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Factors Affecting the Duration of the First Annual Rest*
Factors Affecting the Duration of the First Annual Rest* I. MICHAEL LERNER AND LEWIS W. TAYLOR
University of California, Berkeley, California (Received for publication March 8, 1941)
T
* Assistance in computations was provided under W.P.A. Official Project No. O.P.65-1-08-91. Dist. 11589 Unit B6.
date, age at first egg, length of winter pause, and days broody. They have, however, reported significant negative correlation ratios of rest duration with average clutch size (—.3339), length of laying year ( — .3613), and previous egg production (-.3695). The present study consists of an analysis of simple, partial, and multiple correlations involving the duration of the first annual rest in a flock of White Leghorns. MATERIAL AND METHODS
A population consisting of 289 Single Comb White Leghorn pullets hatched in March and April, 1935, was selected as material for statistical analysis from the University of California flock hatched in that year. The only criterion of selection applied to these birds was the requirement that complete information on the duration of the annual fall rest was to be available. This implied the existence of a well defined indication of cessation of production in the summer, fall, or winter of the second year of life, and a resumption of production following this rest. In this paper the date on which the last egg preceding the fall rest was laid is referred to for the sake of simplicity as the date of last egg. The age of the bird on this date is termed the age at last egg. The date of the first egg after the fall rest is referred to as the date of resumption of lay, and the age of the bird on this date as age at resumption of lay. The number of days elapsing between these dates is designated as the duration of an-
[490]
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on May 31, 2015
HE period of non-production following the first laying year is frequently referred to as the annual molt. Although the close association of cessation of egg production with the process of shedding and replacement of feathers has been adequately demonstrated (Marble, 1930), certain differences in this respect exist between birds with different production characteristics. Marble has found that in cases of higher producing individuals molting begins before production has actually stopped, while in lower producing individuals little if any molt occurs before complete cessation of laying. .Thus there is no complete coincidence of molting with the period of nonproduction between the first and second laying years. Since no molt observations were made on the birds considered in this paper, the period of nonproduction studied is termed here the annual rest. The methods followed in the determination of the duration of this period will be described later. The factors affecting the duration of the annual rest have been investigated only to a limited extent. Marble has found in three flocks of White Leghorns a negative correlation between the duration of annual rest (but not of the duration of actual molt) with egg production. Hays and Sanborn (1930) in studying the records of a flock of Rhode Island Reds found no correlation between annual rest duration and hatching
FACTORS AFFECTING THE DURATION OF T H E FIRST A N N U A L R E S T
may be considered as independent variables, while the duration of the rest may be used as the dependent variable. It may be seen from Table 1 that date of hatch (within the comparatively short hatching season investigated), age at first egg, and length of winter pause bear little TABLE 2.—The relation between length of winter pause
and duration of rest Number of birds
Winter pause character
184 21 13 71
No pause 7-14 days pause 15-21 days pause 2 2 + days pause
Average duration of rest 83.8 93.6 85.7 88.2
+ 2.8 + 8.1 + 7.2 + 5.1
days days days days
or no relation to the duration of the annual rest, thus confirming the findings of Hays and Sanborn. The relation of winter pause DATA AND DISCUSSION length to the duration of annual rest is furTable 1 presents zero-order correlation ther investigated in Table 2 to remove all coefficients between the duration of the doubts with regard to the considerable annual rest and the series of variables though statistically non-significant increase studied. It should be noted that since these in the correlation ratio over the correlation coefficients represent the correlations be- coefficient. Table 2 definitely demonstrates tween factors measured on the same in- the independence of these two variables. dividuals, any implications as to causality Thus date of hatch, age at first egg, and are extremely dangerous. However, all of length of winter pause may be discarded the factors involved are measured prior to from further consideration. The other factors listed in Table 1 all the onset of the annual rest. Hence, at least from the standpoint of order in time, they show significant negative correlations with the dependent variable. Since they are interTABLE 1.—Correlation coefficients and ratios betweenrelated amongst themselves it becomes duration of rest and variables indicated necessary to establish which of the variables involved have a direct influence on the Correlation Correlation Variable correlated with duration of annual rest, and which affect it ratio* coefficient! duration of annual rest only indirectly. This may be done by means -.038 .108 of partial and multiple correlations. .009 .108 .057 .232 Length of winter p a u s e . . . . Table 3 presents the partial, while Table -.220 .308 -.521 .556 4 shows the multiple correlation coefficients. -.525 .550 Date of last egg Since the relationship between biological -.462 .487 Length of laying year -.353 .426 Biological year production. year production and age at last egg, date of last egg, and length of laying year, respect Minimum significant correlation coefficient tively, was found to be curvilinear (respecequals 0.113. * All correlations definitely linear, except those tive correlation ratios of 0.732, 0.616, and involving winter rate and biological year production, 0.582 as against correlation coefficients of the linearity of which is questionable.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on May 31, 2015
nual rest. The term winter pause refers to cessation of lay for a period of seven or more consecutive days beginning prior to March 1 of the bird's first year of life. It should by no means be confused with the first annual rest, which occurs in the second year of life. The number of days between the beginning of first year production and the date of last egg is considered to represent the length of the laying year, and the production during this period is designated as the biological year production. Winter rate is calculated by dividing the number of eggs laid during November, December, January, and February by the number of days in this period less days in pause, days broody, and days before the first egg was laid which may fall within this period.
