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The Condor 86:83-X5 the mean fledging weight was 100.7 g f 3.6 (SD). Thus, 0 The Cooper Ornithological Soaety 1984 the fledgingperiod of the noddy was 13 days shorter than that of the tern. A mean pre-fledging weight loss of 15 g NESTLING METABOLISM AND occurredduring the last week of development. The 0, uptake (1 0, STPD’dayml) of six noddy chicks GROWTH IN THE BLACK suggeststhat fro, reacheda plateauof about4.5 10, STPD. NODDY AND WHITE TERN day-’ at age25 days(Fig. 2). Mass-specific0, consumption (MO,; ml O,,g-‘.h-‘) reached a plateau at age 12 days at a rate of approximately 2.0 cc O,.gml.h-*. Body temper- TED N. PETTIT ature was higher from days 2-9 than any other period (Fig. 2) possiblydue to the higher ambient temperature (30°C) GILBERT S. GRANT duringthis period. Respiratoryfrequency (f breaths’min-I) AND ranged from 40 to 98. G. CAUSEY WHITTOW DISCUSSION The growth rate and asymptote of the growth curve in the White Tern and Black Noddy may be compared with The White Tern (Gygis al&) and Black Noddy (Anous growth characteristicsin two other tropical birds without minutus)are two tropical charadriiform seabirds char- prolonged incubation periods, the Brown Noddy (Anous acterized by prolongedincubation of their eggs,slow em- stolidus)and the Sooty Tern (Sternafuscata)(Table 1). A bryonic growth, and a high total energeticcost of embry- “prolonged incubation period” is defined as an incubation onic development (Whittow 1980, Pettit et al. 1981, Pettit time that exceeds the 95% confidence intervals for the and Whittow 1983). The present study was undertaken relationshipbetween eggmass and incubation time (Whit- to ascertainif post-hatchinggrowth was also slow and if tow 1980). A comparison of average embryonic growth the nestling period was prolonged in the two species.A rates suggeststhat slow embryonic growth is related to a long nestlingperiod reducesthe energyrequirement of the slow “average” growth rate in the chick. Thus, the average chick (Ricklefs and White 1981) but may placean energetic embryonic and chick growth rates of the White Tern and burden upon breeding adults. To evaluate this possible Black Noddv are lessthan those of the Brown Noddy and energeticconstraint in the Black Noddy, the oxygen con- Sooty Tern..Although embryonic growth rates and those sumption of the chick was measuredto estimate the total of the chick may be linked pleiotropically, the constraints energy cost of maintenance activities. on growth are different in the two instances. Thus, em- bryonic growth may be limited by the gasconductance of METHODS the eggshell,while the growth of the chick is determined We studied the growth of White Terns on Midway Atoll largely by the rate of delivery of food to the chick by the (28”13’N, 177”23’W) January to May, 1981. Ten White parent. The duration of the nestling period, however, is Tern chickswere weighed daily with Pesola scales(? 1 g). not necessarilycorrelated to the length of the incubation Growth and metabolism of Black Noddies were studied period among these Hawaiian larids. The Sooty Tern has on Tern Island, French FrigateShoals (23”52 ’N, 165”l SW) the shortest incubation period and the longest nestling January to April, 1982. Six noddy chicks were weighed period of these four species(Table 1). every three days with Ohaus scales (kO.01 g). Oxygen The mass-specific0, consumption (MO,) of the Black consumption (Vo2, 1 0, STPD.day-I) of chicks was de- Noddy hatchling is 1.30 ml O,‘g’.h- and is close to the termined with a closed-system,manometric respirometer. adult rate of 0;uptake (1.28 ml O,.g-l,h-‘; Pettit et al., Oxygen uptake of chicks up to nine days of age was mea- unpubl.). The metabolic rate of hatchlinggulls is also close sured at a constant temperature of 30°C. Thereafter, Vo, to predicted adult mass-specificmetabolism (Dawson et was measuredat ambient room temperaturesranging from al. -1976,Dunn 1976, Dawson and Bennett 1981). MO, of 22°C to 28°C. Carbon dioxide absorbant (sodasorb)and the hatchling White Tern is 1.21 ml’O,‘g-‘.h-’ (Pettit et water vapor absorbant(silica gel) were placedin the cham- al. 1981) and about 80% of measuredadult MO, (1.5 1 ml ber. After a 45-min equilibration period the metabolic O,.g-l’h-‘; Pettit et al., unpubl.). Low rates of embryonic chamber (volume = 38 1)was sealed,and a measuredvol- oxveen consumntionin the Black Noddv and White Tern ume of oxygen was introduced into the chamber in order (Pet%tet al. 1981, Pettit and Whittow 1983) do not persist to maintain the pressure within the chamber at atmo- into low levels ofchick mass-specificoxygen uptake. Thus, spheric pressure.Ambient room and chamber tempera- prolongedincubation is not associatedwith a reducedmet- tures were determined by a Schultheis thermometer abolic level in the developing chick. (iO.l”C) or a Yellow SpringsInstruments (YSI) telether- mometer and urobe f#402) (+O.l”C). The body temper- ature of the chicks was measured before and after each experiment by inserting a YSI probe (#402) into the pro- ventriculus; the average of the two measurementsis re- ported to reflect body temperatureduring the experiment. RESULTS Logistic growth curves were fitted to the data (Ricklefs 1967) for mean weights of White Tern and Black Noddy chicksthroughout the nestlingperiod (Fig. 1). For the tern, W, = 100/l +e-.a96(f-19.0)where I+‘, is the weight of the chick at age t; e is the base of the natural logarithms, 2.72; the numerator is the asymptote of the curve. The correlation coefficientof the linearized data is r = ,986. For the noddy, W, = 115/l +em.lss(r+l’.3),Y= ,988. *GE0&y*, The mean fledging period of five tern chicks was 53.2 days * 2.6 (SD) and the mean fledgling weight was 99.8 FIGURE 1. Growth of Black Noddy and White Tern g + 6.9 (SD). White Tern chicksdid not lose weight before chicks. Mean weights of 10 tern (black dots) and six nod- fledging as did Black Noddies. Among five noddy chicks dy (white dots) chicks are presented along with the fitted the mean fledging period was 40.4 days * 1.5 (SD) and logistic growth curves (see text).

