Fearful and Associated Responses of Caged White Leghorn Hens: Genetic Parameter Estimates1,2

Fearful and Associated Responses of Caged White Leghorn Hens: Genetic Parameter Estimates 1 ' 2 j . v. CRAIG Department of Animal Sciences and Industry, Kansas State University, Manhattan, Kansas 66506 WILLIAM M. MUK Department of Animal Sciences, Purdue University, West Lafayette, Indiana 47907 (Received for publication September 23, 1988)

1989 Poultry Science 68:1040-1046 INTRODUCTION

Fear is an emotional state, which presents difficulties of measurement. For example, Duncan and Filshie (1979) demonstrated in chickens that genetic stocks appearing to be less fearful, in terms of escape and avoidance behavior, exhibited as much tearfulness as another stock when elevated heart rate was used as the criterion. Also, Craig et al. (1984, 1986) found that stocks differing in escape and avoidance behavior either failed to differ or were classified in the opposite way when duration of tonic immobility was used as the measure of tearfulness. Comparisons of caged hens of two stocks differing in escape and avoidance behavior revealed that the stock showing more fearful

'This investigation is part of the Kansas and Indiana contributions to the NC-168 Regional Poultry Breeding Project. ^Contribution Number 89-88-J, Kansas Agricultural Experiment Station, Manhattan, KS, and Journal Paper Number 11737 of the Purdue University Agricultural Experiment Station, West Lafayette, IN.

behavior lost more feathers (Craig et al., 1983; Okpokho et al., 1987). In the earlier study, within-stock comparisons also showed that hens in groups with higher average levels of "nervous" or escape and avoidance behavior lost more feathers. However, Craig and Muir (1986) and Craig et al. (1986), using different genetic material, failed to find evidence that the relative level of escape and avoidance behavior of individual hens within cages was associated with feather loss, level of corticosteroids, body weight, or egg production. Larger group sizes in cages have been associated generally with greater escape and avoidance behavior (Craig et al., 1986; Okpokho et al, 1987; Craig and Milliken, 1989) and with longer duration of induced tonic immobility (Kujiyat et al, 1983). Elmslie et al. (1966) and Hansen (1976) reported the occurrence of "hysteria" among hens of some genetic stocks, when kept in larger-group-size, high density cage environments. As described by Hansen (1976), hysteria included exaggerated elements of escape behavior and was likely to develop only after hens had been kept in cages for at least 15 to 20 wk. When hysteria occurred, extreme feather loss was

1040

Downloaded from http://ps.oxfordjournals.org/ at University of Ulster at Coleraine on May 31, 2015

ABSTRACT Data were obtained from hens kept in three-bird cages in two, full-year experiments. Feather damage and loss scores, several fear-indicating variables, and measures of egg production were analyzed to obtain estimates of heritabilities and genetic and phenotypic correlations. Heritability estimates were slightly biased because data on individual hens were not available for most traits. Heritability estimates of productivity traits were fairly consistent with values obtained by others, except for age at sexual maturity, which was exceptionally low. Estimated heritability of egg number, based on a 15-day palpation period following full-year egg production, was also low. Feather score appeared to be moderately heritable. Heritability estimates involving escape and avoidance behavior and duration of induced tonic immobility indicate that those fear-related traits should respond to selection, if measured appropriately. None of the correlation coefficient estimates involving associations among feather scores, fear-inducing variables, and measures of egg production differed significantly from zero. (Key words: heritability, genetic correlations, fearful behavior, feather scores, production)

GENETICS OF FEARFUL RESPONSES EN CAGED HENS

MATERIALS AND METHODS

Genetic Stock. The North Central Randombred (NCR) population of White Leghorn type