491
492
I. MICHAEL LERNER AND LEWIS W. TAYLOR TABLE 3.—Partial correlation coefficients between duration of rest and variables indicated Variable correlated with duration of rest
Partial correlation coefficient
Winter rate
Age at last egg Date of last egg Length of laying year Biological year production
-.158 -.168 -.148 -.067
Age at last egg
Winter rate Date of last egg Length of laying year
-.504 -.125 -.275
Date of last egg
Winter rate Age of last egg Length of laying year
-.510 -.144 -.306
Length of laying year
Winter rate Age at last egg Date of last egg Age at and date of last egg
-.438 -.055 -.130 -.006
Biological year production
Winter rate
-.130
0.625, 0.476, and 0.430), no partial or multiple correlation coefficients involving these relations are given. It may be seen from the examination of these tables that the primary factors affecting duration of annual rest are either the TABLE 4.—Multiple correlation coefficients with duration of rest as the dependent variable
Independent variables Age at last egg, date of last egg Age at last egg, length of laying year.. Date of last egg, length of laying year. Length of laying year, winter r a t e . . . . Winter rate, biological year production.
Multiple correlation coefficient .535 .523 .538 .536 .544 .481 .358
date of last egg or the age at last egg, or both. The correlation involving the length of laying year is reduced to —0.006 when both age at and date of last egg are eliminated. Furthermore, length of laying year does not increase effectively the multiple correlation coefficient over the zeroorder ones, when age or date of last egg are correlated with duration of molt. The basis for considering winter rate and
biological year production as secondary influences is somewhat more complex. Production itself is determined by maturity (age at first egg), persistency (age at or date of last egg), annual rate (adequately expressed by winter rate, Lerner and Taylor, 1937), and length of winter pause. Broodiness in this flock was too limited in extent to contribute significantly to the variance in production accounted for by the above. Since, as shown above, maturity and winter pause are not involved in the determination of the duration of the annual rest, production is correlated with duration of annual rest through its correlation with persistency and winter rate. It may be seen from Tables 3 and 4 that winter rate does not add significantly to the determination of the duration of rest obtained from age at TABLE 5.—Means and standard deviations of factors determining duration of annual rest
Variable
Mean
Standard deviation in days
Age at last egg Date of last egg Age at resumption of lay.. Date of resumption of lay.
562.2 days October 12 647.7 days January 6
40.9 39.9 38.3 38.1
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on May 31, 2015
Variable eliminated
FACTORS AFFECTING THE DURATION OF THE FIRST ANNUAL REST
or date of last egg. Hence, it must be concluded that the contribution of biological year production to the duration of annual rest is only through persistency, as measured by age at or date of last egg. This interpretation is consistent with the relative magnitudes of correlation coefficients in Table 1. In that table age at and date of last egg show the highest correlations with the duration of rest, followed by the correlation involving length of laying year. This in turn is followed by the correlation involving production, and finally that involving rate. Each of the factors named exercises its influence on duration of rest through the previous mentioned one in this hierarchy. Thus of the different factors considered it is the time of cessation of production that contributes most to the deter-
493
mination of the duration of rest. The question immediately arises whether this is an age or a seasonal effect. The correlation technic used to eliminate the other variables cannot be used to determine this point because age at last egg and date of TABLE 6.—Relation between date of last egg and duration of annual rest Number of birds
August and earlier. September October November December and later
39 62 106 53 29
Mean duration of rest 125.8±9.0 99.7 + 4.4 79.8 + 2.7 63.9 + 2.7 64.7 + 4.5
days days days days days
last egg are very intimately correlated (0.908), so that it is impossible to separate age from date effects.
