The Condor88:499-504 0 TheCooper Ornithological Society 1986 GEOGRAPHICAL ECOLOGY OF CLUTCH SIZE VARIATION IN NORTH AMERICAN WOODPECKERS WALTER D. KOENIG HastingsReservation and Museum of VertebrateZoology, Universityof California, Berkeley, CA 94720 Abstract. Clutch sizes of North American woodpeckersexhibit strong positive latitudinal gra- dients. There is no overall effect of either estimated first egg date or proximity to the coast on clutch size. However, mean clutch size varies inversely with an index of winter productivity and estimates of breeding densities of all woodpecker speciescombined; clutch size is uncorrelated with summer productivity. These results support Ashmole’s hypothesisthat geographictrends in fecundity are determined, at least in part, by seasonalityof resources. Key words: Ashmoles’ hypothesis;clutch size; geographicvariation: Picidae; woodpeckers. INTRODUCTION Specieswithin the family Picidae are widely Since Lack (1947, 1948) first focusedattention distributed and often common throughout on clutch size as a major topic of ornithological North America. Furthermore, despite a fair interest, geographicalvariation has served an degree of ecological diversity within the fam- important role in testing theories that seek to ily, all speciesare altricial, cavity nesting, and explain the evolution of clutch size (e.g., Cody (except for the Acorn Woodpecker Melanerpes 1966). Here I examine clutch size in North formicivorus) monogamous, and thus the im- American woodpeckers(Picidae) with the aim portanceof theselife-history featuresas sources of both describing the geographic patterns of of clutch size variation are minimized. variation and of relating those patterns to hy- MATERIALS AND METHODS potheses concerning the evolution of clutch size. I discusselsewhere clutch size variation Data on clutch size were obtained from col- within this taxon with respect to morphology lectionsin North American museumsand from and diet (Koenig, unpubl.); thus, here I focus an ongoing study of the Acorn Woodpecker at primarily on geographic and climatic corre- Hastings Reservation, Monterey Co., Califor- lations. nia (Koenig and Mumme, in press). In both A primary goal of the analyses performed cases,I used only clutches which were known here investigating the correlation between or thought to be complete based on informa- clutch size and climate is to examine Ash- tion given in the collector’s original data cards. mole’s hypothesis(Ashmole 196 1, 1963). This For each clutch I determined latitude, longi- hypothesis suggeststhat clutch size is deter- tude, estimated date of clutch initiation, and mined by resource abundance during the location in relation to a coastline (whether the breeding season relative to breeding popula- clutch was collected within 1” of a coast or tion density. Ashmole further proposed that inland). Date of clutch initiation was estimated breeding population density was determined as the collection date, minus the number of by winter resource abundance; hence, clutch eggs in the clutch, minus a correction factor size should be positively correlated with sea- based on an 1 l- to 14-day incubation period sonality of resources(i.e., the ratio of summer (Jackson 1977) and the estimated age of the to winter resources) rather than absolute re- clutch as determined by the stageof incubation sourceabundance during any one season.Ash- listed by the collector. For further details on mole’s hypothesishas been shownto be a pow- this estimator, see Koenig (1984a). erful predictor of geographicpatterns of clutch Data were handled in two ways depending size variation both in a seriesofpasserine com- on the goals of the analysis. First, individual munities (Ricklefs 1980) and in the Northern clutcheswere used as independent data points Flicker Colaptes aurutus (Koenig 1984a), and for analysesin which it was desirable that in- has also been implicated as being important traspecific or intrageneric variation be pre- in a variety of other taxa (Grant and Grant served intact. Second, clutches of the same 1980, Cockburn et al. 1983; see also Boyce speciescollected within the same 1” block of 1979). latitude and longitude (latilong block) were combined. Means of clutch sizescollected from all specieswithin a particular latilong block ’ ’ Received 14 February 1986. Final acceptance12 June were averaged to obtain a mean for all wood- 1986. pecker species occurring within that latilong 14991 500 WALTER D. KOENIG TABLE 1. Mean and latitudinal correlations of clutch size in North American woodpecker speciesand genera.’ Latitudinal range Correlation with Species of sample Mean i- SD latitude (I,) n clutches Colaptesauratus 26-54 (28”) 6.20 + 1.56 0.58*** 468 Melanerpescarolinus 25-43 (18”) 4.31 f 0.76 0.14 61 M. aurifrons 14-34 (2ooj 4.65 + 0.80 0.29* M. uropygialis 24-38 (14”) 3.75 * 0.93 -0.02 2; M. erythrocephalus 29-44 (150) 4.82 * 0.80 0.10 71 M. formicivorus 32-39 (7”) 5.06 * 1.06 0.14* 224 M. lewis 33-46 (13”) 5.88 + 1.19 0.32* 51 All Melanerpes 14-46 (32q 4.81 t 1.39 0.24*** 531 Sphyrapicusvarius 38-55 (17”) 4.93 f 1.02 0.42** 46 S. ruber 34-46 (12”) 4.69 f 0.72 0.22 42 S. nuchalis 36-45 (9“) 4.77 * 1.09 - 17 S. thyroideus 34-46 (12”) 5.67 * 0.85 0.12 49 All Sphyrapicus 34-55 (21 g 5.08 * 0.98 0.03 154 Dryocopuspileatus 26-55 (29”) 3.80 & 0.77 0.24* 92 Picoidesvillosus 28-62 (34”) 3.93 i 0.72 0.26*** 159 P. pubescens 28-61(33”) 4.81 f 0.78 0.27*** 205 P. scalaris 25-34 (9”) 3.81 + 0.76 -0.25 58 P. nuttallii 33-40 (79 4.34 + 0.77 -0.05 88 P. stricklandi 32 (0”) 3.50 t- 0.84 6 P. borealis 28-36 (8”) 3.68 ? 0.69 0.16 P. albolarvatus 33-46 (139 4.35 i 0.73 0.09 ;; P. arcticus 43-55 (12”) 3.89 rt 0.85 0.32 27 P. tridactylus 41-61 (20”) 3.87 f 0.64 15 All Picoides 25-62 (377 4.28 f 0.85 0.1F 681 All species 14-62 (484 4.93 * 1.45 0.22*** 1,926 = Correlations with latitude given only when the sample sire IS >25 clutches. *P < 0.05, **P < 0.01. ** P < 0.001. block. Mean latilong clutch size data were then American breeding bird survey program ad- used in analysesinvolving climate and popu- ministered by the U. S. Fish and Wildlife Ser- lation density, as these latter variables could vice (Bystrak 198 1). Mean abundance values not be independently calculated for every were calculated for each specieswithin each clutch, but could be reasonably determined for latilong block from over 18,000 surveys con- geographicareas on the order of latilong blocks. ducted over several thousand different routes. For each latilong block, I added the follow- I assumedthat woodpeckers constitute a rea- ing two climatic variables as determined from sonably cohesive “guild” (Root 1967), and the nearest station listed in Thornthwaite As- thus, that the major competitors of wood- sociates (1964): mean “summer” actual peckers are likely to be confamilial individu- evapotranspiration (AE), defined as the com- als, including conspecifics. Accordingly, I bined AE values for the three consecutive combined the abundances of all woodpecker months yielding the highest total, and mean speciesin analyses involving breeding densi- “winter” AE, defined as the combined AE val- ties. Further details can be found in Koenig ues for the three consecutive months yielding (1984a). the lowest total. Actual evapotranspiration is Taxonomy follows the AOU (1983, 1985). defined as precipitation minus runoff and per- In all, a total of 1,926 complete clutches from colation (Sellers 1965) and is directly related 21 species of North American woodpeckers to primary productivity in terrestrial environ- was analyzed. The Northern Flicker was ex- ments (Rosenzweig 1968). When combined cluded from some of the family-wide analyses into “summer” and “winter” values, AE data becauseI have analyzed it in detail elsewhere hasa theoretically straightforward relationship (Koenig 1984a, b), and becausethe strong lat- to seasonality of resourcesand is thus appro- itudinal trends and relative abundance of this priate for testing Ashmole’s hypothesis (Rick- taxon might otherwise unduly influence the lefs 1980; Koenig 1984a; but see below and results of analysesperformed here. Hussell 1985). Indeed, AE values have been previously shown to be a good predictor of RESULTS geographicvariation in clutch size in both pas- serine communities (Ricklefs 1980) and in the THE LATITUDINAL GRADIENT Northern Flicker (Koenig 1984a). Mean clutch sizes( f SD) for the 2 1 speciesof Data on the comparative abundanceof pop- woodpeckersconsidered are given in Table 1. ulations was determined from the North Also listed in Table 1 are the latitudinal ranges CLUTCH SIZE VARIATION IN WOODPECKERS 501 TABLE 2. Analysis of covariance of clutch size for in- four species(100%) in the first category yielded dividual speciesand for generaof North American wood- significantcorrelations with latitude, while only peckersexcluding Colaptesauratus. two of six (33%) in the second category and Regres- one of four (25%) in the third category did so sion (x2 = 6.7, df = 2, P < 0.05). These results Variable slope F-Value df indicate that the interspecific differencesin the Species correlation between clutch size and latitude Covariates listed in Table 1 are largely an artifact: the Latitude 0.026 35.3*** 1, 1173 likelihood that a significant correlation occurs Est. first egg is a function of the sample size and the lati- date 0.000 0.9 1,1173 tudinal range over which samples were ob- Main effects - tained. ~. Species 22.9*** 19,1173 Proximity to Table 1 also lists mean clutch size for each the coast” - 0.1 1,1173 genusunder consideration. Clutch sizes of all -Explained five genera correlate positively with latitude, (R* = 0.286) - 12.8*** 39, 1173 and only that for sapsuckers(Sphyrupicus) is Genera not significant.