chickens was used. This population, based on crosses among six commercial hybrids, was started in 1971 and maintained as described by Garwood et al. (1980). Fertile NCR eggs were supplied to the Kansas Experiment Station by the North Central Poultry Breeding Laboratory in 1981, and chicks hatched from those eggs formed the Kansas subpopulation of the NCR. Hens in the present study were from third and fourth generation NCR chicks hatched in 1984 and 1985, respectively (henceforth designated as G3 and G4). Each sire was from a different sire of the previous generation, and matings of full and half sibs were avoided. Numbers of single-sire matings were 52, 39, 22, and 23 in Generations 1, 2, 3, and 4, respectively. However, hens used in this study were from 19 and 20 single-sire matings in G3 and G4, respectively. General Procedures. A single hatch of pedigreed chicks was used within G3 and G4. On the day of hatching, chicks were wingbanded, vaccinated for Marek's disease, and placed in 12 and 8 brooding-rearing pens in G3 and G4, respectively of size 305 x 550 cm, with sex ratios and floor space allowances of 1:16 and 1,260 cm 2 and 1:18 and 750 cm2 in G3 and G4, respectively. Each sire family was distributed to at least four brooding-rearing pens. The rearing house was curtain sided, and chicks received daylight throughout the rearing period, which began on April 21 in G3 and May 18 in G4. Artificial lights were on for about 8 h daily, during the natural photoperiod, while caretakers were present. Cockerels were removed at about 16 wk of age. Chicks had their beaks trimmed at 1 wk of age, with about one-third of the upper mandible and a lesser amount of the lower beak being removed. Pullets were moved to cages in a curtainsided house at 17 and 21 wk in G3 and G4, respectively. When placed in the laying house, beaks were checked again and any regrowth was removed. Pullets were placed three per cage in 25.4 cm wide x 45.7 cm deep, commercial type cages, so that feeder and floor space per birds were 8.5 cm and 387 cm2, respectively. Artificial lighting of 13 h per day began at 19 wk for G3 pullets, when natural daylength approximated 13 h. Lights then were increased 15 min/wk until a photoperiod of 15 h was reached. In G4, 16-h photoperiods were provided daily, beginning when pullets were housed.

Downloaded from http://ps.oxfordjournals.org/ at University of Ulster at Coleraine on May 31, 2015

seen, and egg production dropped precipitously (Elmslie et al., 1966; Hansen, 1976). Muir (1985) compared production in single, four-bird, and nine-bird colony cages, while maintaining the same density within the multiple-bird cages. In comparison with birds housed individually, similar reductions in rate of lay were observed in birds housed in either colony size. However, longevity was greatly reduced in nine-bird cages as compared with four-bird cages. The increase in mortality observed in nine-bird cages was attributed to increased escape and avoidance behavior, which increased feather loss, trampling, and eventually brought on hysterical behavior. Craig et al. (1983) indicated that random genetic drift was probably responsible for the large difference found between escape and avoidance behaviors of two White Leghorn strains derived from the same base population and selected for part-year egg mass by the same procedure. Within those strains, significant sire family differences for escape and avoidance behavior were detected also, indicating the presence of within-strain genetic variation. Kujiyat et al. (1984), using the two selected strains mentioned above, two unselected control strains, and crosses between strains within selected and unselected stocks, failed to find evidence that selection for increased egg mass had altered fear-related behavior or that nonadditive genetic effects had an important influence on fearfulness. Additional evidence from the two selected strains and their cross (Craig and Milliken, 1989) also failed to indicate nonadditive genetic effects for escape and avoidance behavior. Gallup (1974) presented evidence from a single-generation, bidirectional, selection study showing that duration of induced tonic immobility, which is frequently used as an indicator of fearfulness, had a realized heritability of .58. Objectives of the present study were to: 1) estimate heritabilities of several measures of fearfulness, feather loss score, body weight gain, and components of productivity for White Leghorn-type hens kept in three-hen cages and 2) estimate genetic and phenotypic correlations between the same measures.

1041

1042

CRAIG AND MUIR

opening at the center of the cage, moved the hand up and down slowly for 10 s, and then scored each hen's reaction for the 10-s period. Scores ranged from 0 (calm, no evasive action) to 4 (running and flying about, squawking, and trying to hide or escape; continuing for the full 10 s). Procedures for the striking-the-cage and metronome tests were modified only slightly from those described by Craig et al. (1983). Each of these tests was used once only. In brief, the method was as follows. Hens were deprived of feed by covering the trough, beginning at 1930 h until tested the following morning (0730 to 1200 h), the fear-inducing stimulus was then applied, feed was uncovered, and latency until each hen fed was then recorded, with a maximum latency of 20 min allowed. With cage widths of 25.4 cm, only two hens could normally feed at any one time. Many hens continued to remain at the feeder for the maximum 20 min allowed for observation. Thus, the third hen, even when attempting to approach the feed trough, was frequently unable to do so. Therefore, it was decided to use latencies until the first and second hens returned to feed as measures of the relative fearfulness of the full-sister group within a cage. The striking-the-cage test was used in G3 only. Other Measures on Individuals. The G3 females were weighed 13 and 55 wk after being placed in cages, and G4 females were weighed before being placed in cages and again 56 wk later. Feathers were scored for damage and loss at the end of the year in cages (55 to 56 wk), using the method of Adams et al. (1978). Scoring was on a scale of 1 to 9, 9 = excellent feathering and 1 = complete loss of feathers on the backs and tops of wings. All hens were individually palpated for the presence of an egg in the uterus shortly after lights were turned on each day over a 15-day period, beginning soon after full-year production records were completed. Measures Based on Cage Means. Egg production records were obtained from 3-day/ wk recordings for cage totals. These records allowed estimation of age at sexual maturity on the basis of age when 50% rate of egg production was first achieved, rate of lay after 50% rate, rate of lay from housing, and daily egg mass per pullet for the full-year period of production. Egg weights taken midway through each of the five 10-wk periods were