-15:00 SCALE FOR OGIVES No. of
birds
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—10:00 DATE OF LAST APR.
MAY
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| JUNE
0
i JULY
1 AUG.
—
1 SEPT.
OCT.
1
1 NOV.
DEC.
1 JAN.
1 FEB.
i MAR.
i APR.
1 MAY
DATE
Fiu. 1. The relationship between absolute day length, change in day length, beginning and end of the annual rest.
-
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on May 31, 2015
Month of last egg
494
I. MICHAEL LERNER AND LEWIS W. TAYLOR
their resting period until this rate is reached again. Thus 88 percent of the birds lay their last egg before the rest within the period during which the length of day decreases by 0.2 or more percent. The resumption of production does not seem to bear such a relationship to day length variation. Some of the birds resume production before the highest rate of decrease in day length is attained. By the winter solstice more than 2 5 percent of the birds are again in lay and 99 percent of the birds resume production before the spring equinox. Interpretation in terms of thresholds of response to such a factor as variation in length of day is extremely difficult. If the absolute length of day is considered as the stimulus, the thresholds for cessation and for resumption of production are not the same in the same bird, since if such were the case, a negative correlation would have been obtained between the dates of last egg and of resumption of lay. Instead a significant positive correlation was obtained. On the other hand it is possible that a single threshold response to relative changes in decreasing day length controls the onset of rest, and that the resumption of lay does not take place until a given modal duration of rest elapses. SUMMARY
The study of single, partial, and multiple correlations in a flock of Single Comb White Leghorns reveals that persistency, as measured by date of or age at last egg before the rest, is the production factor which affects the duration of the first annual rest. Length of laying year and annual production are correlated with duration of rest only by virtue of their association with persistency. Similarly, the correlation of winter rate with duration of rest depends entirely on its correlation with annual production.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on May 31, 2015
A possible approach to this problem is through the study of the age at or date of resumption of production. The means and standard deviations of these variables are presented in Table 5. It would seem that if the seasonal factor were the more important one the variability of the date of last egg and the resumption of production would be less than that of the age at resumption of lay. This does not seem to be the case. Furthermore the date of last egg and the date of resumption are significantly correlated (correlation coefficient of 0.397, correlation ratio of 0.4S3, with borderline significance of difference). This would indicate that the duration of rest is to some extent independent of seasonal effects, in the sense that once the rest has been initiated it tends to continue for a given time. This time, however, as shown by Table 6 may be related to the date of last egg. It thus seems that in the determination of the duration of the rest, the tendency towards a mean length is modified by the time of its initiation. The relationship between the date of last egg and the date of resumption of lay on the one hand and the length of day on the other may be of some interest. It is illustrated in Figure 1, which also shows the rate of change in day length. The curves presented have been smoothed. It may be seen that the length of day begins to decrease towards the end of June. This decrease starting at the rate of about 0.1 percent per day reaches its greatest velocity in the first week of November. The rate of decrease slows down in November and December, so that in the period preceding the winter solstice the daily decrease is only 0.05 percent. The distribution of the dates of last egg is such that about 7 percent of the birds begin their rest before the daily rate of decrease in length of day is 0.2 percent, and about 5 percent of the birds do not enter
FACTORS AFFECTING THE DURATION OF THE FIRST ANNUAL REST
The relation between length of day and duration of the rest is discussed. Eightyeight percent of the birds begin their rest in the period during which length of day decreases by 0.2 or more percent daily. The data do not permit further interpretation of the possible association of duration of rest and changes in the day length.
495
REFERENCES CITED
Hayes, F. A., and R. Sanborn, 1930. Duration of annual molt in relation to egg production. Mass. Agr. Expt. Sta. Bull. 264:73-85. Lerner, I. M., and L. W. Taylor, 1937. Interrelationships of egg production factors as determined for White Leghorn pullets. Journ. Agric. Res. 55:703-712. Marble, D. R., 1930. The molting factor in judging fowls for egg production. Cornell Agr. Expt. Sta. Bull. 503:3-42.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on May 31, 2015
University of California, Berkeley, California (Received for publication March 8, 1941)
T
* Assistance in computations was provided under W.P.A. Official Project No. O.P.65-1-08-91. Dist. 11589 Unit B6.
