The Wilson Journal of Ornithology 122(1):29–38, 2010

BREEDING BIOLOGY AND NESTING SUCCESS OF THE SLATETHROATED WHITESTART (MYIOBORUS MINIATUS) IN MONTEVERDE, COSTA RICA RONALD L. MUMME1 ABSTRACT.—I examined the breeding biology and nesting success of the Slate-throated Whitestart (Myioborus miniatus), a socially monogamous neotropical warbler, for five breeding seasons (2000–2004) in Monteverde, Costa Rica, near the center of its broad geographic range. Nesting was strongly seasonal, extending from late March through the end of June and coinciding with the end of the dry season and the onset of the rainy season in mid-May. Females constructed domed nests on open steep slopes or in banks along roads and trails. Mean clutch size was 2.9 eggs, and the mean incubation period was 14.2 days. Females performed all incubation of eggs and brooding of young nestlings, but both males and females fed nestlings and fledglings. Mean provisioning rate at nests when young were 5–9 days of age was 20.3 feedings/hr, and females fed young at a significantly higher rate than males (11.7 vs. 8.3 feedings/hr, respectively). Nestlings reached mean adult body mass of 9.5 g , day 7–8, and mean age at fledging was 11.3 days. Parents fed juveniles for at least 4 weeks after fledging; the latest record of adults feeding fledged young was for 40-day-old juveniles. Daily nest survival rates showed strong annual variation and generally declined as the nesting season progressed. Mean daily survival rate was 0.968 and estimated overall nest success was 40.3% with a mean of 2.6 young fledging from successful nests. Predation was implicated in ,85% of nest failures. Received 17 February 2009. Accepted 22 July 2009.

monophyletic, although populations from Mexico northwest of the Isthmus of Tehuantepec are genetically distinct from those in southern Mexico, Central America, and South America (Pe´rezEma´n 2005). In Central America, the Slatethroated Whitestart is monogamous, territorial, and often remains paired throughout the year (Skutch 1954, Shopland 1985). It occurs in midelevation montane forest throughout much of southern Central America and South America while a second species of Myioborus replaces it at higher elevation (Ridgely and Tudor 1989, Stiles and Skutch 1989, Curson et al. 1994). In the last decade new data on the breeding biology, nest predation, and survival of tropical and southern hemisphere birds have fueled a vigorous resurgence of interest in avian life history evolution (e.g., Martin et al. 2000, 2007; Martin 2002, 2004; Ricklefs and Wikelski 2002). My objective is to contribute to this growing body of work by presenting data on the breeding biology and nesting success of the Slate-throated Whitestart in Monteverde, Costa Rica. Some data are available for this species from Guatemala, Costa Rica, and Venezuela (Skutch 1954, Shopland 1985, Collins and Ryan 1994), but I present a much more extensive analysis based on 5 years of study of a color-banded population and records from 132 separate nests.

The genus Myioborus comprises 12 species of sexually monomorphic parulid warblers found in montane forests throughout the American tropics and subtropics (Curson et al. 1994). All members of the genus have contrasting black-and-white tails and animated foraging displays that function to startle potential insect prey, which are then pursued and captured in flight (Jabłon´ski 1999, Mumme 2002, Mumme et al. 2006). Species in the genus are termed ‘‘redstarts’’ by the AOU (1998) but ‘‘whitestarts’’ by the International Ornithological Congress Standing Committee on English Names (Gill et al. 2009). Recent phylogenetic analyses indicate that Myioborus is monophyletic (Pe´rez-Ema´n 2005) and part of a larger clade that includes five species from three additional genera: Ergaticus (2 species, Mexico and Guatemala), Cardellina (1 species, Mexico and the southwestern United States), and Wilsonia (2 species, northern United States and Canada) (Lovette and Bermingham 2002, Rabosky and Lovette 2008). The Slate-throated Whitestart (Myioborus miniatus) is the most widely distributed Myioborus species, ranging from the mountains of northern Mexico to the southern Andes of Bolivia and northern Argentina (Ridgely and Tudor 1989, Curson et al. 1994, DiGiacomo 1995, McCormack et al. 2005). Genetic analysis suggests it is

