COMPONENTS
OF AVIAN
ROBERT E. RICKLEFS
BREEDING
PRODUCTIVITY
AND GEORGE BLOOM
ABsTl•CT.--Numbers of nestlingsfledgedper pair per seasonwere calculatedfor 35 speciesof
passerinebirds in four localities from data on length of breeding season, clutch size, nesting
mortality, and length of nest cycle. Mean numbers of nestlingsfledged was 6.42 for 12 woodland
and edgespeciesin Kansas, 4.33 for 8 speciesin lower Sonorandeserthabitat in Arizona, 2.36 for 6
speciesin humid forestand second-growthhabitats in Costa Rica, and 4.80 for 9 speciesin a desert
scrub habitat in Ecuador. The data were analyzed by multiple correlation and stepwisemultiple
regressionto determine the contribution of the variables to variation in the number of young
fledged from one speciesto another in each locality. Variations in seasonlength, clutch size, and
breeding successwere important componentsin variation in numbers of young fledged in Kansas,
but length of nest cycle, being positively correlated with both clutch size and nest success,had the
strongestcorrelationwith the number of young fledged. In Arizona and Costa Rica, variation in
breeding successheld the key to predicting variation in number of young fledged, whereas in
Ecuador seasonlength and clutch size were the critical variables. The analysessuggestthat data on
production of young and the factors that cause production to vary from one speciesto another
wouldadd immenselyto our understanding
of the populationecologyof birds.•Departmentof
Biology,Universityof Pennsylvania,
Philadelphia,Pennsylvania
19174.Accepted1 July 1975.
Ornithologists have pondered the significance of clutch size in birds for nearly
three decadeswithout fully resolvingthe factors responsiblefor its variation among
speciesand regions. Although Skutch (1949) linked the evolution of clutch size to
adult mortality and population turnover rate, population biologists have only recently placed clutch size in a demographic context. For example, not until 1961 did
Drury point out that the production of young by a pair of birds during a breeding
seasondepends on the number of broods raised as well as clutch size. Cody (1971)
and Ricklefs (1973) treated avian demography more fully and demonstrated the use
of life table analysesto compare species.
Although breeding biology and mortality have been investigated in scoresof detailed field studies, completelife tables have been constructedfor only a handful of
species.Of the statisticsrequired to completea life table, the annual production of
young is one of the most difficult to obtain. Observation of individually marked birds
yields relatively few data per hour of effort (for example seeNice 1937, Snow 1958).
Furthermore, estimatesof mean annual productivity obtained from direct observation have large probable errors (wide confidencelimits) owing to the great variation
between individuals. Annual productivity can be calculated indirectly from clutch
size, breeding success,nest cycle length, and length of breeding season (Ricklefs
1970). These data are more readily obtained than direct measuresof breeding productivity. Furthermore, data from different studiesmay be combinedto calculatethe
annual production of a species.
In this paper we calculate annual production (fledglings per pair of adults) for
various passerine birds in two temperate and two tropical localities. The data are
analyzed to determine the factors responsible for differences between species in
annual production. Productivity is calculated by the equation:
productivity = clutch size. nesting success.number
of clutches laid per year.
86
The Auk 94:86 96. January 1977
January 1977]
Breeding Productivity
87
The number of clutchesa female lays each year dependson the length of the nest
cycle (from egg to egg) and the length of the seasonduring which clutches are
initiated. The length of the nest cycle is the average of both failures and successful
nestings. Breeding seasonlength is adjusted for peak periods of egg laying. These
calculations
are discussed in detail below.
MATERIALS
AND METHODS
Life historydata were compiledfor passefinebirdsin four regions,mesictemperatehabitatsin Kansas,
Sonorandesertnear Tucson, Arizona, wet tropical forestand second-growthhabitatsin Costa Rica, and
arid coastalhabitat on the Santa Elena Peninsula,Ecuador. Clutch sizeand length of breedingseasonfor
speciesin Kansaswere obtained from Johnston(1964); nest successand nest cyclelength are from various
studieswithin the easternUnited States(Ricklefs1969a, 1969b).Most of the data in the Kansassample
are means of several years' observations. All data from Arizona were generouslyprovided from an
unpublished 2-year study by George T. Austin. The Costa Rican data are from Skutch's extensive
observations on tropical birds (1954, 1960, 1966). Marchant's (1959, 1960) detailed 4-year study served as
the sole source of data for Ecuador.
