SHORT COMMUNICATIONS ORNITOLOGIA NEOTROPICAL 25: 237–243, 2014 © The Neotropical Ornithological Society NON-RANDOM ORIENTATION IN WOODPECKER CAVITY ENTRANCES IN A TROPICAL RAIN FOREST Daniel Rico1 & Luis Sandoval2,3 1The University of Nebraska-Lincoln, Lincoln, Nebraska. 2Department of Biological Sciences, University of Windsor, 401 Sunset Avenue, Windsor, ON, Canada, N9B3P4. 3Escuela de Biología, Universidad de Costa Rica, San Pedro, San José, Costa Rica, CP 2090. E-mail: [email protected] Orientación no al azar de las entradas de las cavidades de carpinteros en un bosque tropical. Key words: Pale-billed Woodpecker, Campephilus guatemalensis, Chestnut-colored Woodpecker, Celeus castaneus, Lineated Woodpecker, Dryocopus lineatus, Black-cheeked Woodpecker, Melanerpes pucherani, Costa Rica, Picidae. INTRODUCTION tics such as vegetation coverage of the nesting substrate, surrounding vegetation, and forest Nest site selection play’s one of the main roles age (Aitken et al. 2002, Adkins Giese & Cuth- in the breeding success of birds, because this bert 2003, Sandoval & Barrantes 2006). Nest selection influences the survival of eggs, orientation also plays an important role in the chicks, and adults by inducing variables such breeding success of woodpeckers, because the as the microclimatic conditions of the nest orientation positively influences the microcli- and probability of being detected by preda- mate conditions inside the nest cavity (Hooge tors (Viñuela & Sunyer 1992). Although et al. 1999, Wiebe 2001), by reducing the woodpecker nest site selections are well estab- exposure to direct wind currents, rainfalls, lished, the majority of this information is and/or extreme temperatures (Ardia et al. based on temperate forest species and com- 2006). Cavity entrance orientation showed munities (Newton 1998, Cornelius et al. 2008, non-random distribution in 11 of the 17 stud- Sandoval & Barrantes 2009, Acosta-Pérez et ied woodpecker species in America (Table 1). al. 2013). Woodpeckers select nest sites based In woodpecker species with non-random dis- in certain physical characteristics of trees such tributed cavity entrances, the main cause as hardness, size, height, and species (Aitken mentioned for this is to avoid extreme tem- et al. 2002, Adkins Giese & Cuthbert 2003, peratures (Lawrence 1967, Inouye et al. 1981, Sandoval 2008); or certain habitat characteris- Korol & Hutto 1984). Meanwhile, surround- 237 RICO & SANDOVAL ing habitats influenced the cavity orientation due to degradation, modification, or other in species with random distribution of cavities organisms creating them. The purpose was to (Vierling 1997). exclude cavities of uncertain origin. Despite what is known about cavity For each woodpecker cavity found we entrance orientation in temperate wood- took the following measurements: (1) cavity pecker species, the pattern of cavity entrance entrance orientation using a compass; (2) cav- orientation in Neotropical woodpecker’s ity entrance position relative to the forest in a remains poorly understood. Therefore, our 10 m radius from the snag: nest entrance fac- main objective in this study was to analyze if ing an opening (facing a gap or perpendicular woodpeckers show a pattern for cavity to the trail) or closed forest (oriented directly entrance orientation. If cavity entrance orien- to forest); and (3) the size of the woodpecker tation is important for breeding success in that created the nest. We sorted cavities into tropical woodpeckers, we expect to find a two categories of either small or large in over- non-random pattern in the entrance orienta- all size, because cavities vary according to the tion. Furthermore, we expect that orientation entrance size and form, but without seeing will be related with some ambient characteris- the birds inside the cavity it is impossible to tics such as wind currents or sun exposure. determine the exact species that created each Given that woodpecker cavities persist over cavity. Small cavity entrances, with circular time and the large majority are constructed nest entrances and diameters ≤ 7 cm were during the breeding season in Costa Rica created by Chestnut-colored (Celeus castaneus) (Stiles & Skutch 1989, Sandoval & Barrantes or Black-cheeked Woodpecker (Melanerpes 2006, Sandoval 2009), old cavities are a valu- pucherani) (LS unpub. data). Large cavity able resource of information to analyze entrances, with oval shape entrances and entrance orientation (Korol & Hutto 1984). diameter > 7 cm, were created by Pale-billed (Campephilus guatemalensis) or Lineated Wood- METHODS pecker (Dryocopus lineatus) (Skutch 1969, Male- kan 2011). There is a probability that some of We conducted this study from 15–17 July the cavities were not completely constructed 2013 (at the end of the woodpeckers breeding (abandoned during construction process) but season; Winkler & Christie 2002) at La Selva enough excavated to not separate from cavi- Biological Research Station, Heredia prov- ties used as nests. However, those cavities ince, Costa Rica (10°25’ N, 84°00’ W), along a still provide useful information about the ori- 5.4 km length trail that crossed through the entation selected to construct the cavity middle or edge of the forest (trail: Sendero entrance, and were used in the analysis. We Tres Rios -STR-). We walked down the mid- analyzed the cavity entrance orientation in dle of the trail and looked at both sides for both groups separately, because both showed snags (dead trees with or without branches differences in cavity sizes and form, which higher than 2 m and diameters at breast may be one of the factors that influence the height ≥ 10 cm) within 25 m of the trail. At entrance orientation. each snag we recorded the number of wood- Given that it is unknown if more than one pecker cavities that were circular or oval cavity was constructed by the same individual shape and extending into the trunk with the in the same snag, this would cause a pseudo- back wall not being visible from the ground replication problem, because cavity entrance using binoculars. We did not include any cav- orientation may reflect only the selection of ity that we could not assign to woodpeckers, one individual. Differences in the wood hard- 238 SHORT COMMUNICATIONS TABLE 1. List of American woodpecker species where cavity entrance orientation has been studied for patterns. Species Common name Orientation Reference Melanerpes formicivorus Acorn Woodpecker Non-random Hooge 1989 M. erythrocephalus Red-headed Woodpecker Non-random Reller 1972, Conner 1975 M. lewis Lewis’ Woodpecker Random Linder 1994, Vierling 1997 M. uropygialis Gila Woodpecker Non-random Inouye et al. 1981, Korol & Hutto 1984 M. carolinus Red-bellied Woodpecker Non-random Reller 1972, Conner 1975 Sphyrapicus thyroideus Williamson’s Sapsucker Non-random Crockett & Hadow 1975 S. nuchalis Red-naped Sapsucker Both Walters et al. 2002 S. varius Yellow-bellied Sapsucker Non-random Lawrence 1967, Inouye 1976 Picoides pubescens Downy Woodpecker Non-random Lawrence 1967, Conner 1975 P. villosus Hairy Woodpecker Non-random Lawrence 1967, Conner 1975 P. tridactylus American Three-toed Both Leonard 2001 Woodpecker P. arcticus Black-backed Woodpecker Both Dixon & Saab 2000 P. borealis Red-cockaded Woodpecker Non-random Dennnis 1971 P. albolarvatus White-headed Woodpecker Both Milne & Hejl 1989, Kimball et al. 1996, Kozma 2010 Colaptes auratus Northern Flicker Both Lawrence 1967, Conner 1975, Inouye 1976, Stauffer & Best 1982, Ingold 1994 Campephilus magellanicus Magellanic Woodpecker Non-random Ojeda 2006 Dryocopus pileatus Pileated Woodpecker Non-random Bent 1939, Conner 1975 ness within the snag may affect which side is same pair may defend several snags if they are more used for construction of the cavities in close proximity. However, the closest dis- between snags, since woodpecker prefer soft tances between two snags in our study was wood for the construction of cavities (San- approximately 200 m reducing the probability doval & Barrantes 2006), producing another that the same pair defend multiple snags. possible cause of pseudo-replication. To avoid We used the averages between snags to problems with pseudo-replication within the estimate the population cavity entrance orien- raw data, we randomly selected one cavity ori- tation following the Rao’s U spacing test for entation per snag (without replacement) per uniform distribution (U) for binomial or uni- woodpecker category to estimate a between modal distribution, according to the data ori- snag cavity entrance orientation average, fol- entation. We reported the mean angle relative lowing a similar approach used to estimate to magnetic north of the nest entrance and coefficient of variation between individuals in the 95% confidence interval based on 5000 acoustic analysis (Robisson et al. 1993). With bootstraps. This approach produces a more this approach we got a sample size equal to conservative value for the confidence inter- the number of snags, but both woodpecker vals. We used a Chi-square test for uniform cavity types were not present at each snag. distribution with 2 bins to analyze if wood- Another potential independence problem in pecker nest entrances of each group size were our data is the distance between snags. The more allocated in the direction of open or 239 RICO & SANDOVAL FIG. 1. Mean direction and 95% confidence interval of tree cavities entrances using 360° range for: a) small woodpeckers and b) large woodpeckers group. Grey pie pieces are based in 10 bins of equal