METHODS Study Area and Study Population.—The study was conducted during five consecutive nesting

1

Department of Biology, Allegheny College, 520 North Main Street, Meadville, PA 16335, USA; e-mail: [email protected]

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seasons (2000–2004) on a color-banded population of Slate-throated Whitestarts at the Estacio´n Biolo´gica Monteverde (EBM) and surrounding properties in Monteverde, Costa Rica (Nadkarni and Wheelwright 2000). The study area (10u 189 N, 84u 489 W) encompassed 35–45 breeding pairs of Slate-throated Whitestarts on territories established in primary premontane and lower montane wet forest (Haber 2000), old secondary forest, and abandoned pastures with scattered trees and shrubs at elevations of 1,400– 1,700 m. Field work was conducted during January–June 2000, May–June 2001–2003, and April–June 2004. I mist netted and color banded 170 adult Slatethroated Whitestarts during the study, either by attracting birds to song playback or by placing nets near active nests. An additional 172 individuals were color banded as nestlings. Gender of color-banded adults was assigned from either the presence of a brood patch in females or by subsequent behavioral observations, such as incubation or differences in vocal behavior; although both males and females produce song (Shopland 1985), males sing much more vigorously and frequently than females (Mumme 2002). Data Collection.—Breeding territories were surveyed regularly to search for signs of nesting. The date of nest initiation (first egg date) was ascertained directly for nests found during building, but estimated indirectly for nests found with eggs or nestlings by extrapolating backwards using data on the laying sequence, incubation period, and growth and development of nestlings. Nests were usually checked every 3–4 days to ascertain nest fate, although a smaller sample of nests was checked once or twice daily at critical periods to obtain more detailed information about timing of egg laying, hatching, and fledging. I marked eggs daily as they were laid at seven nests and measured egg size with dial calipers. I used a 10-g or 30-g Pesola scale to measure body mass of nestlings at regular intervals from hatching (day 0) through day 8 at 11 nests where hatching date was known. Parental feeding rates at nests were calculated from data collected during 1- to 2-hr nest watches at 24 different nests where at least one member of the pair was color banded and the nests contained 2–3 nestlings that were 5–9 days of age. Nest watches were conducted from an observation blind during mid-morning to early afternoon (0800–1300 hrs) in dry weather. I recorded all feeding visits by males,

females, and birds whose identity could not be ascertained. Nestlings were handled and color banded when 6–8 days of age at all nests. I made regular post-fledging visits to six territories where pairs nested successfully to record developmental state of juveniles and duration of post-fledging parental care. Statistical Analysis.—Data on egg size were analyzed with a two-factor ANOVA in which nest and laying order were used as factors. Statistical comparisons of male and female provisioning rates at nests were made with a matched-pairs t test. Calculations for both tests were performed using R for Mac OS X (Version 2.8.1). Data on nestling growth were fitted to a third-order polynomial curve using Microsoft Excel 2004 for Mac OS X. Daily Nest Survival and Nest Success.—I used the nest survival methods of Version 5.1 of Program MARK (White and Burnham 1999, Rotella 2008) to estimate daily survival rate (DSR) of Slate-throated Whitestart nests. The analysis was based on records from 128 nests where nest fate could be reliably ascertained. I evaluated a series of MARK nest survival models that used combinations of two categorical covariates, year (2001–2004) and nest habitat type (road banks, trail banks, and natural slopes), and two continuously varying covariates, date (calendar date within year) and nest age (number of days since first egg date). Covariates were coded and incorporated into models following Rotella (2008), allowing evaluation of models where nest DSR varied with year, nest habitat type, date, and nest age. Candidate models were compared to the null model of constant nest DSR, S(.), based on Akaike’s information criterion for small samples (AICc). The model with the lowest AICc value was considered to be the model that best fit the nest survival data, and Akaike weight (wi) was used as a measure of the relative support for each candidate model (White and Burnham 1999). Estimates of DSR under specific models were obtained from model beta parameters and back transformation following Rotella (2008). I estimated the cumulative probability of overall nest success by raising DSR to the power 27.6, which is the mean duration of a successful nesting cycle in days. RESULTS Timing of Breeding.—Slate-throated Whitestarts in Monteverde have strong breeding seasonality. First egg dates ranged from 27 March to