The number of speciesusedin the analysesfor each area was limited by the availability of suitabledata,
particularly nestingsuccess.In addition, the period between nest termination (whether by fiedging or by
failure) and the following clutch is not known for most species.For many species,these were represented
by averages for local avifaunas (Ricklefs 1969a, MS).
The annual production of fledglings by a pair of birds (P) was calculated by the equation
P
F 'B
(1)
whereF = expectedrate at which youngare fledgedin a large population(youngfledged/pair'day) and
B = length of breeding season(days).
The length of the breedingseasonis calculatedfrom the number of nestsinitiated each month by an
equation derived from information theory
B = 30 exp(- X Pi log,,Pt)
(2)
(MacArthur 1964, Ricklefs 1966), where e - base of natural logarithms, p• = proportion of clutcheslaid
by a population during month i. This expressionfor breeding seasontakes into account the fact that
breedingis not equally intensein all monthsduring which eggsare found. The number of monthsof
clutch initiation (B) is multiplied by 30 to give the number of days during which clutchesare initiated.
The rate at which the young are fledged (F) is calculated by
F = C.S.I
(3)
where C
clutch size, S = breeding success(proportion of individuals that fledge), I = rate of nest
initiation (clutches/pair'day). Rate of nest initiation is calculated by an equation that takes into account
rate of nest failure and interval between nestings,
I -
m
Ps + m (p•rs + psrs)
(4)
(Ricklefs1970),wherem = nestmortalityrate(proportion
of nestsfailingperday),p., = probabilitythat a
nestsuccessfully
fledgesat leastoneyoung,Ps= probabilitythat a nestfails beforefiedging(Ps= 1 - Ps),
r• - delay beforea new clutchis laid after successful
fiedging,rs = delay beforea new clutch is laid after
nest failure. Nest success(Ps)is related to daily nest mortality rate (m) by the expression
ps- e mr
(5)
where T - length of the nest cycle from clutch initiation to fiedging (days). Rate of nestinitiation (I) does
not take into account the death of adult birds and consequentdecreasein the breedingpopulation during
the breeding season.We feel that discrepanciescausedby differencesbetween localitiesin adult mortality
rate are at least partly balancedby differencesin the length of the breeding season.In the tropics, for
example, low adult mortality rate is balanced by a long breeding seasoncompared to that in temperate
RICKLEFS AND BLOOM
88
TABLE
[Auk, Vol. 94
1
ANALYSISOF THE SENSITIVITYOF PRODUCTION(P, EQUATIONS1-5) TO VARIATION
IN INDEPENDENT
VARIABLES
Constants
Variable
m
T
Range
0.005-0.04
24-36
r•
r•
rs
rs
rs
4 16
5-50
5-50
5-50
5-50
Sensitivity
(b)
m
r.•
rs
T
--0.15
- 1.10
-0.02
10
10
10
10
30
--
-0.15
--0.31
--0.26
--0.18
-0.12
0.02
0.01
0.02
0.04
0.06
------
10
10
10
10
10
30
30
30
30
30
The measureof sensitivity(b) is the sloloeof the logarithmic relationshilobetween productionand the variable in question.
m = daily mortalityrate;r• = intervalbetweenfledgingand relaying(days);rf = interval betweennestfailure and relaying(days);
length of the nest cycle.
localities.The proportionof adults dying during the breedingseasonin tropical localitiesshouldbe more
similar to the proportion of adults dying in temperate localities than indicated by adult mortality rates in
the two regions.
