The Condor 95:282-287 0 The Cooper Omtthological Society 1993

INCUBATION AND FLEDGING DURATIONS OF

YORAM YOM-TOV AND AMOS ARK Department of Zoology, Tel Aviv University,Tel Aviv 69978, Israel

Abstract. The relatively long fledging duration of cavity-nesting has been related to the relative safety of their nesting sites (Lack 1968). We compared the relationships between body and egg massesand incubation (I) and fledging (F) times of 39 speciesof woodpeckersand wrynecks (Picidae) to those of altricial birds in general. We found that Picidae have shorter incubation and longer fledging periods in the comparable range of altricial birds’ body and egg masses.However, the total development duration (I + F) is similar to that of other altricial birds. The representativeF/I ratios of altricial birds within the size range of Picidae are 1.21-1.26, while for 32 picid speciesthey are 2.09 f 0.09 SE. Similar trends appear to exist in other cavity-nesters. We suggestthat the short incubation of Picidae is an adaptation to the apparently poor gasexchange around the eggsunder the incubatingparent in the nest chamber, where oxygen and carbon dioxide levels may be low and high, respectively, and early transition to pul- monary respiration associatedwith an increase in nest ventilation is of advantage. The relatively short incubation may indicate a relatively immature state of hatchingthat requires compensation by a prolonged hatching to fledging duration. Key words: Woodpeckers;hole-nesters; incubation;jledging; allometry.

INTRODUCTION and Li 1992). Lack (1968) attributed this long The study of allometric relationships between duration to the relative safety of the nest cham- various life history parameters and body param- ber in cavity-nesters. However, little work has eters is useful, as it may uncover broad gener- been carried out on the general relationships be- alizations (Schmidt-Nielsen 1984). Residual tween incubation duration and time until fledg- variations in allometric relations must be attrib- ing in Picidae, taking into account body mass uted to other factors, and clear exceptions to gen- and egg mass. eralizations might raise questions as to which We report here the above allometric relation- factors these are. For example, Rahn et al. (1975) ships in woodpeckersand suggestan explanation showed that throughout the class Aves there is for our results. a highly significant relationship between log body METHODS mass and log initial egg mass. However, different taxa may have different slopes and intercepts that We gathered data on female body mass (Mb), represent different evolutionary trends. A sig- incubation (I) and time between hatching and nificant relationship also exists between initial fledging (F = fledging period or nesting period) egg mass and incubation duration for birds in from Short (1982) Fry et al. (1988), and on egg general (Rahn and Ar 1974) but the Procellar- mass (Me) from Schonwetter (1967). Some com- iiformes have, for example, a significantly longer plementary data on North American specieswere incubation duration for their egg mass (Rahn et taken from Ehrlich et al. (1988) and Martin and al. 1984). This may be attributed to their long Li (1992). Data on the Syrian are periods of nest neglect (Boersma 1982). The in- from Bamea (1982). Ordinary Least Squaresre- cubation duration of woodpeckers (Picidae) is gression equations and their statistics were cal- remarkably short (Short 1982), while the time culated for the relationships between log Mb or between hatching and fledging of hole-nesters log Me and log I, log F or log (I + F) for 39 such as woodpeckers is relatively long (Martin speciesand 12 genera of Picidae. Use of multiple specieswithin a taxon for comparative analyses of variables has been criticized, since such speciesshare some common genetic information I Received 8 June 1992. Accepted2 November 1992. z Presentaddress: Deuartment of Phvsioloav.School and therefore they do not provide independent of Medicine, State University of New York ii Buffalo, values (Harvey and Mace 1982). Ideally, the most Sherman Hall, South Campus, Buffalo, NY 14214. satisfactorycomparative analysis should be based

