ORNITOLOGIA NEOTROPICAL 20: 381–389, 2009 © The Neotropical Ornithological Society
BREEDING BIOLOGY OF PALE-EYED THRUSHES (TURDUS LEUCOPS) IN THE CLOUD FOREST OF NORTHEASTERN ECUADOR
Konrad Halupka1 & Harold F. Greeney2
1Department of Avian Ecology, University of Wroclaw, Sienkiewicza 21, 50-335 Wroclaw, Poland. E-mail: [email protected] 2Yanayacu Biological Station & Center for Creative Studies, c/o 721 Foch y Amazonas, Quito, Ecuador.
Resumen. – Biología reproductiva del Mirlo Ojipálido (Turdus leucops) en bosques nublados del noreste de Ecuador. – La biología reproductiva del Mirlo Ojipálido ha sido documentado con observa- ciones sobre unos pocos nidos, aunque es un pájaro con distribución amplio. Presentamos los resulta- dos de estudios realizados de 2003-05 en bosque nublado primario (elevación 1950–2100 m) en las laderas de Los Andes en el este de Ecuador. La temporada reproductiva dura desde diciembre hasta Junio, pero parece que la reproducción esta muy bajo durante ciertos años. Colectamos datos sobre 35 anidaciones. Nidos fueron construidos pegados en los troncos de árboles y también por los filos de car- reteras, con un rango de altura sobre el suelo de 0.8 a 3.2 m. Los nidos eran construidos de musgos, con una copa de fibras negras. Algunos de los nidos eran reutilizados para varias anidaciones. Solo las hembras incubaron dos, o en ocasión un huevo. Huevos eran de color verde-olivo hasta azul claro, con manchas cafés. El periodo de incubación era 14 días. Los pichones crecieron lentamente en com- paración con otros mirlos, saliendo del nido después de 18 días. En promedio, 41% de los nidos fueron exitosos. Documentamos, varias veces, el perdido de solo una porción de la anidada. Dos hembras fueron atacadas durante incubación y estimamos que hasta 15% de las hembras mueren o están heri- dos durante la animación. Concluyamos que la biología reproductiva del Mirlo Ojipálido muestra una mezcla te características de especies tropicales (anidada reducida y crecimiento lento de pichones) y especies templadas (alto nivel de reproducción y de costo a los adultos). La reproducción errática que se documenta en nuestro trabajo podría representar un problema potencial para la conservación ya que es un factor que aumenta la varianza en el número de individuos de la población.
Abstract. – Breeding biology of the Pale-eyed Thrush (Turdus leucops) has previously been docu- mented through observations at only a few nests, despite its wide distribution within humid montane for- ests–a globally important habitat of conservation concern. We present the results of studies carried out from 2003–05 in a primary cloud forest (elevation 1950–2100 m) on the eastern slopes of the Ecuadorian Andes. The breeding season lasts from December to June,Cabañas but it appears that reproduction is greatly reduced during some years. We collected data on 35 breeding attempts. Nests were built in trees and, occasionally, on road embankments. Nest height ranged from 0.8 to 3.2 m. Nests were mossy cups lined with fine rootlets. Some nests were reused for more than one breeding attempt. Females incubated two or, rarely, one egg. Eggs were olive green to light blue with red-brown markings. Incubation lasted 14 days. Nestlings grew rather slowly when compared to other thrushes, and fledged after 18 days. On average, 41% of nests in which incubation began fledged young. We recorded several instances of par- tial brood-loss. Two incubating females were attacked by a predator and we estimate that as many as 15% of females might be killed or injured during incubation. We conclude that the reproductive biology of the Pale-eyed Thrush shows a mixture of traits characteristic of both tropical (small clutch size and slow nestling growth rate) and north-temperate species (high breeding success and parental costs of repro-
381 HALUPKA & GREENEY duction). The erratic breeding documented in our study might be a potential problem for conservation as a factor increasing variance in the population number.
Key words: Pale-eyed Thrush, Turdus leucops, Platycichla, Turdidae, Ecuador, natural history, repro- duction biology.