Values for annual productionwere related to the original variablesusedin the equationfor production
by stepwisemultiple regressionand partial correlation(programsBMD02R and BMD03R, UCLA
biomedical computer programs). These regressionanalyses compute linear, or additive, relationships
betweenvariables, but the factorsdeterminingproductionbear factoffal, or multiplicative, relationships
to each other, as summarized by the expression
p ocC'B.S
T
(6)
Becausein the logarithmicform of equation(6)
logP cclogC + logB + logs - logT
(7)
the relationshipsbetweenvariablesare linear, we haveusedthe logarithmsof the variablesin the multiple
regressionand correlationanalyses.
To determinethe sensitivityof P to changes(or errors) in each of the independentvariables, we
constructeda hypotheticalspeciesfor which we systematicallyaltered each variable, calculatingP by
equations(1) through(5). We then relatedP to eachvariable(x) by a line, havingtheform 1ogP = a + b
logx, fitted by leastsquaresto the data pointsgeneratedfor our hypotheticalspecies.The slopeb of the
relationshipbetweenlogP and logX is the percentagechangein P as a proportionof the percentage
changeinX. Ifb were0.1, for example,P would changeonly 10%as rapidly asX. Valuesofb are listedin
Table 1. Fortunately, the variablesmostproneto measurementerror, m and rs, have the leastinfluenceon
P. Errors of 30% are tolerable becausethey would causelessthan 10% error in P. The influenceofm on P
is smallbecause
of thebalancingeffectsofm onnestingsuccess
and rateof nestinitiation.The influenceof
r., onP is greatestfor specieswith low m, hencewith few nest failures. Slopeb is lessthan - 1.0 for
varyingT becauseincreasingT reducesboth numberof nestingattemptsand nestingsuccess.
B and C
both enter the equation for P as simplefactors, henceboth yield a slopeof 1.0.
RESULTS
Reproduction data for each species are tabulated in Table 2. Means, standard
deviations (SD), and coefficientsof variation (CV = SD x 100 divided by the mean)
for each variable in each locality are presentedin Table 3. The variables exhibit wellknown trends. Breeding seasonsin the tropical localities are longer than in the
temperate localities. Clutch size and nesting successfollow an oppositetrend, being
greater in temperate localities. Length of the nesting cycle (T) has little geographic
variation. Young leave their nests somewhat sooner in Ecuador than elsewhere,
January 1977]
89
Breeding Productivity
TABLE
2
COMPONENTS OF BREEDING PRODUCTIVITY FOR SELECTED PASSERINE BIRDS IN
FOUR LOCALITIES
Season
length
Species
Clutch
size
(months) (eggs)
Nest
Nest
mortal-
period
ity•
Replacement lag
Egg
(days) success
2
r•
rj,
(days)
Young
fledged3
Kansas
Eremophila alpestris
Parus atricapillus
Troglodytesaedon
Turdus migratorius
Sialia
sialis
3.46
2.20
2.77
3.14
3.6
5.4
5.8
3.6
0.0213
0.0090
0.0055
0.0221
24
37
34.5
28.5
0.451
0.716
0.648
0.478
6
10
10
10
6
10
10
10
6.80
6.15
7.54
5.20
3.26
4.9
0.0103
34.5
0.602
10
10
7.39
Sturnus vulgaris
3.34
5.2
0.0068
39
0.768
6
6
9.94
Protonotaria
1.93
4.5
0.0135
27
0.613
6
6
5.58
1.99
2.81
2.15
2.22
2.96
4.2
3.7
4.5
4.1
4.1
0.0256
0.0365
0.0208
0.0076
0.0411
24
25
26.5
29
23.5
0.542
0.416
0.499
0.803
0.357
6
6
10
10
6
6
6
10
10
6
5.68
5.79
4.77
6.09
6.17
4.82
4.12
2.32
5.36
2.68
3.76
2.50
1.97
3.22
2.8
4.0
2.0
3.9
3.2
2.7
2.6