date, age at first egg, length of winter pause, and days broody. They have, however, reported significant negative correlation ratios of rest duration with average clutch size (—.3339), length of laying year ( — .3613), and previous egg production (-.3695). The present study consists of an analysis of simple, partial, and multiple correlations involving the duration of the first annual rest in a flock of White Leghorns. MATERIAL AND METHODS
A population consisting of 289 Single Comb White Leghorn pullets hatched in March and April, 1935, was selected as material for statistical analysis from the University of California flock hatched in that year. The only criterion of selection applied to these birds was the requirement that complete information on the duration of the annual fall rest was to be available. This implied the existence of a well defined indication of cessation of production in the summer, fall, or winter of the second year of life, and a resumption of production following this rest. In this paper the date on which the last egg preceding the fall rest was laid is referred to for the sake of simplicity as the date of last egg. The age of the bird on this date is termed the age at last egg. The date of the first egg after the fall rest is referred to as the date of resumption of lay, and the age of the bird on this date as age at resumption of lay. The number of days elapsing between these dates is designated as the duration of an-
[490]
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on May 31, 2015
HE period of non-production following the first laying year is frequently referred to as the annual molt. Although the close association of cessation of egg production with the process of shedding and replacement of feathers has been adequately demonstrated (Marble, 1930), certain differences in this respect exist between birds with different production characteristics. Marble has found that in cases of higher producing individuals molting begins before production has actually stopped, while in lower producing individuals little if any molt occurs before complete cessation of laying. .Thus there is no complete coincidence of molting with the period of nonproduction between the first and second laying years. Since no molt observations were made on the birds considered in this paper, the period of nonproduction studied is termed here the annual rest. The methods followed in the determination of the duration of this period will be described later. The factors affecting the duration of the annual rest have been investigated only to a limited extent. Marble has found in three flocks of White Leghorns a negative correlation between the duration of annual rest (but not of the duration of actual molt) with egg production. Hays and Sanborn (1930) in studying the records of a flock of Rhode Island Reds found no correlation between annual rest duration and hatching
FACTORS AFFECTING THE DURATION OF T H E FIRST A N N U A L R E S T
may be considered as independent variables, while the duration of the rest may be used as the dependent variable. It may be seen from Table 1 that date of hatch (within the comparatively short hatching season investigated), age at first egg, and length of winter pause bear little TABLE 2.—The relation between length of winter pause
and duration of rest Number of birds
Winter pause character
184 21 13 71
No pause 7-14 days pause 15-21 days pause 2 2 + days pause
Average duration of rest 83.8 93.6 85.7 88.2
+ 2.8 + 8.1 + 7.2 + 5.1
days days days days
or no relation to the duration of the annual rest, thus confirming the findings of Hays and Sanborn. The relation of winter pause DATA AND DISCUSSION length to the duration of annual rest is furTable 1 presents zero-order correlation ther investigated in Table 2 to remove all coefficients between the duration of the doubts with regard to the considerable annual rest and the series of variables though statistically non-significant increase studied. It should be noted that since these in the correlation ratio over the correlation coefficients represent the correlations be- coefficient. Table 2 definitely demonstrates tween factors measured on the same in- the independence of these two variables. dividuals, any implications as to causality Thus date of hatch, age at first egg, and are extremely dangerous. However, all of length of winter pause may be discarded the factors involved are measured prior to from further consideration. The other factors listed in Table 1 all the onset of the annual rest. Hence, at least from the standpoint of order in time, they show significant negative correlations with the dependent variable. Since they are interTABLE 1.—Correlation coefficients and ratios betweenrelated amongst themselves it becomes duration of rest and variables indicated necessary to establish which of the variables involved have a direct influence on the Correlation Correlation Variable correlated with duration of annual rest, and which affect it ratio* coefficient! duration of annual rest only indirectly. This may be done by means -.038 .108 of partial and multiple correlations. .009 .108 .057 .232 Length of winter p a u s e . . . . Table 3 presents the partial, while Table -.220 .308 -.521 .556 4 shows the multiple correlation coefficients. -.525 .550 Date of last egg Since the relationship between biological -.462 .487 Length of laying year -.353 .426 Biological year production. year production and age at last egg, date of last egg, and length of laying year, respect Minimum significant correlation coefficient tively, was found to be curvilinear (respecequals 0.113. * All correlations definitely linear, except those tive correlation ratios of 0.732, 0.616, and involving winter rate and biological year production, 0.582 as against correlation coefficients of the linearity of which is questionable.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on May 31, 2015
nual rest. The term winter pause refers to cessation of lay for a period of seven or more consecutive days beginning prior to March 1 of the bird's first year of life. It should by no means be confused with the first annual rest, which occurs in the second year of life. The number of days between the beginning of first year production and the date of last egg is considered to represent the length of the laying year, and the production during this period is designated as the biological year production. Winter rate is calculated by dividing the number of eggs laid during November, December, January, and February by the number of days in this period less days in pause, days broody, and days before the first egg was laid which may fall within this period.