INTRODUCTION and nestling periods, and nestling growth and development. The Pale-eyed Thrush has been only recently moved to the genus Turdus. Previously it was, METHODS together with the Yellow-legged Thrush (T. flavipes), included in genus Platicichla; however, Data were collected from 2003 to 2005 in the DNA analyses changed taxonomic position northeastern Ecuadorian Andes, on the pri- of both species (Klicka et al. 2005, Voelker et vate birding reserve of Cabañas San Isidro, al. 2007). next to the Yanayacu Biological Station and Pale-eyed Thrushes are distributed from Center for Creative Studies (00°35’S, the Guyana Shield (Guyana, Venezuela and 77°53’W), at elevations of 1950–2200 m a.s.l.. Brazil; Milensky et al. 2005) throughout most The reserve encompasses about 1500 ha of of the Andes, on both slopes, from Venezuela primary cloud forest adjoining the 188,000 ha (Verea et al. 1999) and Colombia (Hilty 1977, Antisana Reserve. Donegan & Dávalos 1999, Renijfo 1999, Nests were found by systematically Londoño 2005), south through Ecuador searching the understory vegetation, espe- (Marin & Carrion 1991), Peru (Davis 1986), cially epiphytes on tree trunks, in the vicinity and Bolivia (Brumfield & Maillard 2007). of places where some activity of thrushes Despite their wide geographic distribution, (singing, nest building, alarm calling, feeding, Pale-eyed Thrushes are restricted to a rather etc.) were observed or by accidentally flushing narrow altitudinal band of tropical cloud for- adults from nests. In 2004 and 2005, between ests that are currently the focus of much mid-February and mid-May, the study site was attention as endangered, biologically diverse explored in the morning and afternoon (4–10 habitats (Hamilton 1995, Bruijnzeel & Hamil- hours daily) and effort was directed toward ton 2000). Thus, research on the breeding finding nests of thrushes. During the remain- biology of all cloud forest species is impor- ing months of 2004 and 2005, as well as in tant from a conservation perspective. In addi- 2003, nests were found opportunistically in tion, more data are needed to link empirical the vicinity of Yanayacu Biological Station. knowledge and theoretical explanations of lat- The amount of data collected at each nest itudinal variation in avian life histories varied. We are convinced, however, that the (Stutchbury & Morton 1999). subsample of nests described in more detail is To date, the breeding biology of Pale-eyed representative for the general population. A Thrushes is known from descriptions of four few remote nests were only visited once. nests in Ecuador and Colombia (Marin & Other nests were checked every 1–5 days until Carrion 1991, Londoño 2005). Here we they failed or fledged. Nests were visited present the results of observations of 35 more frequently (every 1–2 days) near the breeding attempts in northeastern Ecuador. expected hatching or fledging dates and when We describe breeding phenology, habitat and collecting data on nestling growth. Nest visits nest-site selection, parental behaviour, nest were as brief as possible and, when nestling success, clutch sizes, duration of incubation mass exceeded 40 g, we observed nests from
382 BREEDING OF PALE-EYED THRUSHES a distance to avoid causing premature fledg- ysis. If the resulting 19-point data subset did ing. After the nest fledged or was depredated, not cover at least 12 days, the resampling pro- we measured nest and nest-site characteristics cedure was repeated until this minimal time (cf. Clement & Hathaway 2000) to the nearest span criterion (i.e., the time when the s- 1 cm using a measuring tape. Density of can- shaped growth pattern becomes apparent) opy at the nest was measured with a forest was met. Thereafter, using the least squared spherical densitometer (manufactured by RE method, the logistic curve was fitted to the Lemon, Bartelsville, OK, USA). Canopy den- data and the resulting equation recorded. This sity was scored as a percentage of sky visible re-sampling procedure was iterated 2000 using the average of four measurements taken times. Following the Central Limit Theorem 6 m from the nest, at 90° intervals around the (cf. Crowley 1992), mean values and standard substrate tree. deviations of the resampling distributions of We measured eggs to the nearest 0.1 mm the growth curve coefficients approximate the using standard