3.2
3.6
0.0248
0.0315
0.0195
0.0235
0.0166
0.0295
0.0437
0.0209
0.0321
28.5
30
27
29
28
23.5
22.5
21
23.5
0.384
0.373
0.500
0.408
0.441
0.510
0.368
0.591
0.377
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
5.11
6.27
2.25
8.25
3.50
5.76
2.95
4.14
4.95
Myiozetetes granadensis
Elaeniafiavogaster
Turdus grayi
Ramphoceluspasserinii
Thraupis episcopus
3.09
4.14
3.64
4.51
3.46
2.5
2.0
2.5
2.0
2.0
0.0220
0.0473
0.0478
0.0330
0.0250
38
34
29
25
32.5
0.408
0.167
0.161
0.423
0.440
11
13
19
34
12
15
15
14
15
15
2.42
1.32
1.42
2.84
2.54
Tiaris olivacea
5.93
2.5
0.0428
28.5
0.267
19
15
3.64
citrea
Dendroica petechia
Agelaius phoeniceus
Quiscalus quiscula
lcterus spurius
Spizella pusilla
Ecuador
Pyrocephalusrubinus
Muscigralla brevicauda
Euscarthmus meloryphus
Mirnus longicaudatus
Polioptila plumbea
Sporophila peruviana
Sporophila telasco
Rhodospinguscruentus
Poospizahispaniolensis
Costa Rica
Arizona
Auriparusfiaviceps
Campylorhynchus
brunneicapillus
Mimus polyglottos
2.53
3.0
0.0170
36
0.441
8
5
2.99
4.09
1.92
3.5
3.5
0.0248
0.0582
38.5
26.5
0.478
0.216
6
8
5
5
6.80
2.27
Toxostoma
4.09
2.8
0.0441
32
0.242
13
5
3.45
2.49
2.57
2.24
1.80
3.0
3.0
3.4
4.0
0.0329
0.0213
0.0401
0.0189
26
24
23.5
24
0.440
0.583
0.420
0.625
7
8
4
8
9
5
5
5
3.85
5.27
4.84
5.15
curvirostre
Pipilofuscus
Amphispiza bilineata
Aimophila carpalls
Aimophila cassinii
Nest mortality = proportionof nestsfailing each day (seeRicklefs 1969b).
Proportionof eggslaid that eventuallyleavethe nestas fledgedyoung,
Young fledged: numberper pair per seasoncalculatedfrom the otherbreedingparametersby the equationspresentedin the text.
which is perhaps attributable to the absencefrom the sample of hole-nestingspecies
with long nest periods. Number of young fledged per seasonis greatest in Kansas
(6.42), intermediate in Arizona (4.33) and Ecuador (4.80), and least in wet, tropical
Costa Rica (2.36).
How typical are the small samplespresentedin Table 3 of the localitiesfrom which
they were taken and of the regions they represent?In Kansas the average season
length in our sample (2.69) is similar to that in a sample of 58 species(2.70 months)
reported on by Johnston(1964); clutch size and nest length are similar to mean values
reported elsewherefor temperateregions.The 57% eggsuccessof the Kansasbirds is
reasonablefor a temperate sample having one-third hole-nestingspecies(Nice 1957,
RICKLEFS
90
AND BLOOM
TABLE
[Auk, Vol. 94
3
SUMMARY OF BREEDING PRODUCTIVITY DATA FOR FOUR LOCALITIES
Number
Localit),
of species
Variable •
Mean
SD 2
CV 2
Kansas
12
Season
Clutch
Period
Success
2.69
4.47
29.38
0.57
0.56
0.73
5.47
0.14
22
16
19
25
Arizona
8
Fledged
6.42
1.38
21
Season
2.72
0.89
33
Clutch
Period
Success
3.28
28.81
0.43
0.40
5.90
0.14
12
20
33
Fledged
1.46
34
4.13
2.25
31.17
0.31
1.02
0.27
4.61
0.13
25
12
15
42
Fledged
2.36
0.88
37
Season
3.42
1.18
34
3.11
25.89
0.44
0.65
3.28
0.08
21
13
18
1.84
38
Costa Rica
6
Season
Clutch
Period
Success
Ecuador
9
Clutch
Period
Success
4.33
Fledged
4.80
t Season
- lengthof the egg-laying
period,adjustedfor unequaldistributionof clutches,in months(equation2); clutch- number
of eggsper set;period- time betweenfirst eggof a clutchand fledging,in days;success
- proportionof eggsthat eventuallyproduce
fledglings:
fledged= numberof youngraisedto the time of nestcleaving
per pair per season.