491
492
I. MICHAEL LERNER AND LEWIS W. TAYLOR TABLE 3.—Partial correlation coefficients between duration of rest and variables indicated Variable correlated with duration of rest
Partial correlation coefficient
Winter rate
Age at last egg Date of last egg Length of laying year Biological year production
-.158 -.168 -.148 -.067
Age at last egg
Winter rate Date of last egg Length of laying year
-.504 -.125 -.275
Date of last egg
Winter rate Age of last egg Length of laying year
-.510 -.144 -.306
Length of laying year
Winter rate Age at last egg Date of last egg Age at and date of last egg
-.438 -.055 -.130 -.006
Biological year production
Winter rate
-.130
0.625, 0.476, and 0.430), no partial or multiple correlation coefficients involving these relations are given. It may be seen from the examination of these tables that the primary factors affecting duration of annual rest are either the TABLE 4.—Multiple correlation coefficients with duration of rest as the dependent variable
Independent variables Age at last egg, date of last egg Age at last egg, length of laying year.. Date of last egg, length of laying year. Length of laying year, winter r a t e . . . . Winter rate, biological year production.
Multiple correlation coefficient .535 .523 .538 .536 .544 .481 .358
date of last egg or the age at last egg, or both. The correlation involving the length of laying year is reduced to —0.006 when both age at and date of last egg are eliminated. Furthermore, length of laying year does not increase effectively the multiple correlation coefficient over the zeroorder ones, when age or date of last egg are correlated with duration of molt. The basis for considering winter rate and
biological year production as secondary influences is somewhat more complex. Production itself is determined by maturity (age at first egg), persistency (age at or date of last egg), annual rate (adequately expressed by winter rate, Lerner and Taylor, 1937), and length of winter pause. Broodiness in this flock was too limited in extent to contribute significantly to the variance in production accounted for by the above. Since, as shown above, maturity and winter pause are not involved in the determination of the duration of the annual rest, production is correlated with duration of annual rest through its correlation with persistency and winter rate. It may be seen from Tables 3 and 4 that winter rate does not add significantly to the determination of the duration of rest obtained from age at TABLE 5.—Means and standard deviations of factors determining duration of annual rest
Variable
Mean
Standard deviation in days
Age at last egg Date of last egg Age at resumption of lay.. Date of resumption of lay.
562.2 days October 12 647.7 days January 6
40.9 39.9 38.3 38.1
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on May 31, 2015
Variable eliminated
FACTORS AFFECTING THE DURATION OF THE FIRST ANNUAL REST
or date of last egg. Hence, it must be concluded that the contribution of biological year production to the duration of annual rest is only through persistency, as measured by age at or date of last egg. This interpretation is consistent with the relative magnitudes of correlation coefficients in Table 1. In that table age at and date of last egg show the highest correlations with the duration of rest, followed by the correlation involving length of laying year. This in turn is followed by the correlation involving production, and finally that involving rate. Each of the factors named exercises its influence on duration of rest through the previous mentioned one in this hierarchy. Thus of the different factors considered it is the time of cessation of production that contributes most to the deter-
493
mination of the duration of rest. The question immediately arises whether this is an age or a seasonal effect. The correlation technic used to eliminate the other variables cannot be used to determine this point because age at last egg and date of TABLE 6.—Relation between date of last egg and duration of annual rest Number of birds
August and earlier. September October November December and later
39 62 106 53 29
Mean duration of rest 125.8±9.0 99.7 + 4.4 79.8 + 2.7 63.9 + 2.7 64.7 + 4.5
days days days days days
last egg are very intimately correlated (0.908), so that it is impossible to separate age from date effects.