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WHITTOW, G. C. 1980. Physiologicaland ecologicalcor- Present addressof secondauthor: North Carolina State relatesof prolongedincubation in seabirds. Am. Zool. Museum of Natural History, P.O. Box 27647, Raleigh, 20:421-4X% North Carolina 2761 I. Received 1I February 1983. Final acceptance26 September 1983. Department of Physiology,John A. Burns Schoolof Med- icine, University of Hawaii, Honolulu, Hawaii 96822.

The Condor 86:85-87 to be much lower than in other yearsand our netting efforts 0 The CooperOrnithological Society 1984 were relatively unsuccessful. At ThousandOaks there was a consistentvearlv nattem SEX AND AGE RATIOS IN to the sex ratio data (Table 1). Significantly fewer males were capturedthan females (626 vs. 897, x2 = 48.22, P < WINTERING WHITE-CROWNED 0.001). The proportion of males (41%) was lower than in SPARROWS the Owens Vallev samole or in one netted to the north near Bakersfield(Kern Co.) by Hardy et al. (1965) again supporting the trend for fewer males to winter at lower MARTIN L. MORTON latitude (Table 2). Given that Z. 1.gambelii consistently return to the same wintering area (Mewaldt 1964, 1976; Cortopassiand Me- Differential migration of the sexesoccurs in many species waldt 1965) the latitudinal cline in sex ratio is maintained of North Ame&an songbirds.The resultant asymmetry becauseimmature males stop their southward migration in distribution on the wintering groundshas a consistent sooner than immature females. Presumably this benefits pattern in that males tend to occur farther north than males becauseit would facilitate their early return to the females (Ketterson and Nolan 1976). In Gambel’s White- breeding area and procurement of a territory (King et al. crowned Sparrow (Zonotrichia leucophrysgambelit) the 1965). In addition, they would experience a shorter, less latitudinal cline thus generated is quite marked. Popula- hazardous migration. Natural selection should favor the tions near the northern limit of the winter range in Wash- sameresponse in femalesexcept that they are smaller than ington contain about 80% maleswhereas those to the south males and may be displacedfrom food by them (Parsons near the Mexican border may be only about 25% males and Baotista 1980). Intersexual behavioral dominance is (King et al. 1965). Major informational gaps still exist, probabiy greaterin harsher(more northern) climates (Ket- however, concerningthe winter distribution of Z. 1.gam- terson and Nolan 1979). Female Z. 1.gambelii also cannot belii. Especially lacking are large unbiased samples from fast as long as males (Ketterson and King 1977) and pre- discretelocations. Herein are presentedadditional data on sumably are less likely to survive severe storms. sex ratios in wintering Z. 1.gambelii as well as evidence In large netted samplesthe percentageof immatures in that they experience differential mortality with both age wintering flocks declines from about 70% in southern and sex. Washington, 46”N, to about 50% in southern California, 34-35”N (Table 2). Since the proportion of immatures at a given location remains unchangedthroughout the winter METHODS (King et al. 1965), they apparentlyexperience a differential Wintering populations were sampled by mist-netting at increasein mortality rate over adultsas migration distance two locationsin California, one in the Owens Valley (Inyo increases.This seems sensiblebecause immatures are in- Co.) and the other near Thousand Oaks (Ventura Co.). experienced navigators and more are prone to go astray The Owens Valley is a deep, narrow corridor in eastern during migration (Stewart et al. 1974). Guidance could be California that is borderedon its westernedge by the Sierra provided by adults but there is no assurancethat such Nevada. Birds were captured there from fencerows and always occurs.The first flocks that form on the breeding on fallow lands between Olancha and Bishop, a distance of 130 km. At Thousand Oaks, 50 km northwest of Los Angeles, Z. 1. gambelii (and a few Z. 1. pugetensis)were captured on agricultural lands lying 4 km or less to the west of the city. All birds were sexed by laparotomy and aged by crown color. TABLE 1. Sex and age of Zonotrichia leucophrysgam- belii captured from two wintering areas in California.

RESULTS AND DISCUSSION Adults Immatures In the Owens Valley significantly more males were cap- Location YeaI Males Females Males Females tured than females (366 vs. 281; x2 = 11.17, P < O.OOj, Table 1). This nrooortion of males f57Oh)is considerablv Owens Valley lower than thai reported for Washington populations by 1968 100 72 108 76 Kinget al. 1965 (80%) and Lewis et al. 1968 (77%) (Table 1969 24 9 2), thus supporting the putative latitudinal cline in sex 1970 i: 46 49 6; ratio. It is higher, however, than ratios obtained from Total 200 142 166 139 museum specimensalso collected east of the Sierra Ne- Thousand Oaks vada (38%, Emlen 1943; 45%, King et al. 1965) and does 1977 58 not lend support to Emlen’s (1943) suggestionthat the 67 35 49 1978 65 mountains divide wintering populations with highly dis- 94 54 85 1979 76 parate sex ratios favoring males to the west and females 73 59 75 1980 41 to the east. 65 59 108 1981 71 The small sample collected in 1969 (Table 1) has sig- 110 57 100 1982 nificancein that the Owens Valley had received more snow - 37 - 27 34 Total than usual.Numbers of Z. 1.gambelii in the area appeared 446 291 451