vernier calipers and egg mass general population’s growth parameters and to the nearest 0.001 g using a digital micro- estimates of standard errors. gram balance that was calibrated before each At each approach to the nest, we recorded weighing using a standard 50 g weight. Nest- the distance at which the incubating/brood- lings were weighed (± 0.25 g) every 1–3 days ing adult flushed from the nest, and any asso- with a Pesola spring balance. The growth ciated behaviours. If the nest was predated, curve relating body mass to age was fitted to and the date of predation could not be exactly the logistic function: determined, failure was assigned as the day in Mass(age) = A/(1+b exp(-K age)) the mid-point between the last two visits. If where A is the asymptotic body mass and K is nestlings on the penultimate nest check the growth rate coefficient (Ricklefs 1976). appeared ready to fledge, but were gone with The age (in days) at which the curve inflected no signs of predation (e.g., blood, feathers, or was calculated as ln(b)/K and the inverse disturbed nest site) at the next visit, the nest measure of growth rate was calculated using was considered successful. the formula 4.4/K. We followed Ricklefs’ Descriptive statistics are presented as (1976) convention of making the day of means ± SD, unless otherwise stated. Survival hatching day one. rates of nests and incubating adult birds were To minimize disturbance, we last weighed estimated using the Mayfield (1975) method nestlings when 14–16 days old and body and standard errors for Mayfield statistics masses were about 40 g. Because nestling were calculated with the algorithm described body mass could still increase after this, the by Johnson (1979). asymptotic body mass might be slightly underestimated and the growth rate coeffi- RESULTS cient overestimated (cf. Remeš & Martin 2002). Breeding phenology. Pale-eyed Thrushes Logistic curve coefficients were calculated appeared to breed in our study area rather with a resampling method (Crowley 1992), erratically. Only 4 nests were found from Jan- allowing us to avoid problems with pseu- uary to May 2003. No nests were found from doreplication (data from siblings should not February to May 2004, even though adult be treated as independent). To do this, we ran- birds were frequently observed and heard and domly selected only one recorded body mass other sympatric thrushes did breed (Greeney from each of the 19 nests included in the anal- & Halupka 2008). However, from December
383 HALUPKA & GREENEY
2004 to June 2005, with the same effort Nest trees had a mean diameter at breast invested in nest searching, we collected data height of 41 ± 19.2 cm (n = 14). Eleven nests on 31 breeding attempts. were on the side of the trunk, and supported During the 2004–2005 breeding season, by epiphytes, protruding bark, or both. One the earliest nest, with incubation already nest was in a large tree fork, and two were in underway, was found on 2 December. Nest- shallow hollows on the side of a trunk. Within lings from the latest nests fledged from 1 to a 6-m-radius around nest trees, an average of 15 June. During seven years of nest searching 6.5 ± 2.8 trees reached the canopy layer of the in the area (HFG unpubl.), no active Pale- forest and the mean number of smaller trees eyed Thrush nests have been found from and tree ferns in the sub-canopy was 10.6 ± June to November, and most clutches were 7.3. The mean percentage of sky visible (mea- initiated in December and January. Thus, sured with a spherical densitometer) was 35.5 breeding attempts are initiated during about ± 6.8%. These figures were very close to sta- six months of the year in our area. tistics calculated for randomly selected patches within the forest, where nests were Nests, nest sites, and nest patches. Of 32 nests, 27 not detected (KH unpubl.). (84.4%) were in trees and five on the ground For two nests, two consecutive clutches on roadside embankments. Nest height were initiated in 2005. Unfortunately, even ranged from 1.15 to 3.20 m for nests in trees though we knew the date of fledging of young (mean = 1.96 ± 0.50 m, n = 27) and from 0.8 from the first broods (7 March and 7 April), to 2.3 m for nests on banks (mean = 1.58 ± nests were not checked again until 25 March 0.55 m, n = 5). Cups were generally circular, and 18 April, respectively, and incubation was but the side of the cup was often thinner, or already underway at both nests. Therefore, we even lacking, where the rim adjoined the sup- do not know how many days elapsed between porting substrate. Mean outer cup diameter fledging and re-nesting. Also, individuals were was 13.2 ± 1.02 cm, mean inner cup diameter not marked so we do not know if the same was 8.1 ± 0.86 cm, and mean cup depth was pair initiated both clutches. 