Abstracts

Abstract: The breeding success of a double-brooding colony of Boat-billed HeronsCochlearius cochlearius was studied in pasturelands of Costa Rica. Mean clutch size in the first clutches (2.9 eggs/nest) was higherthan in second and repeat clutches (2.3 eggs/nest). Breeding success was similar in the first attempt and secondattempts(20.7 % and 21.7 %, respectively). In both attempts earlier nests enjoyed a higher breeding success.Starvation of the youngest chicks within the nest and destruction of nests by bad weather conditions were themain factors related to nestling death. No effects of human activity on the reproduction of the breeding colonywere observed. Rev. Biol. Trop. 54 (4): 1131-1134. Epub 2006 Dec. 15.

Key words: boat-billed Heron, Cochlearius cochlearius, human activity, clutch size, predation, starvation.

The Boat-billed Heron Cochlearius cochlearius (Linnaeus, 1766) inhabits the wooded or mangrove fringes of freshwater creeks, lakesand marshlands from Mexico south to Peru,Bolivia and northeastern Argentina (Hancockand Kushlan 1984).

This species breeds solitarily or in colonies(Carbonell 1993), and two peaks of activityhave been observed during the nesting season of some colonies (Dickerman and Járez 1971, Carbonell 1993). Similar results were recorded by Gómezet al.(2001), who assessed the second peak of activity to the occurrence ofsecond and repeat clutches.

Previous studies of some Boat-billed coloniespointed out that human presence appearedto affect breeding success. In fact, Juarez(1967) reported the occurrence of nest desertionand predation when humans came closeto the colony and also reported on the effectsof mangrove pruning and egg robbery on thebreeding success of these herons. Further,Wetmore (1965) pointed out that this speciesdisappears from areas occupied by humans. The capacity of a species to tolerate the effectsof habitat transformation can be an importantfactor to consider in their conservation.Information on the status of the populationsin transformed areas and the factors related totheir survival can help us to determine conservationguidelines.

This paper presents some data on the breeding of a colony of Boat-billed Heronsin pasturelands of Costa Rica and comparesthe breeding success of the first and secondattempts. We do a comparison of the results ofother studies that have been done and discusswhether or not human presence is compatiblewith the conservation of this species.

Materials and methods

The study was conducted in Santa Clarade San Carlos (Province of Alajuela, 10°21’ N,84°30’ W, 150 masl) located on the northernCaribbean Slope of Costa Rica. In this areapastures prevail, since stockbreeding is the major economic activity, although agricultureplots and human settlements are also present.Scattered along the many rivers and creeks arepatches of forest.

During almost two years, the studied colonywas observed in the surroundings of a smalllagoon in the village of Santa Clara. A narrowstrip of forest along the border of the lagoonwas the colony of Boat-billed Herons, wherethey normally roosted and foraged, but it wasalso the site of a chicken farm and the owner’shouse. As a result, there was continuous humanactivity meters away from the bird colony.

Villagers informed us that the colony hadbred in this area for some years, using in 1998and 1999 two mango trees (Mangifera indica)next to the chicken farm at the edge of thelagoon for nesting.

The present study covers the breeding seasonof 1999. A sketch of the trees with the locationof each nest was mapped out to facilitatethe identification and monitoring of the nestson visits every one to four days. The contentof each nest was recorded by climbing up thetrees. Visits were avoided on rainy days andfemales were not disturbed if they remained ontheir nest during inspection to prevent damage.For some nests, the exact date of the laying ofthe first egg was known. For the rest, the datethe first egg was laid was calculated by backdating,assuming 28 days (Juarez 1967, Gómezet al. 2001) for laying and incubation.

Thirty nests were found from February toJuly and were distributed in two marked periodsof laying, with 15 nests in each period. Nests inthe first period were in use mostly during thedry season; second period clutches started atthe beginning of the rainy season and developedwhen precipitation was high. The absence of other individuals in the area, as well as theconcordance between the maximum numberof adults counted (30 individuals) and the 14nests with eggs simultaneously in each period,revealed the occurrence of second clutches andrepeat attempts (Gómez et al. 2001).

The survival of the nestlings was recorded at20 days of age because the chicks still remainednear by the nest; thus, we could determine to which nest they belonged (Hernández-Vázquezand Fernández-Aceves 1999).

A logistic regression model to examine thedependency of nesting success on laying dateand clutch type (first versus second and repeatclutches) was used. Logistic regression modelshave been used to analyze factors affectingnesting success (Staus and Mayer 1999) andare appropriate when response variables arebinary (eg. nest success or failure). Analyseswere conducted with SPSS for Windows 8.0.

Results

Clutch size ranged from two to threeeggs, but clutches of three eggs were morecommon in first clutches (90 %, N= 10), while clutches of two eggs predominated in second attempts (73 %, N= 15). In fact,first clutches were significantly larger (t-test=22.9, p<0.001). We lacked information on theclutch size of five nests from the first periodbecause three nests failed before clutch sizecompletion and the other two nests could notbe inspected before hatching. Twenty percentof first clutches and 40 % of second attemptsfailed before hatching.