Downloaded from http://ps.oxfordjournals.org/ at University of Ulster at Coleraine on May 31, 2015

Experimental Design. In G3, 19 sire families had sufficient pullets to fill four cages each with three full sisters from progeny of each of two hens and to allow one or more additional full sisters to be kept as spares in holding cages. Two rows of cages were used. Each hen's daughters were subdivided, so that three were placed in a randomly assigned cage within each of the two rows. In G4, five sire families were placed in each of four rows of cages, each having either three or four dam families. Thus, a total of 20 sire families was involved. As in G3, each dam family was subdivided, so that three full sisters were present in each of two cages. In each generation, additional full sisters were kept as "spares" in comparable 3-hen cages, but hens of different sire families were mixed in the spare cages. When an experimental or spare female died, she was replaced on the same day or the following day in most cases, so that group size, feeder space, and floor area would remain constant throughout the study. As spare females were used, they were replaced in turn by excess females of the NCR population kept in floor pens in the same house. Estimates of Fearfulness. Each hen was tested once only for duration of induced tonic immobility. This was done 40 to 42 and 42 to 45 wk postcaging in G3 and G4, respectively. The methodology has been described in detail (Craig et al, 1984). In the present study, only duration of the immobile phase until the hen turned from her back and stood up ("righting" time) was used. A maximum latency of 50 min was allowed for a hen to recover. Very few (about 3%) of all hens remained immobilized for this length of time. Three procedures for evaluating escape and avoidance behavior were used; those are designated as Hansen's test, striking the cage, and metronome. For all three methods, hens were tested in their home cages, at least 46 wk postcaging. Hansen's test was used with G4 hens only. It involved scoring each hen for responses elicited by an observer in terms of descriptions given by Hansen (1976). Hens in each cage were scored on 4 days, with intervening intervals of 3 or 4 days. Mean scores were computed for each hen and used in analyses. Briefly, the test was as follows. The observer stood directly in front of the cage, placed the fingers of one hand through the vertical

1043

GENETICS OF FEARFUL RESPONSES IN CAGED HENS

Yijkl. = H + Gi + S(i)j + d(ij)k + r(ijk)i where: Yjjki, is the mean value of the variable averaged over the three pullets per cage; \i is the overall mean; Gj is the effect (fixed) of the i* generation (year); s(i)j is the effect (random) of the j 1 * 1 sire within the ft1 generation; d(ij)k is the effect (random) of the k * dam mated to the j 1 * 1 sire within the i1*1 generation; r (ijk)l is the average pullet effect plus the effect of the 1th replicate cage within the k10 dam mated to the j * sire of the i * generation. Variance components for the random effects were estimated by setting the mean squares for each source of variation equal to their expectations and solving (Grossman and Gall, 1968). As averages over three pullets per cage (replicate) were used as the experimental unit, the replicate component (op contains the average mean bird-to-bird variation (or/3), as well as the cage effect (erf). Phenotypic variances (or) were calculated on an equivalent individual bird basis as:

°p = °J + «d + 3°r2

=°J + °d +

3c

£ + °b

These estimates are inflated by 2c~. Additive genetic variances and covariances were calculated as twice the sum of the sire plus dam components of variance and covariance, respectively. Heritabilities were calculated on an individual bird basis as: h2 = 2(o^ + o2)/ (0 2 + o j + 3
As the denominator contains 3a~, these estimates are biased downward. Phenotypic correlations were similarly affected. However, genetic correlations were not biased by this effect. The degree of bias could be examined for those variables measured on an individual bird basis because o

and or were then

separable. Where estimable, o~ was found to be at least 10 times smaller than a., thereby minimizing its effect. Standard errors for heritabilities and all correlations were based on formulas given by Grossman and Norton (1981) and Grossman and Norton (1974), respectively. RESULTS AND DISCUSSION