2 SD = standarddeviation;CV = coefficientof variation = (SD/mean)-100.
Lack 1954). It should be remembered that nest successdata used in the calculations
for Kansas were taken widely from studies in the eastern United States, and that
hole-nestingspecies,which lay large clutchesand have high nesting success,are
disproportionately represented.
So little comparative information is available for birds from arid temperate
habitats that we cannot assessthe validity of the Arizona sample. The data were
collected in a small study area of uniform habitat over a 2~year period. Clutch size
and nest cycle length are undoubtedly reasonablebecausethesemeasuresare usually
uniform within species.
Mean values for seasonlength, clutch size, nestcyclelength, and eggsuccessin the
Costa Rican sample resemble means for larger samples taken more broadly from
Skutch's work. But the average length of the breeding season, even in a larger
sample of 37 Costa Rican species(4.53 months, Ricklefs 1966), is shorter than that
reported for other tropical localities (5.3-7.8 months). This factor by itself suggests
that in mosttropical localitiesthe number of young fledgedper pair per seasonwould
be 20-75% greater than our estimate for the Costa Rican sample, assumingclutch
size, nest mortality rate, and interval between broods were similar. This would bring
the productivity of tropical birds into the range calculated for birds in Arizona and
Ecuador, but it would still be below the productivity of birds in Kansas.
The sample from Ecuador reported on here includesmost of the speciesfor which
Marchant (1959, 1960) had gathered data, and it thus cannot be compared more
broadly.
Variation within the samplesis difficult to compare statistically between one locality and another becauseeach variable has a different mean in each locality. Coefficients of variation (SD, expressedas a percent of the mean) do, however, suggest
some interesting patterns. Breeding seasonlength varies more from one speciesto
January 1977]
Breeding Productivity
TABLE
91
4
CORRELATION COEFFICIENTS AMONG LOGARITHMS OF INDEPENDENT VARIABLES THAT
CONTRIBUTE TO ANNUAL BREEDING PRODUCTIVITY 1
Variable
Season
Kansas
Arizona
Costa Rica
Ecuador
Clutch
Kansas
Arizona
Costa Rica
Ecuador
Period
Kansas
Arizona
Clutch
Period
Success
Fledge
Fledge
4
- 0.15
-0.52
0.03
0.47
0.18
0.722
-0.64
0.64
- 0.27
-0.29
-0.18
- 0.54
0.57
0.29
0.45
0.782
(0.883)
(0.993)
(0.973)
(0.983)
0.803
- 0.26
0.16
0.24
0.64
0.29
- 0.22
-0.36
0.512
0.29
0.13
0.782
(0.733)
(0.993)
(0.933)
(0.953)
0.642
-0.01
(- 0.612)
(-0.933)
0.773
-0.18
Costa Rica
0.01
Ecuador
Success
- 0.44
Kansas
Arizona
Costa Rica
Ecuador
-0.30
0.30
0.44
0.752
0.74
- 0.25
(0.31)
(- 0.933)
(0.743)
(0.963)
(0.993)
(0.983)
• Number of species(n) are 12, 8, 6, and 9 for Kansas,Arizona, CostaRica, and Ecuador, respectively.
Degreesof freedomfor
statistical tests of correlation coefficients - n 2 p < 0.05.
2.
4 Valuesin parentheses
are partial correlationcoefficients
obtainedwith BiomedicalComputerProgramBMD 03R.
another in dry localities than in wet localities. In both Arizona and Ecuador variation
in breeding seasonlength is increased by groups of specieswith relatively short
breeding seasonsin which reproductionis restrictedto brief periodsof abundant food
following irregular and highly seasonalrains. Clutch size exhibits little variation
within any locality. Its coefficient of variation is greatest in Ecuador probably because speciesthat are resident in the arid coastal lowlands have larger clutchesthan
speciesthat migrate into the regionseasonallyto breed. Ranked in order of increasing
variability of egg success,the localitiesare: Ecuador, Kansas, Arizona, and Costa
Rica. Variation in nesting successis likely related to variation in nest construction
and placement. A comparison of these factors in New York and the Panama Canal
Zone revealed that the tropical sample was the more variable (Ricklefs 1969b).