Mumme N BREEDING BIOLOGY OF MYIOBORUS MINIATUS

FIG. 1. Estimated first egg dates for 132 Slate-throated Whitestart nests in 2000–2004 in Monteverde, Costa Rica. Indistinct bars indicate that during 2001–2003 an unknown number of nests initiated in late March through early May either fledged young or failed before they could be discovered.

17 June with 6 May being the median date of nest initiation (Fig. 1). However, field work did not begin until after 10 May in 2001–2003, and several pairs had already fledged young from early nests. Thus, the true peak of nesting was likely earlier than 6 May (Fig. 1). I found 132 nests during the study: 60 (45.5%) during building, 28 (21.2%) during egg laying or incubation, and 44 (33.3%) during the nestling stage. Pairs frequently renested when initial nesting attempts failed by making a second (n 5 16; 12.1% of nests) or third (n 5 1; 0.8% of nests) attempt. However, only one certain case of true double brooding was observed; in 2001, one pair was feeding fledged young from its first nest on 29 May and initiated a new nest on 31 May. The young from the second nest fledged successfully on 26 June. Nest Building, Egg Laying and Incubation.— Slate-throated Whitestarts built domed nests in road banks (n 5 70; 53.0%), trail banks (n 5 19; 14.4%), or on steep slopes and natural banks in open areas such as pasture edges, ravines, washouts, and tree fall gaps (n 5 43; 32.6%). Nests away from roads and trails were relatively more difficult to find, making it likely they were under-represented in the sample of 132 nests. I observed nest-building behavior for 31 pairs where at least one member of the pair was color banded and gender was known; in all 31 cases

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females alone carried nest material and constructed the nest. Males did not directly participate in nest building, but they frequently accompanied their mates to the nest during building. Clutch size was 1–4 eggs with a mean 6 SE of 2.89 6 0.04 (n 5 82) and a strong modal clutch size of 3; 87.8% of nests contained three eggs (Fig. 2A). Laying occurred early in the morning, generally within 2 hrs of dawn. Eggs were laid on consecutive days in 13 (76.5%) of 17 nests where egg laying was monitored daily. Final clutch size was three eggs in all 13 of those nests. However, in four nests, a 1-day (n 5 3) or 2-day (n 5 1) gap occurred between laying of the first and second egg, and final clutch size was two eggs in three of the four nests. Thus, the mean 6 SE interval between laying of the first egg and clutch completion was 2.12 6 0.08 days (n 5 17 nests). Mean 6 SE egg size in seven three-egg clutches was 16.67 6 0.18 3 13.36 6 0.07 mm (n 5 21). Length and width varied significantly among nests (two-factor ANOVA F6,12 5 26.71 and 9.29, respectively, both P , 0.001), but neither length nor width was significantly related to laying order (two-factor ANOVA F2,12 5 0.50 and 0.19, respectively, both P . 0.6). Females alone incubated the eggs, and the incubation period was 14–15 days (mean 6 SE 5 14.2 6 0.1 days; Fig. 2B). Hatching was generally synchronous, and hatching success of eggs was high; of 160 eggs from 55 nests that survived through the end of incubation, 150 (93.8%) hatched. Nestling and Fledgling Periods.—Nestlings averaged 1.4 6 0.1 g (mean 6 SE) at hatching (n 5 3 nests) and reached mean adult body mass of 9.5 g , day 7–8 after hatching (Fig. 3). Both males and females provisioned nestlings, and female brooding of young nestlings ended when nestlings reached 5–6 days of age. Overall provisioning rates at nests with nestlings of age 5–9 days was 20.3 6 1.0 feedings/hr (n 5 24 nests); females provisioned young at a significantly higher rate (11.7 6 0.8 feedings/hr) than males (8.3 6 0.5 feedings/hr; two-tailed paired t test, t 5 4.23, df 5 23, P , 0.001). Age at fledging was 11.3 6 0.2 days (mean 6 SE; Fig. 2C). Juveniles began to forage on their own by day 30, but parents continued to feed juveniles for at least 4 weeks after fledging; the latest record of adults feeding dependent young was of 40-day-old juveniles that had fledged 29 days earlier.