Means of the fear-related measures, units for analysis, and weeks after caging when measures were obtained are presented in Table 1. Measures of duration of induced tonic immobility were similar in G3 and G4. However, latency to recover from exposure to the metronome was about three times greater in G4 than in G3. Perhaps the fact that the striking-the-cage test preceded the metronome test by 1 wk in G3, but was not used in G4, may be at least partially responsible. Those tests are similar in most elements, but differ in the presence of the primary fear-inducing object at close range (human or metronome). The human observer is present in both tests, but further removed in the metronome test. Habituation to the general test situation could have occurred in G3, thereby reducing the response to the metronome. Information on body weights, egg production traits, feathering, and mortality are given in Table 2. In general, productivity was at expected levels for this population, which had been unselected for about 15 yr. For full-year production, rates of lay after sexual maturity were 73 and 74% for G3 and G4, respectively. Feather damage and loss were considerable. Scores of 4.5 and 3.5 were obtained at 55 and 56 wk after caging for G3 and G4, respectively. Mortality was 14% in G3 and 16% in G4 for the full-year period of egg production. Heritability estimates are given in Table 3. Estimates of genetic and phenotypic correlations are given in Table 4. Generally, for correlations, standard errors were greater than

Downloaded from http://ps.oxfordjournals.org/ at University of Ulster at Coleraine on May 31, 2015

used in estimating egg mass. Deaths were recorded daily. Statistical Procedures. In order to estimate correlations between measures of fear and productivity, all variables measured on an individual were put on a per cage basis by averaging over pullets within a cage, because most production variables were measured on a per cage basis. Fear-estimating variables, measured as time to return to "normal" behavior or condition, were transformed by logs to stabilize variances and normalize the distribution. The following model was used to partition sources of variation and covariation:

1044

CRAIG AND MUIR TABLE 1. Means of fear-related measures and weeks postcaging1 G4

G3 Fear-related measures

282

40 to 42

Week 289 1.8

42 to 45 46 to 47

51 110 176

52

49 80.7 182.4

27.6 63.4

'Chickens used werefroma subpopulation from North Central Randombred (NCR) eggs obtained in 1981, hatched from the third (G3) and fourth (G4) generation.

twice their estimates, with some exceptions. Estimates are not given for the striking-thecage measure of fearfulness. This variable was only measured in the first generation and produced negative estimates of genetic variance, hence generic correlations with this trait could not be estimated, and heritabilities would be negative. Heritability estimates of productivity traits measured in this study are fairly consistent with values obtained by others (Kinney, 1969), except for age at sexual maturity (AGE 50), which was exceptionally low. Egg number, based on the 15-day palpation period of hens

after a full year of egg production, was also low. Feather score (method of Adams et al., 1978) was moderately heritable. Feather scores obtained by this method are influenced by high-density environments; more feathers are damaged and lost with higher densities and in cages holding more hens (e.g., Craig and Milliken, 1989). Craig et al. (1983) found that sire family variance associated with feather score approached significance for hens in cages holding 10 to 14 birds. Feather score was heritable under the conditions of relatively moderate density used in the present experi-

TABLE 2. Means for body weights, egg production traits, feathering, mortality, and weeks postcaging1

G4

G3 Measures Individual hens BW (1st), g BW (2nd), g Weight gain, g Eggs laid, 15 days, n Feathering score: 1 to 9 Cage basis Age at 50% rate (A), wk Rate of lay after A, % Rate of lay postcaging, % Egg weight, x 5 ages, g 2 Egg mass/day, g Mortality, %

X

1499 1,633

Weeks

x"

Weeks

13 55

1,376 1,691

0 56

13 to 55 53 to 55

0 to 56 53 to 55

55

315 9.2 3.5

23

6

24

3

72.9 67.6 57.1 39.5 14.2

7 to 53 3 to 53

74.2 70.3 58.2 40.9 16.0

4 to 51 1 to 51

34 9.7 4.5

3 to 53 3 to 53

56

1 to 51 1 to 51

'Chickens used were from a subpopulation from North Central Randombred (NCR) eggs obtained in 1981, hatched from the third (G3) and fourth (G4) generation. 2 Eggs were weighed five times at 10-wk intervals, beginning at 8 and 6 wk postcaging in G3 and G4, respectively.