To examine the relationships among the independent variables (season length,
clutch size, nest cycle length, and egg success)and their individual contribution to
variation in number of young fledged per season, we converted variables to
logarithms and calculated correlation coefficientsfor each pair of variables at each
locality (Table 4 and Fig. 1). The correlation between two variables indicates the
degreeto which the value for one can be predictedby the correspondingvalue for the
other. Clutch sizeand seasonlength are not significantlycorrelatedin any of the four
localities;eggsuccessand seasonlength are inversely, but insignificantly,related in
all localities. Relationshipsbetween other pairs of variables follow no obvious patterns. Among independent variables, statistically significant correlationsoccur between seasonlength and nest cyclelength in Arizona and Ecuador. Nest cyclelength,
clutch size, and egg successare positively correlated with each other in Kansas, but
not elsewhere.The well-known associationof large clutches,long nestlingperiods,
and high nest successamong hole-nestingspecies(see Table 2) probably underlies
thesesignificantcorrelationsin Kansas. (Hole-nestingspeciesare not included in the
92
RICKLEFSANt) BLOOM
[Auk, Vol. 94
•Kansas
Fig. 1. Left: correlation coefficientsof logarithms of independent variables with the logarithm of
number of young fledged per pair per season.Thickness of bars indicates correlation coefficient (0 to 1),
which is proportional to the contribution of the independent variable to variation in number of young
fledged. Solid bars indicate positive coefficients,open bars negative coefficients.Right: coefficientsof
correlationbetween independentvariables, portrayed in the same manner as the left portion of the figure.
B = seasonlength, C = clutch size, S = egg success,T = nest period, P = number of young fledged.
other samples,largely becausethey are absent from arid habitats without trees, and
they are difficult to observe in the tropics.)
Factors correlated with interspecific variation in number of young fledged differ
from one locality to the next. In both Arizona and Costa Rica egg successis the only
factor whose variation among speciesis significantly correlated with variation in
number of young fledged.In both localitieseggsuccessis the most variable factor in
the sample. In Ecuador seasonlength and clutch size are equally and highly correlated with variation in breeding productivity. As the two independentvariables are
not strongly correlated with each other, they must affect the number of offspring
produced in different ways. Again, seasonlength and clutch size are the most variable factors.
In Kansas the number of young fledged is most highly correlated with length of
nest cycle. If this relationship reflected the direct effect of nest cycle length on
breeding productivity, we would expect the correlation to be negative, but it is
positive becausenest cycle length is strongly related to both clutch size and nesting
January 1977]
Breeding Productivity
93
success(Table 4 and Fig. 1). Nest cyclelength apparentlyrepresentssomecombination of the two that is more closelyrelated to breeding productivity than is either
clutch size or nesting successalone. Season length contributes independently to
variation in number of young fledged in the Kansas sample.
To avoid the confusioncausedby correlationsamong independentvariables, we
calculatedpartial correlationcoefficientsbetweenthe number of youngfledgedand
eachindependentvariable (Table 4). The partial correlationcoefficientindicatesthe
relationshipbetween two variables when all other variables are held constant,thus
revealing their true relationship.As all independentvariables contributeto the calculation of production, their partial correlation coefficientswith production are all
highly significant,as we would expect, with the exceptionof nest cycle length in
Costa Rica. In the last case, variation in period length is inversely related to the
interval between a brood and the following clutch. Becauseinterval between broods
was not included in the partial correlation analysis, its relationship to nest cycle
length would confound the latter's correlation with production.