5.2 ± 1.2 cm (n = 19 nests). The outer layer of 5 nests collected from trees and dissected, Eggs and incubation period. At 27 nests where consisted of green moss and there was a vari- incubation had begun, two had one-egg able amount of soil at the bottom of the cup clutches and 25 had two-egg clutches. Eggs or, in one nest, a large amount of wood dust. were olive green to light blue, with brown to The cup lining consisted of thin, dark root- red-brown blotches. Spots were irregular, lets. The green moss in outer walls blended heavier, and overlapping near the blunt end of well with the naturally growing moss and the egg. other epiphytes on tree trunks, therefore Eggs (n = 36 from 21 nests) ranged from nests were well camouflaged when observed 25.8 to 32.6 mm in length (mean = 28.99 ± from below or the side. However, nests were 1.50 mm) and from 18.6 to 21.5 mm in width generally poorly concealed from above and (mean = 20.12 ± 0.69 mm). Two recently laid only a few were partially sheltered by the eggs in one nest weighed 6.07 g and 5.98 g, stems of ferns or other epiphytes. For most respectively. In two other nests with two-egg nests, incubating or brooding adults were eas- clutches where incubation had just begun, ily seen by observers approaching the nest, mean egg mass was 7.2 and 6.4 g, respectively. indicating that adults also had a clear and At five nests, eggs (n = 9) were weighed at 3 unobstructed view from the nest. to 6-day intervals during incubation to docu-
384 BREEDING OF PALE-EYED THRUSHES ment loss of mass (= water loss; Rahn & Ar feathers found on the empty nests). Thus, the 1974), and the mean mass loss was 71.4 ± daily survival rate of nests during the incuba- 34.9 mg (range = 21–124 mg) per day, or, tion stage was 0.958 (SE = 0.016; 160 success- using the mean fresh egg mass as the initial ful days and seven days with a failure). Using mass, 1.09 ± 0.63% per day. The length of the an incubation period of 14 days (including the incubation period (measured from clutch hatching day), we calculated that 55% of nests completion to hatching of entire clutch) at the survive until hatching. The daily risk of death only nest where we had detailed laying data to the incubating bird at the nest was 1.2% was 14 days. In two other nests found after (SE = 0.8; 165 nest-days without predation, incubation had begun, we recorded 13 and 14 two fatal days). Over the entire incubation days until hatching. stage, the adults’ risk of being killed or injured was 15.5%. Development of nestlings. At nine nests with 2-egg During the nestling period, out of 22 clutches, both eggs hatched within 24 hours nests, three were depredated and two failed of laying. Primary feather pins broke skin when the nestlings died of unknown causes. around day 5–6 and emerged from sheaths In the latter nests, nestlings were underfed when nestlings were 10–12 days old. Eyes and cold and we observed partial losses (of an opened on days 9–10, and nestlings fledged egg in one nest and a newly hatched chick in when 18.6 ± 1.2 days old (n = 5 nests). In the second) several days before the nests were three other nests fledging was apparently pro- deserted. Partial losses also were observed in voked by the approach of an observer and two other nests that both eventually fledged nestlings left the nest at ages of 16, 18, and 19 one nestling. In these nests the difference in days. body mass between nestlings gradually During the period of fastest growth (day 5 increased and the smaller chick eventually to day 9), the increase in body mass was died (15 and 11 days after hatching). The approximately linear and nestlings (n = 32 smaller chicks’ body masses at the last check from 19 nests) on average gained 2.7 g/day where they remained alive were, respectively, (SE = 0.35). The asymptotic body mass 75.2% and 74.2% of their nest-mate’s mass. attained by nestlings close to fledging was Daily survival rate of nests during the 43.1 g (SE = 