Eleven 20 day-old nestlings from the seven successful first clutches were counted.Six out of the eight clutches that failed werethe last to be laid. In the second period, twelvechicks were fledged from the eight successfulclutches. Second attempts presented a similarfailure pattern, since five out of the sevenfailed nests were the last to be laid. In fact, theoverall regression model fits the data (Hosmerand Lemeshow Goodness-of Fit statistic= 5.83,df= 7, p=0.56) and is significant (χ²= 17.86,p<0.001). Within each period, the risk of failureincreased significantly as the season progressed(B= -0.2, SE= 0.07, Wald= 8.48, df= 1,p=0.0036). Clutch type variable was excluded from the significance model.

The first nest from the second period hadeggs before successful nestlings from the firstclutches had fledged, and when failure of theeight first clutches had occurred.

At the end of each breeding period, offspringremained in a stable group close to themango trees. The juvenile group of the firstclutches had nine individuals, while the juvenilegroup of the second and repeat clutcheshad eight individuals. Thus, in relation to themean clutch size, similar breeding success(20.7 % and 21.7 % respectively) resulted.

Although clutches of three eggs were usual,mainly among first clutches, no pair fledged thethree chicks. Since asynchronous hatching aswell as great differences in size among individualswithin a nest was found, starvation ofthe youngest chicks in the nest must have beenone of the main causes of failure. Nevertheless,no nestling was observed dead in the nest; theymay have fallen in the lagoon, been removed bytheir parents, or predated. However, predationwas not observed, although potential predatorssuch as squirrels (Sciurus granatensis), greattailed grackles (Quiscalus mexicanus) and boas (Boa constrictor) were observed in the vicinityof the mango trees. It was also recorded thata chick fell from its nest during an episode ofcharacteristic aggressive behaviour that occurswhen adults or chicks come too close to thenests of other individuals. In addition, fournests from the first attempt and five fromthe second disappeared, coinciding with gustywinds or strong rainstorms.

Discussion

The mean clutch size in Santa Clara issimilar to that reported in Mexico by Juarez(1967), although we did not find clutches offour eggs as they did. In fact, clutches of threeeggs must not be scarce in Costa Rica as waspointed out by Stiles and Skutch (1989). Theirdata may come from nests of the last part of thenesting season, since clutch size reduction is acommon pattern in species with several consecutivebroods (Morrison 1998, Olmos andSilva 2001), as is the case with the Boat-billedHeron (Juarez 1967) .

The first pair that nested in the secondperiod came from the group that failed in itsfirst attempt. If successful breeders in secondattempts were those whose first clutches failed, and could incorporate as early breedersin the second period, at the end of the breedingseason most pairs should have raised at leastone brood.

The breeding success of the Boat-billedHeron in Santa Clara is higher than that reportedin the Mexican mangroves and in protected areas of Costa Rica (Juarez 1967, Carbonell1993), but coincide with the findings of otherstudies (Juarez 1967, Hernández-Vázquez andFernández-Aceves 1999), which show thatasynchronous hatching and adverse climaticconditions were the main factors related tonesting death.

Juarez (1967) reported that human activities,such as egg robbery and mangrove pruning,were also important causes of nest failure.Hernández-Vázquez and Fernández-Aceves(1999) did not find reproductive differencesbetween the breeding successes of two coloniesof Boat-billed Herons located at differentdistances from a human settlement. The resultsof Santa Clara suggest that the conservationof this species is compatible with human presencesince they have used this particular site in Santa Clara, Costa Rica for years and thattheir breeding success is higher there than atother locations studied (Juarez 1967, Carbonell1993). Therefore, the evidence does not supportthe view that this species abandons sitesthat are at a close proximity to human occupancy(Wetmore 1965). Perhaps, the absence of disruptive activities that directly impactreproduction and the presence of some of thecharacteristics in the area, such as the littlepatch of forest along the part of the lagoonwhere herons can roost and take refuge, mayhave allowed the Boat-billed Herons to the successfullyestablish in Santa Clara.

Acknowledgments

We are grateful to the Sede San Carlos of the Instituto Tecnológico de Costa Rica and the Instituto Clodomiro Picado (UCR) for logistical support. Funding was provided by a grant to JG from Agencia Española de Cooperación Internacional, and a Salvador de Madariaga grant to JAGD from the Ministerio de Educación y Ciencia. JSM received a postdoctoral grant from Generalitat Valenciana.

Resumen

Estudiamos el éxito reproductivo de una colonia del ave Cochlearius cochlearius en una zona ganadera de Costa Rica. El promedio de huevos por nido fue mayor durante el periodo de primeras puestas (2.9 huevos/nido) que durante el periodo de segundas puestas y reposiciones (2.3 huevos/nido). El éxito reproductor de los dos periodos fue similar (20.7 % y 21.7 %, respectivamente). En ambos tuvieron más éxito las parejas que comenzaron la puesta antes. Las principales causas de mortandad fueron inanición de los pollos más jóvenes del nido y la destrucción de los nidos debido a condiciones climáticas adversas. No notamos efectos adversos por la actividad humana.

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