Acta Biológica Colombiana

The research has faced significant challenges due to financial constraints, especially in developing countries (Ayalew and Xianzhi, 2019). Nonetheless, there remains pressure on researchers to increase productivity and publish their findings in reputable outlets (Jordão, 2019). To address this, rapid inventory techniques have been proposed and recommended for tropical birds, offering an alternative to time-consuming long-term inventories (Herzog et al., 2002; Stevens et al., 2019). It is essential to improve these methods by analyzing abiotic environmental factors that affect bird species records, making them both efficient in terms of species accumulation and cost-effective. For instance, studies have demonstrated the importance of temperature for the occupancy of Australian birds (Einoder et al., 2018) and its role in determining species richness along elevation gradients in Bolivia (Montaño-Centellas et al., 2021). However, there is no universal rule as temperature (and precipitation) did not show significant associations with species richness in tropical mountains (Hanz et al., 2018). It is often necessary to consider novel covariables in the analysis. For example, the slope of a mountain was found to drive differences in bird communities in the Brazilian Atlantic Forest, with fewer species found in steeper terrains (Cavarzere et al., 2021).

Birds, being conspicuous, predominantly diurnal, and responsive to environmental changes, have received substantial attention in ecological studies (Pizo and Tonetti, 2020) and they serve as a valuable taxonomic group for planning conservation actions (Stralberg et al., 2019). Therefore, meticulous study of birds is of paramount importance for obtaining rapid and high-quality data (Vergara-Tabares et al., 2020). Few studies tried to evaluate the influence of abiotic factors on bird detection. Thus, I wished to investigate whether temperature, wind speed and atmospheric pressure drive the detection of bird species to suggest climatic conditions that will optimize resources while ensuring efficient surveys. I hypothesize increasing values of temperature, wind speed and atmospheric pressure should account for fewer bird species.

Chapada Diamantina, located in the state of Bahia, northeastern Brazil, is situated within the Caatinga, an exclusive Brazilian biome. It represents the northern section of the Espinhaço Range, which stretches south to the state of Minas Gerais; encompassing an area of 60000 km² it covers inland regions in Bahia, ranging from 10° to 15° S and 40° to 44° W (Nóbrega and Boas, 2020). The average annual temperature is 19.5° C and the precipitation varies between 500 and 1300 mm per year, with the majority of rainfall occurring from November and March. The elevations in this area can reach up to 1200 m (Nóbrega and Boas, 2020). Across the four study areas, namely Damacena, Maniçoba, Pedra, and Veredão, which are at least 6 km apart, the vegetation exhibits considerable similarity and is predominantly characterized by caatinga. The landscape features scattered small trees, with some reaching a height of up to 7 m, and in certain transects, shrubs and pastures are also present, particularly in Veredão, where the vegetation does not form a continuous canopy.

Bird surveys were conducted in the municipality of Mulungu do Morro, Bahia, from September 2 to 5, 2013. The surveys utilized 5-species-lists, in which the observer completes lists of five species without repeating species in the same list. Species that are detected again can be annotated in sequential lists. Five-species-lists were used because it took longer to complete lists with 10 or more species. By doing this, more lists, which are the sampling effort, were accumulated (Herzog et al., 2002). Each locality was visited during a single morning. Birds were identified visually with 8 x 40 mm binoculars, and aurally. Censuses started at 6 am and lasted for 4 h, except at Damacena, where the survey began at 5 am. At each hour three abiotic covariables were collected: temperature, humidity, and atmospheric pressure. Measurements were obtained using a digital thermometer, hygrometer, and barometer, respectively. In addition, the average wind speed was acquired from a nearby anemometer station. The collinearity among the covariables was verified with a Pearson test and “humidity” was excluded for being highly and negatively correlated with “temperature” (p = 0.05, r = 0.95). Wind speed values were log10-transformed. Competing models were then created considering individual covariables and their interactions. Model comparison was performed using Akaike’s Information Criteria (AIC) corrected for small samples. Plausible models were identified based on a ΔAICc value < 2.

A total of 66 5-species lists¹ were accumulated, with an average of 17.0 ± 5.5 SD (ranging from 11 to 24 list per locality). In total, 69 bird species were recorded, including seven Caatinga endemics (Appendix 1). Species richness varied per locality from 26 to 48 (with a mean of 38.5 ± 10.8). Temperatures exhibited a wide range, from 18.9 to 31.2º C (24.1 ± 4.2), while atmospheric pressure (mean of 901.4 mmHg ± 1.4) and wind speed (mean of 8.7 m/s ± 2.4), showed a narrower amplitude (Table S1).

Appendix 1: Bird species recorded in the municipality of Mulungu do Morro, Bahia, northeastern Brazil, and their endemic status in the Caatinga. Localities: 1 = Damacena, 2 = Maniçoba, 3 = Pedra, 4 = Veredão

Individually, there was a tendency for species richness to decrease with increasing temperature and atmospheric pressure, while no clear trend was observed regarding wind speed (Fig. 1). However, an additive interaction between temperature and wind speed was detected, which was the most plausible model (Table S2).

Temperature is a well-known abiotic factor that influences the detection and occurrence of birds (Einoder et al., 2018; Montaño-Centellas et al., 2021; Ramesh et al., 2022). Although hourly variations of bird records suggest forest species are more detectable during the first morning hours (Blake, 1992), I was unable to find research specifically examining the effect of temperature on the record of higher or lower species richness over time. It has been suggested that birds tend to avoid warmer microclimates (Jirinec et al., 2022). Also, the energy storage stochasticity hypothesis proposes that birds have surplus energy reserves in the morning due to overnight temperature fluctuations (McNamara et al., 1987). Thus, birds invest this left-over energy singing (Hutchinson, 2002; Barnett and Briskie, 2007), enhancing their conspicuousness during the early hours of the day. Such observations corroborate the notion that in this Brazilian arid highland scrub, as time progresses, the rising temperatures during the early morning hours may result in fewer bird records. The effect of atmospheric pressure on bird species is also straightforward, as birds are highly sensitive to its variations (Kreithe and Keeton, 1974). I tended to record fewer bird species during low atmospheric pressure, which aligns with the fact that migratory movements are often reduced when atmospheric pressure drops (Liechti et al., 2018). Individually, the wind speed did not exhibit a significant effect on species richness. However, lower species richness observed at Veredão suggests that high wind conditions compromise bird detection (Wiest and Shriver, 2016). In Veredão, the vegetation was more open, lacking a continuous canopy, therefore more susceptible to gusts of wind.

Figure 1: Linear regressions with shaded 95 % confidence intervals between species richness and (a) temperature, (b) atmospheric pressure, and (c) wind speed.

Given the significance of bird surveys in ecological studies and conservation planning, it is crucial to employ rapid inventories effectively, considering suitable abiotic conditions, to yield efficient and timely results. For future censuses in Brazilian arid highland scrubs, which are prone to temperature increases in the morning and high wind speeds, it is recommended to assess the temperature (below ~20° C) and wind speed (< 10 m/s) before initiating fieldwork. This approach will help to optimize resources and ensure maximum efficiency bird surveys.

Table S1: Hourly records of bird species and measurements of abiotic factors in the municipality of Mulungu do Morro, Bahia, northeastern Brazil.

Table S2: Model selection showing the values of Akaike’s Information Criteria corrected for small samples (AICc), delta AICc, and weights and cumulative weights of each model.