5.5), or 69% of the mean body nestling stage was 0.983 (SE = 0.008, 22 nests mass (62.5 g; range = 59.5–65.0 g) of three observed, 286 successful days, 5 days with adult males captured near our study area (C. failure). Using a 17-day nestling period (with- Dingle pers. com.). Growth rate expressed by out the hatching day), this yields an estimated the K parameter of the logistic equation 75% nest survival from hatching to fledging. equaled 0.36 (SE = 0.05). The growth curve Combining these data with estimates from inflected at day 7 post-hatching, and nestlings incubation, 41% of nests where incubation took 12.2 days to grow from 10% to 90% of begins are predicted to fledge at least one the asymptote. young. The mean number of fledglings per successful nest (n = 15) was 1.7 ± 0.5. Breeding success. The survival rate of nests dur- ing egg laying was not estimated because we Adult behaviour at nests. Only females were had too few data. During incubation, two of observed incubating eggs (n = 11 nests at 24 nests were abandoned and five were depre- which sex of the incubating parent was deter- dated, including two where the incubating mined at least once) and brooding young (n = female was apparently killed (with many 6 nests). At three nests, diurnal brooding con-
385 HALUPKA & GREENEY tinued until 9–12 days after hatching, when Marin & Carrion (1991) consisted only of the contour feathers of nestlings were emerg- moss and rootlets. These observations sug- ing from their sheaths. Sporadic, non-quanti- gest there is variation within and between fied video recordings at several nests (HFG populations of Pale-eyed Thrushes in choice unpubl.) revealed that both adults provi- of nest materials. sioned nestlings. Growth rates of nestling Pale-eyed During incubation, modal flush distance Thrushes in our study (K = 0.36) were similar at 14 newly discovered nests was 0.5 m (range to those of nestlings of a sympatric thrush, = 0.5–2 m). This sudden and conspicuous the Andean Solitaire (K = 0.33; Greeney & flushing from the nest, in conjunction with Halupka 2008). Coefficients reported for nine alarm calling and, in several cases, distraction North American thrush species (genera displays (flying in a slow and fluttery manner), Catharus, Sialia, Hylocichla, and Turdus) are however, was not the rule. Some females left higher (range: 0.44–0.69; Remeš & Martin nests when observers were at much greater 2002). Growth rates of Pale-eyed Thrushes distances and, even though nest checks during also appear to be rather slow compared to incubation were frequent, such females were other tropical species. For passerines with an rarely seen on the nest. During nest checks, adult body mass less than 100 g, Ricklefs adults gave alarm calls while well hidden in (1976) reported a range of K values between vegetation close to the nest. Alarm calls were 0.278 and 0.604. Even compared to other given even during the early stages of incuba- tropical thrushes, including species from tion. montane forests in Venezuela (Ricklefs 1976), Pale-eyed Thrushes in our study grew rela- DISCUSSION tively slowly. Greeney & Halupka (2008) sug- gested that similarly slow growth of Andean Most nests of Pale-eyed Thrushes in our Solitaires might be due to their rich in fruits, study were located in trees. Similarly, two low-protein diet, and the relatively low tem- nests found by Londoño (2005) were also in peratures at about 2000 m a.s.l., where they trees. We found five Pale-eyed Thrush nests breed. The same factors may contribute to on the ground on road embankments, and the slow growth of nestling Pale-eyed Marin & Carrion (1991) also reported nests Thrushes because they breed during the same on embankments. While the reason for such period of the year as Andean Solitaires and variability in nest site selection is unclear also feed nestlings a high percentage of fruits (there were many potential nesting trees in (Londoño 2005, HFG pers. observ.). The the forest adjoining the road), it appears brood reduction observed at some nests also that ground nests are unusual in the forest suggests that food resources may have interior, as we found no nests on the numer- been limiting for Pale-eyed Thrushes in our ous vertical banks along streams in our study study. area. Estimated nest success for Pale-eyed Four of the five Pale-eyed Thrush nests Thrushes in our study was relatively high we examined contained soil (not simply (41% of nests in which incubation begins decaying nest material) and the remaining nest were predicted to fledge at least one young), had a layer of wood dust. In contrast, two comparable to values from North America nests from Colombia examined by Londoño (41% average for shrub or low-foliage nesting (2005) and a single nest from the western species; Martin 1995) and higher than that for slope of Ecuadorian Andes described by three thrush species that breed in montane