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FIG. 3. Growth in body mass of nestling Slate-throated Whitestarts in 11 nests of known hatch date in Monteverde, Costa Rica, 2000–2003. Each point represents the mean mass of nestlings from a single nest with either two or three young.

Daily Nest Survival and Nest Success.—Estimated nest daily survival rate (DSR) for the 128 nests used in the Program MARK analysis was 0.968 with a 95% confidence interval (CI) of 0.958–0.975. Estimated nest success, based on an average nest exposure period of 27.6 days (2.1 days between laying of the first egg and clutch completion, 14.2 days incubation, and 11.3 days during the nestling period), was 40.3% (95% CI, 30.5–49.9%). An average (6 SE) of 2.61 6 0.07 young fledged from 74 successful nests. The MARK model of nest DSR that best fit the data included interaction terms between covariates year and date (Table 1). This model was 2.31 AICc units better than the second best model, which included the same two covariates in an additive model. These results suggest (1) there is considerable annual and seasonal variation in nest DSR, but (2) the precise nature of the seasonal effect of date is year-dependent. Estimated DSR varied annually from a low of 0.953 (95% CI, 0.927–0.970) in 2004 to a high of 0.989 (95% CI, 0.965–0.996) in 2003, corresponding to estimated nesting success of 26.3 and 72.7%, respectively (Fig. 4A). Nest DSR generally declined over the course of the nesting season, but the nature of the r FIG. 2. Variation in clutch size (A), length of the incubation period (B), and nestling period (C) in Slatethroated Whitestarts in Monteverde, Costa Rica.

Mumme N BREEDING BIOLOGY OF MYIOBORUS MINIATUS

TABLE 1. Models of nest survival for 128 nests of Slate-throated Whitestarts in Monteverde, Costa Rica, 2000–2004, based on analysis with Program MARK. Models are ranked from best to least supported according to values of DAICc, the difference in Akaike’s information criterion for small samples between the listed model and the top model. The covariates used in each model are shown in parentheses. Other columns show the number of parameters estimated in each model (K), model deviance, DAICc, and Akaike weight (wi), a measure of the relative support for each model. Model

K

Deviance

DAICc

wi

S(year 3 date) – interaction S(year + date) – additive S(year) S(date) S(.) – constant survival S(nest habitat type) S(nest age)