Downloaded from http://ps.oxfordjournals.org/ at University of Ulster at Coleraine on May 31, 2015

ndividual hens Tonic immobility, s Hansen's test, x, scores: 1 to 4 rage basis Striking the cage, s One hen feeding Two hens feeding Metronome, s One hen feeding Two hens feeding

Week

1045

GENETICS OF FEARFUL RESPONSES IN CAGED HENS TABLE 3. Heritabilities and standard errors for production and behavioral traits SE

.279 .374 .067 .061 .104 .338 .084 .146 .295 .278 .187 .081

.095 .101 .036 .076 .077 .096 .079 .080 .104 .102 .088 .100

!?

O N C-> r~ >n oo Os •vt VO t— >n CM - n VO 1 I 1

O

8

s

•—'

en oo oo r^ •o OO m o o en en *—' m in o en r1 1 1 i

s

m o t- oo

Eg

00 vO

S8 6-5

SO OO 00 w> en ON so

w

1— f-4 CM ON

1

m en m >n

CN O

&

5 CN

© 1

1 1

a

-.50

Heritability

TI FS GAIN EGG NO. MET1 MET2 MORT AGE 50 EHD 50 EHD EMS HS

-

~1

!

K 8 CN K »n VO CM *—' 1

so Os 0> CN © ^- CN c-- CN 1 1 1

O v e n h m C3 m P~ ^- r- o\ r- OO r- rn OO 1 1

-1.2

TI = Tonic immobility latency; FS = feather score, averaged over three pullets; GAIN = weight gain; EGG NO. = eggs laid in 15 days; MET1 = metronome, 1st to feed; MET2 = metronome, 2nd to feed; MORT = mortality percentage, full-year; AGE 50 = age at 50% hen-day production; EHD 50 = rate of lay after 50% production; EHD = hen-day production from housing; EMS = egg mass per pullet per day; HS = Hansen's score.

ON

§s

.3-2

1°

" en m OO O m oo CN CN CN 1 1 1

- /

zi

en

o Tf

£ *- ~ O

—1» en

1

O Q en <1 O C O N CN O — Tt m 1

en f— oo CSOO rH —. ,—

<-•

1

VO o CN t CN en O, — (N 1

8 fi

OS CN

s

s8

oo en Ov O 1

O 1

Z

-H «li.
1

o ^ f en en ©

B=

o zn

r- O .£) o O

1

32

IS

If!

v p VO

5

en ~-

oo

8s

m 1^ m O ci o 1 1

o o in m

I

r-i ON

~

o o

8

^

It

I

0 \ Os CN IN 0 \ ^ H m r00 o CN en CN CN O 1 1 1 1 1 1 1

•*t

—

1

o,

I

8

1

1

S oo

o

m CN CM OS oo W"l m • * c o o 1 1

so

so so rO en en

S1 8

I

1

o o

o

I

-

respecliv ely

O

1

047

1

the diago

45E=f= S

en en t r— »n as m

a\ o o — l

as B Q O Vi H H n uO l l S m H H tt, W A '£ < U U U I

*

o

elations abo e and be

CN T f m

e

S

enotypic

oo

•n

enet ic an

so en

.464

s. ment as compared with the higher densities used in commercial practice in the United States. One of the metronome measures (MET2) indicated that escape and avoidance behavior should respond to selection, if measured appropriately (Table 3). Although score based on Hansen's test was not indicated as being heritable in this study, evidence from the earlier study of Craig et al. (1983) indicated that additive genetic variation was present for Hansen's scores as well as for the metronome test, when applied to hens in cages holding 10 to 14 birds. The heritability estimate for tonic immobility indicated that tearfulness, as measured on caged hens by this criterion, also should respond to selection (Table 3). This estimate, obtained with adult White Leghorn hens, is about half as large as the realized heritability obtained by Gallup (1974), whose selection study was based on responses of 3-wk-old chicks, described as Production Reds. Although the present study gives several estimates of heritability that are significant, none of the correlations differed significantly from zero (Table 4), with the exception of genetic and phenotypic correlations between the two metronome measures of tearfulness and between rate of lay and egg mass. Lack of significance is common with estimates of