386 BREEDING OF PALE-EYED THRUSHES forests in Argentina (23–29%, calculated from plantation (Londoño 2005). Additionally, Tables 2 and 5 in Auer et al. 2007) and for research carried out in Colombia (Renijfo understory open-cup nesting birds in a low- 1999) suggests that Pale-eyed Thrushes are land humid forest of Panama (32–38%; Table tolerant of forest fragmentation. Conversely, 2 in Robinson et al. 2000). The survival however, the erratic inter-annual breeding rates of Pale-eyed Thrush nests were also documented in our study is a noteworthy con- comparable to those of Andean Solitaires in sideration for future evaluations of the con- the same study area (39%; Greeney & servation status of this species, as it is a Halupka 2008). In both species, parent birds potential factor increasing demographic sto- choose similar nesting sites and behave in chasticity (Simberloff 1988). similar ways when approached during incuba- Most characteristics of the breeding biol- tion or brooding, thus such a high similarity ogy of the Pale-eyed Thrush agree with gener- of the breeding success statistics is not sur- alizations made for other tropical birds prising. (Stutchbury & Morton 1999). Namely, the We also found high predation rates on Pale-eyed Thrush has a long breeding season, incubating females of both species, with daily small clutch size, and slow-growing nestlings. risks of 1.2% in the Pale-eyed Thrush and Two traits, however, seem to be unique. First, 2.5% in the Andean Solitaire. Extrapolated Pale-eyed Thrushes in our area show breeding over the entire incubation period, this sug- success comparable to temperate species. Sec- gests that 15.5% and 32% of females, respec- ond, our data demonstrate that the cost of tively, would be attacked. Our statistics might reproduction in this species, intensified by the be overestimated because the presence of vulnerability of incubating females, may be adult feathers at the nest does not necessarily relatively high. Although these differences indicate a fatal attack. On the other hand, we suggest that the nesting strategy of the Pale- only used the incubation period in our calcu- eyed Thrush shows characteristics of both lations and the true exposure period would high-latitude and tropical species, further also include time when nestlings are brooded, studies are needed to confirm these patterns thus producing even higher estimates. It is in other parts of its range. often argued that tropical birds typically have relatively low annual mortality, at least when ACKNOWLEDGMENTS compared to closely-related species from the temperate zone (Ricklefs 1997, Stutchbury & We thank C. Dingle, R. Gelis, R. Lynch, and J. Morton 1999). Thus a high incidence rate of Simbaña for help with field work. C. Busta- predation attempts in our population might mante and M. Lysinger kindly allowed us to support alternative models of life histories in collect data in the birding reserve of Cabañas tropics (Karr et al. 1990, McGregor et al. 2007, San Isidro. We also thank anonymous review- Blake & Loiselle 2008). ers for suggestions on how to improve the The current conservation status of Pale- manuscript and M. Torralva for helping with eyed Thrushes is that of "least concern" the translation of abstract to Spanish. KH was because they are widely distributed and, where supported by the State Committee for Scien- they occur, common (BirdLife International tific Research, Republic of Poland (2PO4F 2008). Pale-eyed Thrush nests have been 03226). HFG is supported by Matt Kaplan found in areas outside of mature cloud forest, and John V. Moore through the Population including secondary growth (Marin & Carrion Biology Foundation, as well as by Field 1991) and a Chinese Ash (Fraxinus chinensis) Guides Inc. and the Maryland Audubon Soci-
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