10 6 5 2 1 3 2

333.48 343.88 347.89 354.11 357.23 354.73 356.94

0.000 2.312 4.307 4.495 5.613 7.126 7.326

0.606 0.191 0.070 0.064 0.037 0.017 0.016

seasonal decline varied from year to year (Fig. 4B). Neither nest site habitat type (road banks, trail banks, or natural slopes) nor nest age appeared to have a major influence on DSR; models incorporating these covariates received little support and were ranked lower than the null model of constant DSR (Table 1). Thus, DSR appears to be largely independent of both nest site characteristics and nest stage. Apparent predation of the entire clutch (n 5 19) or brood (n 5 26) was the leading cause of nest failure, accounting for 83.3% of all observed nest failures (n 5 54). Abandonment of intact nests with eggs (n 5 3) or young (n 5 2) accounted for 9.3% of nest failures. An additional three nests (5.6% of failures) were abandoned after apparent partial nest predation reduced the nest contents to either one egg (n 5 2) or one nestling (n 5 1). I also observed a single case of nest failure that may have been a result of male egg removal; at one nest where the banded male had disappeared and been replaced by an unbanded immigrant, the three eggs were found intact underneath the empty nest. The female was observed building a new nest with her new mate 6 days later. DISCUSSION Breeding Biology.—My results are generally comparable to those of other studies of the breeding biology of the Slate-throated Whitestart and other Myioborus species. The March–June breeding season coinciding with the end of the dry

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season and onset of the rainy season is consistent with the seasonality reported for Slate-throated Whitestarts in Costa Rica (Skutch 1954, Shopland 1985, Stiles and Skutch 1989) and Venezuela (Collins and Ryan 1994), and the Collared Whitestart (M. torquatus) at higher elevations in Monteverde (Shopland 1985). The Brown-capped Whitestart (M. brunniceps) in subtropical Argentina (26u S) is also strongly seasonal with a September-December nesting season coinciding with a rainy season that begins in November (Auer et al. 2007). However, recent work on the Spectacled Whitestart (M. melanocephalus) suggests this species has a much more extended MayDecember breeding season in Ecuador (00u 369 S) that occurs during relatively dry periods (Greeney et al. 2008). Placement of Slate-throated Whitestart nests in banks along roads and trails, and on open steep slopes is consistent with other reports (Skutch 1954, Shopland 1985, Collins and Ryan 1994). Skutch (1954) reported the male participated in nest building at one of four nests where he observed nest construction. In contrast, I observed nest building at 31 nests where at least one member of the pair was color banded and gender was known, and males did not carry nest material or assist in nest construction. Skutch’s report, however, is based on observations of unbanded birds, and he did not see both members of the pair arrive at the nest simultaneously with nest material. He wrote ‘‘I had proof that both birds worked at the nest, since I saw the second fly up to it with material in its bill, before the first, which had already added its contribution, had left the clearing’’ (Skutch 1954:360). However, the ambiguities in his description, and the possibility the bird building the nest had slipped away unseen by Skutch before returning with more nest material, suggest the possibility of nest construction by males of this species should be viewed skeptically until more persuasive evidence is presented. Nest construction in other species of Myioborus is either by the female alone or, in cases where gender could not be ascertained, a single adult (Marshall and Balda 1974, Barber et al. 2000, Greeney et al. 2008). The average clutch size of 2.9 eggs is similar to that reported by others in Costa Rica (Skutch 1954, 2.8 eggs, n 5 11 nests; Shopland 1985, 2.8 eggs, n 5 33 nests), but slightly higher than the mean clutch size of 2.4 eggs (n 5 7 nests) reported for Slate-throated Whitestarts at

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FIG. 4. Annual and seasonal variation in daily nest survival rate and overall nesting success of Slate-throated Whitestarts from Monteverde, Costa Rica, 2000–2004. (A). Annual variation in estimated daily survival rate and estimated nesting success. Mean 6 95% confidence intervals are shown. (B) The effect of date on predicted daily nest survival rates for 2000–2004, based on the most strongly supported model of nest survival, the S(year 3 date) interaction model (Table 1). Predicted daily survival rates are shown only for dates when active nests were being monitored.