A stepwise multiple linear regressionanalysis was performed for each locality
using logarithms of the original data. This analysiswas intended to provide a complete descriptiveequation of the form
pcr aCwBxS"
Tz
(8)
where a is a constantand w, x, y, and z are exponentsof variables C (clutch size),B
(breeding seasonlength), S (egg success),and T (nest cycle length). Becauseclutch
size and breeding seasonlength enter into the equation for number of young fledged
as simplemultiplying factors,we would expectexponentsw and x to be 1. Nest cycle
length and eggsuccessenterthe equationfor number of youngin a more complicated
manner becausethe averagelength of a nest cycle dependson the time to failure,
replacementdelay, and proportion of nestslost. The exponentsy and z therefore
shoulddiffer from 1 (seeMaterials and Methods).
Resultsof the stepwiseregressionanalysisare presentedin Table 5, where we have
listedthe stepat which eachvariable was enteredinto the regression,its contribution
to the residualvariance, and its regressioncoefficient(exponentin equation8). The
order in which the independentvariables entered the regressionand magnitude of
exponentsin the equation for productivity do not exhibit any noticeablegeographic
pattern. Where the exponentsof seasonlength and clutch size differ from 1.0, these
deviations are caused by correlations of clutch size and seasonlength with other
independent variables. The coefficient for egg successvaries between 0.6 and 0.8
becausenesting lossesare partly compensatedby the ability of the birds to renest
quickly after nest failure. The constantof regressionpresentedfor each locality in
Table 4 is the natural logarithm of the constanta in equation 8. These constants
correspondto values of a of 21.9 in Kansas, 8.4 in Arizona, 0.36 in Costa Rica, and
12.0 in Ecuador.
DISCUSSION
Comparisonsof breedingproductivity presentedin this paper indicate that despite
the longerbreedingseasonavailable to tropical species,small clutches,poor nesting
success,and relatively long breeding cyclesreduce their annual breeding productivity
below that of temperate passerines.Even in tropical localities with long breeding
RICKLEFS
94
AND BLOOM
TABLE
[Auk, Vol. 94
5
SUMMARY OF A STEPWISE MULTIPLE REGRESSION ANALYSIS OF FACTORS
CONTRIBUTING TO BREEDING PRODUCTIVITY
Variable
Constant
t
Season
Clutch
Period
Success
Total r22
Kansas
Step entered3
2
4
1
Increasein r22
0.22
0.16
0.41
3.08
I. 01
I. 00
Step entered
1
2
Increase in r 2
Coefficient
2.48
0.66
1.05
0.21
0.98
2
3
4
1
0.35
I. 10
0. I0
1.0I
0.00
0.13
0.54
0.79
Step entered
2
3
4
Increase in r 2
Coefficient
0. I9
1.21
0. I2
1.52
Coefficient 4
- 0.98
3
0.08
0.87
0.70
Ecuador
4
0.04
-0.83
3
0.09
0.73
0.99
Costa Rica
Step entered
Increase in r 2
Coefficient
- 1.03
0.99
Arizona
2.12
0. I2
-0.92
I
0.56
0.64
0.98
• Logarithm of the constanta of equation 8.
2 The valueof r2 (thesquareof the correlation
coefficient)
represents
the amountof variationin numberof youngfledgedaccounted
for by including the variable in questionin the regression
• The order(decreasing)
in whichthe variablescontributeto variationin numberof youngfledged.
4 With regardto the independentvariables,the coefficients
representthe exponentsw, x, y, and z in equation8.
seasons,an average productivity of four young fledged per pair is a reasonable
maximum. This value approachesthe productivity of speciesin arid localities, in
both temperate and tropical rYgions(4.3 for Arizona,.4.8 for Ecuador), but it falls
short of the 6.4 young per pair calculatedfor a sampleof passerinesin Kansas.
