protein catabolism produced 16.4% of the energy loss at night. Conse- quently, the nonprotein respiratory quotient is 0.744, i.e., catabolism of carbohydrates produced 7.4% and catabolism of lipids 76.2% of the total heat production of fasting birds.

Carcass analysis.--The total body weight of adults decreased at the start of the molt and remained low during the molt. Only in the premigratory period (October)

April 1979]

Fig. 4. _+ one SE.

Chaffinch Molt Energetics 259

1.5

0.5

0.5

_ I

June July Aug Sept Oct Amount of energy lost in the urine during molt by adults and juveniles. Vertical lines indicate

did it begin to increase (Fig. 5). The nonfat body weight and the nonfat body weight minus feathers were higher during molt than before and after molt. In juveniles the total body weight increased slowly during molt, decreased near the end of molt, and then increased again in the premigratory period (Table 1).

The water content of the body became very high during molt in both adults and juveniles and was correlated with the rate of feather regeneration and feather weight (Fig. 6). In juveniles, the increase in water content was greatest in the skin and feathers and progressively less in the stomach, liver, breast muscle, and brain (Table 1). Increase in body water during molt occurs generally in redpolls (Carduelis fiammea), but in Bullfinches (Pyrrhula pyrrhula) it is localized in the feathers (Newton 1968).

The fat content of the molting adult birds was low and the daily variation was small (Fig. 5). The difference in fat content between birds collected in the evening and at daybreak varied only between 100 and 200 mg (0.95-1.9 kcal). The energy obtained by the oxidation of this fat overnight (9 h) equals about one-third of the total catabolism at thermoneutrality. Additional energy must have been derived from proteins, carbohydrates, and food in the alimentary tract. The glycogen level in molting birds, however, is low, and the accumulation of reserve glycogen in the evening is small compared to that before and after molt.

Nonfat dry body weight minus feathers increased at first but then declined during

26O DOLNIK AND GAVRILOV [Auk, Vol. 96

23 22

•21 2O

•18 • 17

IG

14

5 4

• 5

2 • 0

6 50

Fig. 5.

4

June July Aug Sept Oct Body composition of adults during molt. 1, total body weight; 2, fat-free body weight; 3, fat-

free body weight excluding feathers; 4, weight of body water; 5, lean dry weight excluding feathers; 6, weight of protein; 7, weight of body fat; 8, weight of glycogen. Open circles, morning; closed circles, evening.

molt in a similar fashion in both adults and juveniles. The protein content varied during molt in the same manner. In adults, it constituted 76.5-83.2% of the dry nonfat weight minus feathers; in juveniles, 80.0-84.0%. Before and after molt, the protein content at dusk was 150-250 mg higher than at dawn. Before the start of


2.2

•2.0 -

'*-- 1.6-

1.2 -

I I I I i

1.0 • • I ,• , I I • June July Aug Sept Oct

Fig. 6. Plumage weight during molt in adults (closed circles) and in juveniles (open circles). Full lines are for fresh weights, broken lines for dry weights.

molt, protein accumulated in the body, and its utilization during the night was minor. During the first part of molt, the dusk-dawn change in protein level reached 500 mg and constituted about 70% of the nightly energy loss. Newton (1968) also found a greater daytime accumulation of lean dry weight in molting compared with nonmolting Bullfinches. Increases of body water content and nightly protein utilization during molt may be connected with the active synthesis of proteins for feathers.

DISCUSSION

The increase in metabolized energy during molt in Chaffinches results from the use of productive energy in feather formation and the heat lost through increased conductance of plumage. The additional energy cost for body temperature regulation may be calculated from total productive energy of molt if the net energy for new feather formation were measured.

The amount of productive energy in birds molting at a constant temperature of 26øC (140 kcal) is perhaps a better estimate of the net energy cost of feather synthesis. Selection of 26øC for this measurement is somewhat arbitrary, but the ambient temperature must be sufficiently high to impose no cost on the bird for temperature regulation.

The increase of 0.045 kcal h -• (1.1 kcal day -•) in resting metabolism at night in the zone of thermoneutrality is a second way to calculate the net cost of feather

262 DOL•4I}C AND GAVRILOV [Auk, Vol. 96

TABLE 1. Total body weight and water content (percent of fresh tissue weight, X -+ SE) in several parts of the body in juvenile Chaffinches during molt.