10u 219 N, 67u 419 W in Venezuela (Collins and Ryan 1994). The clutch size of the congeneric Painted Whitestart (M. pictus) in the Chiricahua Mountains of Arizona is larger, averaging 3.1 eggs (Barber et al. 2000). Mean egg size, 16.7 3 13.4 mm, was similar to that reported by Skutch (1954) (17.5 3 13.4 mm) and Collins and Ryan (1994) (16.3 3 12.3 mm). Egg size was not related to laying sequence, unlike many species of birds (e.g., Slagsvold et al. 1984, Lifjeld et al. 2005). Incubation periods of 13–

15 days have been reported for Slate-throated Whitestarts in both Costa Rica (Skutch 1954) and Venezuela (Collins and Ryan 1994), but I observed incubation periods of 14–15 days. However, the mean incubation period I recorded in Monteverde, 14.2 days, is nearly identical to that reported by Skutch (1954) (14.4 days, n 5 5 nests) and by Collins and Ryan (1994) (14.3 days, n 5 4 nests). The percentage of eggs surviving through the end of incubation that hatched successfully was 93.8%, relatively high for a tropical bird (Koenig

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TABLE 2. Life-history traits of the Slate-throated Whitestart and three related north temperate zone warblers, the Redfaced Warbler, Canada Warbler, and Wilson’s Warbler. The three related species were selected based on the phylogeny of Lovette and Bermingham (2002). Data for Slate-throated Whitestarts based on this study. Data for other species derived from Martin and Barber (1995), Conway (1999) and Ammon and Gilbert (1999). Life-history trait

Slate-throated Whitestart

Red-faced Warbler

Breeding range

Tropical montane forest

Nest site

Ground

Subtropical montane coniferous forest Ground

Mean clutch size 2.9

4.2

Incubation period 14–15 days Nestling period 10–12 days (mean 11.3 days) Post-fledging 29 days after fledging dependence

13 days 11–13 days (mean 12 days) No data

1982). The low incidence of hatching failure at Monteverde, a montane cloud-forest site with cool ambient temperatures during the nesting season (Clark et al. 2000), is consistent with the hypothesis that one of the major causes of loss of egg viability and hatching failure is exposure to high ambient temperatures during the laying period (Stoleson and Beissinger 1999, Beissinger et al. 2005, Cooper et al. 2006). Duration of the nestling period averaged 11.3 days, somewhat shorter than reported by Skutch (1954) (12.4 days, n 5 4 nests) elsewhere in Costa Rica, but similar to that found for Monteverde by Shopland (1985) (11.7 days, n 5 16 nests). The relatively greater role of females in feeding nestlings was also noted by Skutch (1954) at a nest of this species in Guatemala. Nestling growth appears to be slightly more rapid than that of Venezuelan nestlings (Collins and Ryan 1994), and the average provisioning rate I observed at nests (20.3/hr) is about twice the rate at which food is delivered to Brown-capped Whitestart nests in subtropical Argentina (Auer et al. 2007). Few comparative data are available on duration of post-fledgling care of young, but both Skutch (1954) and Chipley (1976) noted the apparently long period of juvenile dependence by Slatethroated Whitestarts. My observation of adults feeding 40-day-old juveniles, 29 days after fledging is comparable to Shopland’s (1985) observation of Collared Whitestarts feeding young 28 days after fledging. Nesting Success.—Daily survival rate (DSR) of Slate-throated Whitestart nests in Monteverde was 0.968, yielding an estimated overall nesting

Canada Warbler

Wilson’s Warbler

Northern boreal forest Northern boreal and humid Pacific forest Ground Usually ground, some low in vegetation 4.3 3.9 (Pacific coast) 4.4 (NE US) 5.3 (Alaska) 11–12 days 11–12 days 10–11 days 10–11 days No data