O

MlIf

° R O.

a II

&'a §,

S | b ..So,

!* I •3 5 .S "^

u

•a _ ua .yC CN ? ..6 0

=1

Downloaded from http://ps.oxfordjournals.org/ at University of Ulster at Coleraine on May 31, 2015

Trait1

1046

CRAIG AND MUIR

genetic correlations. However, by averaging over several such published estimates, Kinney (1969) succeeded in providing reliable estimates. These correlation estimates are presented so that others, if available in the future, may be similarly combined to provide more reliable values. REFERENCES

Downloaded from http://ps.oxfordjournals.org/ at University of Ulster at Coleraine on May 31, 2015

Adams, A. W., J. V. Craig, and A. L. Bhagwat, 1978. Effects of flock size, age at housing, and mating experience on two strains of egg-type chickens in colony cages. Poultry Sci. 57:48-53. Craig, J. V., T. P. Craig, and A. D. Dayton, 1983. Fearful behavior by caged hens of two genetic stocks. Appl. Anim. Ethol. 10:263-273. Craig, J. V., S. K. Kujiyat, and A. D. Dayton, 1984. Tonic immobility responses of White Leghorn hens affected by induction techniques and genetic stock differences. Poultry Sci. 63:1-10. Craig, J. V., and G. A. Milliken, 1989. Further studies of density and group size effects in caged hens of stocks differing in fearful behavior. Poultry Sci. 68:9-16. Craig, J. V., and W. M. Muir, 1986. Fear and featiier loss of hens in 3-bird cages; associations with other traits. Poultry Sci. 65(Suppl. 1):28. (Abstr.) Craig, J. V., J. V. Vargas, and G. A. Milliken, 1986. Fearful and associated responses of White Leghorn hens: Effects of cage environments and genetic stocks. Poultry Sci. 65:2199-2207. Duncan, I.J.H., and J. H. Filshie, 1979. The use of radio telemetry devices to measure temperature and heart rate in domestic fowl. Pages 579-588 in: A Handbook on Telemetry and Radio Tracking. C. J. Amlaner and D. W. MacDonald, ed. Pergamon Press, Oxford, England, U.K. Elmslie, L. J., R. H. Jones, and D. W. Knight, 1966. A general theory describing the effects of varying flock

size and density on the performance of caged layers. Pages 49CM95 in: Proc. 13th World's Poultry Congress, Kiev, Ukraine, U.S.S.R. Gallup, G. G., Jr., 1974. Genetic influence on tonic immobility in chickens. Anim. Learn. Behav. 2:145-147. Garwood, V. A., P. C. Lowe, and B. B. Bohren, 1980. An experimental test of the efficiency of family selection in chickens. Theor. Appl. Genet. 56:5-9. Grossman, M., and G.A.E. Gall, 1968. Covariance analysis with unequal subclass numbers: component estimation in quantitative genetics. Biometrics 24:49-59. Grossman, M , and H. W. Norton, 1974. Simplification of the sampling variance of the correlation coefficients. Theor. Appl. Genet. 44:332. Grossman, M , and H. W. Norton, 1981. An approximation of the minimum-variance estimator of heritability based on variance component analysis. Genetics 98: 417-426. Hansen, R. S., 1976. Nervousness and hysteria of mature female chickens. Poultry Sci. 55:531-543. Kujiyat, S. K., J. V. Craig, and A. D. Dayton, 1983. Duration of tonic immobility affected by housing environment in White Leghorn hens. Poultry Sci. 62:2280-2282. Kujiyat, S. K., J. V. Craig, and A. D. Dayton, 1984. Fearrelated responses of White Leghorn hens of several genetic stocks in five-bird cages and associations with quantitative traits. Poultry Sci. 63:1679-1688. Kinney, T. B., 1969. A summary of reported estimates of heritabilities and of genetic and phenotypic correlations for traits of chickens. Agricultural Handbook No. 363. U.S.D.A./Agric. Res. Service, U.S. Gov. Print. Off., Washington, DC. Muir, W. M., 1985. Relative efficiency of selection for performance of birds housed in colony cages based on production in single bird cages. Poultry Sci. 64: 2239-2247. Okpokho, N. A., J. V. Craig, and G. A. Milliken, 1987. Density and groups size effects on caged hens of two genetic stocks differing in escape and avoidance behavior. Poultry Sci. 66:1905-1910.