-15:00 SCALE FOR OGIVES No. of
birds
+.40-
300—
\
.^ /
-14:00
/ —13:00
>
/
V
1
1 1
200 —
1
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1
1 — 12:00
1
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A
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/ \
—IV.00
\
/ \
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/
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V/ /
—10:00 DATE OF LAST APR.
MAY
\
+.20-
+.10 -
1 -.10-
/
1 I lI
\50-/
-.20-
/
-.30-
\
-y
EGE__
1 OGIVE t
1
/
0
/1 ! 1
\ \\
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100
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1
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1/ Ik
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\
1
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250/—
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//
| JUNE
0
i JULY
1 AUG.
—
1 SEPT.
OCT.
1
1 NOV.
DEC.
1 JAN.
1 FEB.
i MAR.
i APR.
1 MAY
DATE
Fiu. 1. The relationship between absolute day length, change in day length, beginning and end of the annual rest.
-
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on May 31, 2015
Month of last egg
494
I. MICHAEL LERNER AND LEWIS W. TAYLOR
their resting period until this rate is reached again. Thus 88 percent of the birds lay their last egg before the rest within the period during which the length of day decreases by 0.2 or more percent. The resumption of production does not seem to bear such a relationship to day length variation. Some of the birds resume production before the highest rate of decrease in day length is attained. By the winter solstice more than 2 5 percent of the birds are again in lay and 99 percent of the birds resume production before the spring equinox. Interpretation in terms of thresholds of response to such a factor as variation in length of day is extremely difficult. If the absolute length of day is considered as the stimulus, the thresholds for cessation and for resumption of production are not the same in the same bird, since if such were the case, a negative correlation would have been obtained between the dates of last egg and of resumption of lay. Instead a significant positive correlation was obtained. On the other hand it is possible that a single threshold response to relative changes in decreasing day length controls the onset of rest, and that the resumption of lay does not take place until a given modal duration of rest elapses. SUMMARY
The study of single, partial, and multiple correlations in a flock of Single Comb White Leghorns reveals that persistency, as measured by date of or age at last egg before the rest, is the production factor which affects the duration of the first annual rest. Length of laying year and annual production are correlated with duration of rest only by virtue of their association with persistency. Similarly, the correlation of winter rate with duration of rest depends entirely on its correlation with annual production.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on May 31, 2015
A possible approach to this problem is through the study of the age at or date of resumption of production. The means and standard deviations of these variables are presented in Table 5. It would seem that if the seasonal factor were the more important one the variability of the date of last egg and the resumption of production would be less than that of the age at resumption of lay. This does not seem to be the case. Furthermore the date of last egg and the date of resumption are significantly correlated (correlation coefficient of 0.397, correlation ratio of 0.4S3, with borderline significance of difference). This would indicate that the duration of rest is to some extent independent of seasonal effects, in the sense that once the rest has been initiated it tends to continue for a given time. This time, however, as shown by Table 6 may be related to the date of last egg. It thus seems that in the determination of the duration of the rest, the tendency towards a mean length is modified by the time of its initiation. The relationship between the date of last egg and the date of resumption of lay on the one hand and the length of day on the other may be of some interest. It is illustrated in Figure 1, which also shows the rate of change in day length. The curves presented have been smoothed. It may be seen that the length of day begins to decrease towards the end of June. This decrease starting at the rate of about 0.1 percent per day reaches its greatest velocity in the first week of November. The rate of decrease slows down in November and December, so that in the period preceding the winter solstice the daily decrease is only 0.05 percent. The distribution of the dates of last egg is such that about 7 percent of the birds begin their rest before the daily rate of decrease in length of day is 0.2 percent, and about 5 percent of the birds do not enter
FACTORS AFFECTING THE DURATION OF THE FIRST ANNUAL REST
The relation between length of day and duration of the rest is discussed. Eightyeight percent of the birds begin their rest in the period during which length of day decreases by 0.2 or more percent daily. The data do not permit further interpretation of the possible association of duration of rest and changes in the day length.
495
REFERENCES CITED
Hayes, F. A., and R. Sanborn, 1930. Duration of annual molt in relation to egg production. Mass. Agr. Expt. Sta. Bull. 264:73-85. Lerner, I. M., and L. W. Taylor, 1937. Interrelationships of egg production factors as determined for White Leghorn pullets. Journ. Agric. Res. 55:703-712. Marble, D. R., 1930. The molting factor in judging fowls for egg production. Cornell Agr. Expt. Sta. Bull. 503:3-42.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on May 31, 2015