Before Start of Middle of End of molt molt molt molt

Number examined 12 12 24 12 Total body weight, g 19.8 -+ 0.5 20.9 -+ 0.6 21.0 -+ 0.6 20.0 -+ 0.5 Liver 72.3 -+ 0.7 73.0 -+ 0.2 73.5 -+ 0.3 72.3 -+ 0.1 Breast muscle 71.5 -+ 0.2 72.9 -+ 0.3 72.5 -+ 0.2 71.4 -+ 0.1 Skin 53.0 -+ 1.0 59.0 -+ 0.5 60.0 -+ 1.2 54.0 -+ 1.5 Feathers 18.0 -+ 2.0 32.0 -+ 2.0 27.0 _+ 1.0 19.0 -+ 0.5 Brain 78.5 -+ 1.0 79.0 -+ 0.8 78.9 -+ 0.3 78.7 -+ 0.5 Stomach 68.0 -+ 0.5 73.8 -+ 0.3 74.8 -+ 0.1 71.6 -+ 0.1

formation. The period of productive energy expenditure continues for 70 days (Fig. 2), so the additional cost of feather formation over this period should be 77 kcal. However, this may be a minimal estimate. As indicated by growth bars on feathers, formed as a result of diurnal metabolic variations in the rate of synthesis (Wood 1950), feather formation at night is less rapid than during daytime.

A third approach is to calculate the difference between metabolized energy during molt in the thermoneutral zone (23ø-33øC) and existence metabolism at the same temperatures in the premolting period. If we assume that existence metabolism before molt continues at 11.2 kcal day -• through the zone of thermoneutrality, then the difference between this value and metabolized energy in the first and middle parts of molt (Fig. 1; lines 1,2) would be 2.0 kcal day -•. Through a 70-day period, the net cost of feather formation is 140 kcal day -1, the same as in the direct measurement at 26øC.

The extrathermoregulatory cost during molt is significant and can be roughly estimated as follows. The total productive energy of molt at 7øC is 273 kcal, at natural temperatures (average: 17.2øC), 240 kcal. The extra cost for body temperature regulation is thus 273 - 140 = 133 kcal and 240 - 140 --- 100 kcal, respec- tively, or 3.23 kcal øC •. At 0øC, metabolized energy is 26.7 kcal day -1 in the first part of molt (Fig. 1:1) and 22.3 kcal before molt (Fig. 1:4). The difference of 4.4 kcal may be allocated to the cost of feather formation (2.0 kcal) and the extra cost for body temperature regulation (2.4 kcal).

Our results show that protein metabolism is greatly accelerated during molt. The index of plumage regeneration (Dolnik and Gavrilov, in press), the wet and dry weights of plumage (Fig. 6), and the amount of total productive energy (Fig. 2), all show clearly that synthesis of material for new feathers is intensive during the first part of molt. Development of feathers is more intensive in the middle part of molt, while during the latter part of molt both synthesis and development decrease. In the first part of molt productive energy (Fig. 2) and standard metabolism in the thermoneutral zone (Fig. 3) are higher than in the middle part of molt, but thermal conductance is lower in the first period than in the second (Fig. 3). This suggests that in the first part of molt a larger proportion of the productive energy is being expended for feather formation than in the middle part of molt.

One of the apparent enigmas in our results is the contrast between the large net productive energy required for feather formation (140 kcal) and the small amount of material involved in feather formation. The net energy equivalent of the plumage that is replaced is only 7.7 kcal in adults (1.4 g feathers x 5.5 kcal g-l). The effi-


ciency ratio of feather synthesis (5.5% of net productive energy) contrasts with the efficiency of, for instance, the formation of eggs and somatic tissue during growth (about 80%, Brody 1945).

Another surprise is the very high level of protein catabolism during molt, espe- cially in the first part of molt. The activation of protein catabolism is indicated by the increase of protein content in the body before molt and decrease during molt, the substantial difference in the level of protein between dusk and dawn, the respiratory quotient of 0.88, the increase of urinary nitrogen, and the increasing water content of the body and its parts. This very active protein catabolism results in an extremely small final protein production, 1.4 g of feathers through 70 days.