PW INCUBATION AND FLEDGING DURATIONS OF WOODPECKERS 283 on comparing phylogenies. However, since ac- can not solely account for the very curate evolutionary histories of speciesare rarely short incubation period, since incubation is short available, taxonomic classificationscan form an even for the size of these small eggs.Most of the ad hoc basis of comparative studies(Harvey and variation in incubation duration is also ex- Purvis 1991). Here, we analyze data allometri- plained at the generic level, suggestingvery little tally at both the speciesand genus levels. Since independence among individual species (Table most published analysesare based on species,we 3). compare the results within the general avian Time betweenhatchingandfledging. Time from trends mainly at the species level. However, a hatching to fledging (F) in Picidae is not signif- nested-ANOVA test (Sokal and Rohlf 1969) was icantly correlated either with egg mass or body conducted to account for the relative variation mass (Table 2), and the variation explained at at the speciesand genera levels. the generic level is 53% (Table 3). We found no published relationship between F and body mass RESULTS AND DISCUSSION for altricial birds in general, but by using the F We obtained data on Mb, Me, I, F, and the ratio vs. incubation duration (I) relationship (Ar and F/I for 2 speciesof wrynecks and 37 speciesof Yom-Tov 1978; F = 1.053.11.061;rz = 0.632), and woodpeckers(Table 1). the relationship of body mass to eggmass (Rahn Relationship of egg mass to body mass. Picid et al. 1975) we estimated the general equation specieshave significant positive correlations be- to be: F = 10.97.Mb0.177. tween egg mass and body mass (Table 2). The Fledging vs. incubation durations. From the relationship between egg mass and body mass is equation of Ar and Yom-Tov (1978) altricial similar to that given for Piciformes (including 3 birds, within the range of incubation durations Bucconidae) by Rahn et al. (1975), and for a of woodpeckers,should have F/I ratios of 1.2 l- larger sample of mainly African Picidae by Payne 1.26. Fledging and incubation periods in Picidae (1989). Our relationship has, however, one of the are essentially uncorrelated (r* = 0.042 for the lowest slopes(on log-log scales;Table 2 vs. Rahn specieslevel and 0.072 for genera level; Table et al. 1975) among all avian orders tested. There- 2). The F/I ratio for the 32 speciesof Picidae in fore, in spite of the similar intercepts of the re- Table 1 averages2.09 ? 0.49 (X + SD). On the lationship between the logarithm of eggmass and genus level, it averages 1.96 f 0.39 (Table 2). that of body mass in birds in general, Picidae However, only 4 1% of the variation is explained have small eggsin relation to their body mass. on the generic level (Table 3). Note that the in- This means they have relatively small hatchlings cubation durations of some other cavity-nesting which may take longer to reach final size and birds are also substantially shorter in relation to fledge.The nested-ANOVA test showsthat most their fledging periods. An example is provided of the variation in either body mass or egg mass by Australian parrots, where F/I = 1.96 f 0.41 is explained as differences among genera rather in 53 species. All of these are cavity-nesters, than of specieswithin genera (Table 3). whereasthe only parrot nesting in the open (Pe- Relationshipof incubationduration to bodyand zoporuswallicus) has an F/I ratio of 1.14 (Saun- egg masses.Egg and body massesin picid species ders et al. 1984). Similarly, the mean F/I ratio are significantly correlatedwith either incubation among 8 African hornbill species(Bucerotidae) duration or the sum of incubation and fledging is 2.15 f 1.12 (Yom-Tov and Ar, unpubl. data). times (Table 2). However, incubation duration From data given by Martin and Li (1992) we seems to depend little on body mass (Table 2), calculated average F/I and SD values for open- when compared to the equation for birds in gen- nesters,non-excavators and excavators as 0.9 t eral: I = 9.105.Mb”.L67 (Rahn et al. 1975). In- 0.1, 1.2 -+ 0.2, and 2.0 -t 0.4, respectively. cubation durations of Picidae are shorter than Time from laying to fledging. The combined those of similar-sized altricial birds, especially development period (I + F) in Picidae correlates among larger species(Table 2; Fig. 1). The same significantly with body mass and egg mass, and is revealed when analysesare based on eggmass. the correlations are closer at the generic level Large eggs(from large species;Tables 1, 2) tend (Table 2), although only 23% of the variation in to have shorter-than-expected incubation when (I + F) is explained (Table 3). The slope of (I + compared with the general avian trend (Rahn F) vs. body masson log-log scales(0.072 f 0.020) and Ar 1974). The relatively small eggsize of the does not differ significantly from the slope for I 284 YORAM YOM-TOV AND AMOS AR

TABLE 1. Female body and initial egg masses(Mb and Me, respectively), incubation and fledging durations (I and F, respectively) of wrynecks and woodpeckers.For sourcesof the data see text.