24–25 days after fledging

success of 40.3%, comparable to the 45% nest success reported by Skutch (1954) for 11 nests. Data on overall nest DSR are not available for other species of Myioborus, but Auer et al. (2007) report the daily nest predation rate for Browncapped Whitestarts in subtropical Argentina was 0.023. I found that ,85% of nest failure was due to predation, and the comparable figure of daily nest predation rate for Monteverde Slate-throated Whitestarts would be 0.028. Two other studies of passerines in Monteverde have reported data on nest DSR and nesting success. Sargent (1993) found that nest DSR for Yellow-throated Euphonias (Euphonia hirundinacea), a species that nests in many of the same road banks favored by Slate-throated Whitestarts, was 0.972, yielding an estimated nest success of 33.6%. Nest DSR and estimated nesting success for cavity-nesting House Wrens (Troglodytes aedon) in Monteverde were considerably higher, 0.991 and 72.8%, respectively (Young 1994). My observed nest DSR of 0.968 in the Costa Rican highlands is greater than found in many studies of lowland neotropical birds (e.g., Robinson et al. 2000, Ryder et al. 2008) but comparable to an average nest DSR of 0.977 at 400 m elevation in Braulio Carrillo National Park, Costa Rica (Young et al. 2008). Comparison to North Temperate Zone Warblers.—Recent phylogenetic analyses of the Parulidae and Myioborus (Lovette and Bermingham 2002, Pe´rez-Ema´n 2005, Rabosky and Lovette 2008) indicate the three North Temperate Zone warblers most closely related to the Myioborus whitestarts are Red-faced Warbler (Cardellina

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rubrifrons), Wilson’s Warbler (Wilsonia pusilla), and Canada Warbler (W. canadensis). Molecular evidence suggests the Hooded Warbler (W. citrina) is not part of the clade that includes Myioborus and the other Wilsonia (Rabosky and Lovette 2008). The Slate-throated Whitestart in Monteverde, in contrast to its relatives in the North Temperate Zone, displays the standard suite of life-history traits—small clutch size, prolonged incubation, and perhaps prolonged nestling and post-fledging care (Table 2)—that characterize tropical birds (Martin 2004, Russell et al. 2004). Differences in duration of incubation are particularly striking, as the incubation period is 1–3 days longer for Slatethroated Whitestarts than for Cardellina or Wilsonia (Table 2). Recent studies have shown that longer incubation periods of tropical birds are at least partially a result of slower inherent developmental rate of eggs (Tieleman et al. 2004, Robinson et al. 2008), although proximate differences in nest attentiveness and incubation temperature may have a role as well (Martin et al. 2007). The nestling period of Slate-throated Whitestarts in Monteverde is only slightly longer than for Wilsonia and apparently shorter than for Cardellina (Table 2). It also appears to be slightly shorter than for other Myioborus whitestarts (Barber et al. 2000, Auer et al. 2007, Greeney et al. 2008). The relatively short nestling period in Monteverde merits further study, but may be affected by the relatively high rate at which food is delivered to nests (20.3 feedings/hr), the relatively rapid rate of nestling development, and the moderately high daily nest predation rate (0.028) (Martin et al. 2000). Several studies have indicated the post-fledging period of juvenile dependence on parents is typically greater in tropical birds than in related temperate species (Skutch 1949, Russell et al. 2004, Tieleman et al. 2006). Data from Slatethroated Whitestarts are consistent with this hypothesis and suggest a slightly longer period of post-fledging care than for Wilson’s Warbler (Table 2). However, additional data are needed to reach a firm conclusion about the relative duration of post-fledging parental care in Myioborus versus related temperate-zone warblers. ACKNOWLEDGMENTS I thank Pawel Cygan, Mark Galatowitsch, Piotr Jabłon´ski, Lydia Petell, Sarah Sargent, Tadek Stawarczyk, Ursula Valdez, and Scott Wissinger for field assistance. Marvin

Hidalgo of the Estacio´n Biolo´gica Monteverde (EBM) allowed me to work on EBM properties, and many other Monteverde landowners, including James Forrest, Steven and Betsy Kendall, Sharon Kinsman, Alan and Karen Masters, Jim and Phoebe Richards, Bruce Young, and Willow Zuchowski provided me with access to their land. Alicia Mora and Francisco Campos of the San Jose´ office of the Organization for Tropical Studies provided generous assistance with permits. Jack Eitniear and two anonymous reviewers provided helpful criticisms of an earlier version of the manuscript. Financial support was provided by grant 7194-02 from the Committee for Research and Exploration of the National Geographic Society, and by the Allegheny College Academic Support Committee.

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