It may be that this disparity results from food consumption for the extraction of certain amino acids rather than for its energy content. Feather synthesis requires certain amino acids. Newton (1968) noted a concentration of sulfur-containing cystine and cysteine in feathers at 6.8-8.2 g per 100 g total protein, compared with only 0-6.3 g per 100 g in animal proteins and 0-2.9 g per 100 g in vegetable proteins. The new plumage resulting from the postnuptial molt in Chaffinches contains 105 mg of cystine and cysteine. This amount of sulfur-containing amino acids is contained in 3.62 g of digestable proteins of the food mash used in our experiments, or in 30.2 g of digested food. If it is assumed that cystine and cysteine are essential (i.e., not synthesized in the body in significant amounts from other precursors-- which may not be strictly correct), then the amount of food supplying the minimal quantities of these acids required in the replacement of the plumage contains 141 kcal of metabolized energy, or essentially the same as estimated by other methods.

It is thus possible that the primary cause of increased food consumption during molt is the consumption of proteins for feather synthesis. All other components of food should be oxidized, and the energy of oxidation should serve as compensation for the extra cost of body temperature regulation, as a secondary effect of the greater food consumption. The increase in body temperature and decrease in the caloric equivalent of body weight loss during the first part of molt indicate that additional heat production from oxidation is higher than need be for temperature regulation during this period of molt.

Through all other seasons (migration, wintering, egg formation) birds accumulate fat reserves for compensation of increased energy expenditure. Only molt is associated with a low level of fat. In the Chaffinch and several other species (Spiza americana, Zimmerman 1965; Zonotrichia leucophrys, King et al. 1965; Pyrrhula pyrrhula, Newton 1968; Acanthis fiammea, Evans 1966; and Passer domesticus, Barnett 1970), the start of molt is associated with a lowering of the fat level, an increasing of the water content of the body, and an increase in daily variation of non-fat components of the body. It is a paradox if the increased food intake during molt serves only for thermoregulation. However, it seems an adaptive reaction if the heat production of oxidation of non-sulfur-containing components of food can compensate for a major part of the extra heat loss in molting birds.

During the first part of molt, the increased food consumption serves primarily for the intake of the required amounts of amino acids high in sulfur, secondarily pro- viding heat for compensation of increased conductance. This may be why in Chaf- finches during the first part of molt, heat production is higher than that required for thermoregulation. Body temperature and water loss increase at this time. During the daytime, food gives proteins for synthesis and replenishing of body protein degraded during the night. During the night, growth of feathers may be based partly

264 DOLNIK aND G^VRILOV [Auk, Vol. 96

on the degradation and distribution of the body proteins. The great daily fluctuation of dry non-fat body components is shown in Chaffinches and other species. Fat reserves cannot serve for synthesis during the night or during food deprivation, and fat reserves are minimal in all species through the molt period.

In all species the molt period is associated with increased water content in the body. The additional water may be necessary for degradation, redistribution, and resynthesis of proteins, for the increased blood volume perfusing the active feather pulps, and for compensation of the additional water loss in respiration.

ACKNOWLEDGMENTS

We are greatly indebted to John D. Chilgren, Charles R. Blem, James R. King, and S. Charles Kendeigh for help in the preparation of this paper. Carol Pietruszka prepared the illustrations.

LITERATURE CITED

BARNETT, L. B. 1970. Seasonal changes in temperature acclimatization of the House Sparrow, Passer domesticus. Comp. Blochem. Physiol. 33: 559-578.

BLACKMORE, F. H. 1969. The effect of temperature, photoperiod and molt on the energy requirements of the House Sparrow, Passer domesticus. Comp. Blochem. Physiol. 30: 433-444.

BOEME, L. R. 1952. Pevchie ptitsi [Song-birds]. Moscow, Publ. House Sovetskaia Nauka: 263. BRODY, S. 1945. Bioenergetics and growth. New York, Reinhold Publ. Co. DOLNIK, V. R. 1965. Bioenergetika linki vurkovikh ptits kak adaptatsia k migratsii [Bioenergetics of

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bioenergetics of molt in the chaffinch (fringilla coelebs)