Species and Mb Me F/I mean of genus C&9 (9) ratio Wrynecks Jynx torquilla 36 2.9 12 21 1.75 J. ruficollis 52 3.0 13 25 1.92 Jynx mean 44.0 2.95 12.5 23.0 1.84 Woodpeckers Picumnus minutissimus 13 1.30 13 P. olivaceus 13 14 25 1.79 Picumnus mean 13.0 1.3 13.5 25 1.85 Melanerpes lewis 110 5.6 13 31 2.38 M. erythrocephalus 80 5.3 12 28 2.33 M. formicivorus 76 5.1 11 31 2.82 M. chrysauchen 56 4.3 10 35 3.50 M rubicauillus 44 4.7 10 32 3.20 M. uropi&allus 65 4.8 13 M. aurifrons 84 5.0 13 30 2.31 M. carolinus 79 4.6 13 25 1.92 Melanerpes mean 14.5 4.93 11.9 30.3 2.55 Sphyrapicus varius 49 3.7 12 27 2.25 S. thyroideus 54 3.8 13 24 1.85 Sphyrapicus mean 51.5 3.75 12.5 25.5 2.04 Dendropicos ficescens 27 2.7 11 Picoides moluccensis 16 1.8 12 P. canicapillus 26 2.8 12 P. minor 21 2.3 11 21 1.91 P. mahrattensis 37 3.2 13 P. medius 51 4.3 12 23 1.92 P. leucotus 121 7.0 15 27 1.80 P. syriacus 72 5.1 11 25 2.27 P. major 84 5.2 13 21 1.62 P. nuttallii 37 3.1 14 29 2.07 P. pubescens 28 3.2 12 21 1.75 P. borealis 47 4.3 11 25 2.27 P. stricklandi 42 14 P. villosus 66 4.4 14 29 2.07 P. alboiarvatus 64 4.4 14 26 1.86 P. tridactylus 61 4.4 13 24 1.85 P. arcticus 75 4.8 13 25 1.92 Picoides mean 53.4 4.02 12.7 24.7 1.94 Colaptes auratus 142 6.8 13 28 2.15 Celeus brachyurus IO 13 Dryocopus pliaetus 290 11.0 18 27 1.50 D. martius 330 11.8 12 28 2.33 Dryocopus mean 310.0 11.40 15.0 27.5 1.83 Campephilus principalis 448 13.0 14 35 2.50 Picus viridis 205 9.1 18 21 1.17 P. canus 140 1.5 18 24 1.33 Picus mean 172.5 8.30 18.0 22.5 1.25 Chrysocolaptes lucidus 160 7.5 15 25 1.67

alone (0.077 k 0.025). Hence, it is not surprising an equation for the relationship between (I + F’) to find that F does not correlate significantly with and body mass for altricial speciesin the appro- body mass (Table 2; Fig. 1). By using the equa- priate massrange is calculated as I + F = 20.304. tions of Rahn et al. (1975) for the relationship Mb0.16’. A similar equation, I + F = 19.338. between incubation duration and body mass,and Mb0.16’, was calculated by Yom-Tov (1987) for assuming 1.231= F after Ar and Yom-Tov (1978), 58 speciesof Australian passerines.Comparing INCUBATION AND FLEDGING DURATIONS OF WOODPECKERS 285

TABLE 2. Valuesfor Ordinary LeastSquares regression equations relating breeding parameters in Picidaein the form of log y = log a + b.log x. Valuesare for species;values in parenthesesare for genera.

Dependent variables, y Independent variable, x Egg mass, g Incubation, d Fledging, d Inc. + Fledg., d Bodymass, g 0.362 * 1.097 9.373 * 1.110 20.312 t 1.155 28.761 * 1.073 a i SE (0.291 * 1.129) (9.734 * 1.160) (19.813 ? 1.199) (30.190 + 1.097) b ? SE 0.603 & 0.022 0.077 i 0.025 0.058 ?Z0.033 0.072 ? 0.020 (0.639 -c 0.027) (0.072 i 0.033) (0.063 + 0.039) (0.063 f 0.020)

; (K) (:;) (:A) (:A) r2 0.958 0.209 0.098 0.305 (0.985) (0.247) (0.248) (0.563) P 0.0001 0.003 0.087 n.s. 0.00 1 (0.0001) (0.052 n.s.) (0.143 n.s.) (0.012) Eggmass, g 10.485 i 1.072 21.738 -t 1.110 31.051 ? 1.065 * SE a (11.320 i 1.100) (22.756 f 1.114) (34.566 f 1.058) b + SE 0.134 i 0.044 0.114 * 0.062 0.145 * 0.038 (0.106 -c 0.055) (0.090 f 0.060) (0.092 f 0.032)