Latitudinal variation in breeding productivity representsa shift in the balance
between opposingfactors. On one hand clutch size and nesting seasonincreaseand
nest cycle length decreasesas latitude increases,tending to increaseproduction; on
the other hand seasonlength decreasesas latitude increases,tending to reduceproduction. The latitudinal increase in breeding productivity may not, perhaps, continue into arctic regions.If we considera typical arctic speciesas having a breeding
season too short for more than one successfulbrood, and a clutch size of six, the
maximum possibleproductivity is somewhatlessthan that calculatedfor the Kansas
sample.A peak in annual productionof youngin temperatelatitudesarisesfrom the
fact that decreasein the logarithm of breeding seasonlength as latitude increasesis
most pronouncednorth of temperateregionswhereasincreasesin the logarithm of
clutch size and nestingsuccessas latitude increasesoccur primarily south of temperate regions(Fig. 2). As the logarithmof productivityis approximatelythe sumof the
logarithmsof clutchsize, breedingseasonlength, and nestingsuccess,a maximum is
obtained in temperate regions.
Whereas variation in annual production between one region and another can be
related to geographictrends in factors that influence reproduction, variation within
regionsis not so readily explained. The factor or factors contributing most to variation in production are generallythe most variable factors within the sample:nesting
successin Arizona and Costa Rica, and seasonlength and clutch size in Ecuador. In
Kansas variation in both productivity and the variables determining productivity is
relatively low.
January 1977]
Breeding Productivity
95
lO
8
ProductivityA
6
4
1
0.8
O.6
o.1
TROPICAL
0
TEAAPE?ATE
35
ARCIIC
70
Degrees
of Latitude
Fig. 2. Latitudinal variation in breeding productivity and its components.Reasonablevalues were
estimatedfor eachlatitudinalbelt (seetext). Productivitywas calculatedby equations(1) to (7) (A) and by
summingthe logarithmsof clutch size, seasonlength, and egg success(B).
We cannot judge to what extent the analysespresentedin this paper reflect important aspectsof the environment for breeding birds. But our resultssuggestthat in the
Arizona and Costa Rican localities componentsof the environment that affect nesting
success--whether related to diversity of predators or nesting sites--are more
heterogeneousthan componentsof the environment that determine clutch size and
seasonlength. In Ecuador food resource,expressedin both seasonlength and clutch
size, appears to be the most variable componentof the environment of breeding
birds. No singlefactor predominatesin its contribution to variation in reproductive
output in Kansas. Direct measurements in different localities of variation among
speciesin nest site and food resourcewould be extremely interesting in comparison
with these analyses.
Knowing the number of young fledged per pair each year and annual adult mortality, we can calculate the survival of young birds from fledging to the onset of
reproduction. We must assume(a) that population size and age structure remain
constant, (b) that young breed in their first year, and (c) that half the fledglings are
females. Because,under theseconditions,first-year birds must replace adult losses,
mortality between fledging and reproduction must reduce the size of the cohort of
potential recruits to the number of adults lost from the population each year. For
example, if passerinesin Kansas suffer 50% annual adult mortality (Lack 1954,
Farner 1955)and produce6.4 young per pair per year (3.2 young per individual), 3.2
fledglingsmust be reduced to 0.5 adults if population size is to remain constant. A
survivorship of 15.7% (0.5/3.2) from fledging to reproduction is thus required to
balance the population.
96
RICKLEFS AND BLOOM
[Auk, Vol. 94
Annual adult survivorshipin the tropics is poorly known, but available studies
suggestmortality rates of 10-30% per year, with 20% being a reasonableaveragein
the absenceof more extensive data (Snow 1962, Ricklefs 1973, Snow and Lill 1974).
If 1.5 young fledgedper adult, a survivorshipof 13% (0.2/1.5) during the first year
would be required to balance the population.
The calculationspresentedin this paper border on demographicspeculation,but
we have pointed out likely trends in production and population turnover rates as a
function of latitude. These trends must be verified by direct observationof annual
adult survival, age at first reproduction, and annual reproductive performance in
different localities, particularly in the tropics and arctic. Population and life table
information of this kind is needed for the development and testing of models of
demographicadaptations, particularly modelspertaining to the evolution of reproductive effort (Williams 1966, Gadgil and Bossert 1970, Cody 1971).
ACKNOWLEDGMENTS
We thank Richard Brewer for commentson the manuscript.The study was supportedby grants from
the National ScienceFoundation and the John Simon Guggenheim Foundation to R.E.R.
LITERATURE
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