; (::) (Z) (Z) r2 0.217 0.106 0.348 (0.297) (0.217) (0.516) P 0.004 0.079 n.s. 0.0006 (0.083 n.s.) (0.175 n.s.) (0.0192)

these equations with the equation we calculated ative safety of their nest sites. However, this may for Picidae (Table 2) indicates that overall de- not apply to Picidae becausethe sum of I and F velopment from laying until fledging tends to be does not differ appreciably from altricial birds in longer in small woodpeckers,and shorter in large general (Fig. 1). We suggestan alternative expla- ones, when compared to the general trend in al- nation, that the shorter incubation of woodpeck- tricial birds (Fig. 1). However, this can also be ers may represent an adaptation for breeding in partially explained by the effect of the variability cavities. In such cavities, gas exchange may be on the slope (Riggs et al. 1978) in the narrower poor, oxygen concentrations are low, and carbon range of ordinates of our data set. Body mass dioxide levels may rise considerably (Withers explains only about 3 1% of the variation in (I + 1977, Birchard et al. 1984, Boggs et al. 1984, F) among speciesand about 56% among genera Howe et al. 1987, Ar and Piontkewitz 1992), (Table 2). The “average” picid in our data set especially at night when the incubating does has a body mass of 89 g and a developmental not leave the nest. During late incubation, when period (I + F) of 40 days, similar to the value of 4 1 days calculated from the equation of Yom- Tov (1987). Little of the variance in (I + F) can TABLE 3. Resultsof2 levelnested-ANOVA test (lev- el O-species;level 1-genera; level 2 -subfamilies) be explained as a genuine difference among the showingvariation explainedon the genuslevel. different genera (26%; Table 3).

Concluding remarks. The main difference be- Van- tween altricial birds in general and the Picidae Vanable ation* df F P is that the point in time at which picids hatch Bodymass 0.94 10.1 45.77 KO.001 within their total, normal developmental period Eggmass 0.88 9.1 21.79

(I+F) = 2 8.8 . Mb.”’ I 3 9.4 - Mb.’ *‘

_..,...... !;-(;+F) n ;;‘ _... , ...... ~.&q...~~... n . n . . .A * Pa * $2 A:. AA la

A . . A .

10 100 1000

BODY MASS (grams) FIGURE 1. A comparisonof the relationshipsbetween incubation durations(dots, solid heavy line, I), hatching to fledging durations (triangles), the sum of the two (squares,broken heavy line, [I + Fj), and speciesmass in Picidae to the generaltrend of altricial birds (dotted fine lines marked Ia, Fa and [I + F]a for incubation, fledging and their sums). Equations are for picid species.See text for details. the needs of the developing embryos for oxygen and aid respiratory gas exchange (Howe et al. increase and add to that of the incubating parent 1987, Ar and Piontkewitz 1992). Rahn et al. in the nest, the poorly ventilated atmosphere in (1974) suggestedan inverse relationship between the deeper part of the nest below the incubating incubation duration and oxygen uptake rate dur- parent may stressthe embryos. A hypoxic and ing pre-internal pipping, provided that total ox- hypercapnic nest atmosphere reduces the diffu- ygen uptake and the degree of development at sive gradients for respiratory gasesbetween the hatching are the same in all cases.The only pre- nest and the respiratory organ of the embryo, internal pipping value reported for Picidae is that and may restrict the oxygen uptake because the of the ( embryo does not have a mechanism for venti- major). It amounts to 132 ml 0, [STPD]/day latory compensation (Ar et al. 199 1, Tazawa et (Berger et al. 1992) compared to the predicted al. 1992). A solution might be to shorten incu- value of 80-90 ml O,/day (Hoyt et al. 1978, Hoyt bation and, therefore, shift as soon as possible 1987). However, it is not known whether the from diffusive respiration acrossthe eggshelland total amount of oxygen consumed during em- the parallel, relatively inefficient, embryonic cir- bryonic development, which may represent the culation to convective respiration of the hatch- degreeof altriciality at hatching (Vleck and Vleck ling, using lungs, and to serial-type circulation. 1987), is different in Picidae from other altricial Thus, perhaps excluding the respiratory system, species. Data presented by Carey et al. (1980) the chick hatches relatively immaturely as may indicate such a pattern. The Northern Flicker be judged from the extreme altriciality the hatch- (Colaptesaurutus) eggcontains only 3.3 W/gram lings exhibit (naked, eyelids fused, low thermo- of energy, compared with eggsof altricial birds regulatory capacity; Weathers et al. 1990). Com- in general (5.0 kJ/gram), and the Eurasian Wry- pensation may be achieved by increasingthe time neck (Jynx torquilla) has only 16% yolk com- of development up to fledging. During this pe- pared with the 23.5% average for the eggs of riod, any departure of a parent from the nest to altricial birds. In sum, eggrespiration represents provide food or climbing by the hatchlings up only a small fraction of total oxygen consump- the nest cavity would force ventilation in the nest tion rate in the nest (Ar and Piontkewitz 1992). INCUBATION AND FLEDGING DURATIONS OF WOODPECKERS 287

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