Roadside Car Surveys: Methodological Constraints and Solutions for Estimating Parrot Abundances across the World

Abstract

Parrots stand out among birds because of their poor conservation status and the lack of available information on their population sizes and trends. Estimating parrot abundance is complicated by the high mobility, gregariousness, patchy distributions, and rarity of many species. Roadside car surveys can be useful to cover large areas and increase the probability of detecting spatially aggregated species or those occurring at very low densities. However, such surveys may be biased due to their inability to handle differences in detectability among species and habitats. We conducted 98 roadside surveys, covering > 57,000 km across 20 countries and the main world biomes, recording ca. 120,000 parrots from 137 species. We found that larger and more gregarious species are more easily visually detected and at greater distances, with variations among biomes. However, raw estimates of relative parrot abundances (individuals/km) were strongly correlated (r = 0.86–0.93) with parrot densities (individuals/km2) estimated through distance sampling (DS) models, showing that variability in abundances among species (>40 orders of magnitude) overcomes any potential detectability bias. While both methods provide similar results, DS cannot be used to study parrot communities or monitor the population trends of all parrot species as it requires a minimum of encounters that are not reached for most species (64% in our case), mainly the rarest and more threatened. However, DS may be the most suitable choice for some species-specific studies of common species. We summarize the strengths and weaknesses of both methods to guide researchers in choosing the best–fitting option for their particular research hypotheses, characteristics of the species studied, and logistical constraints.

The Role of Monk Parakeets as Nest-Site Facilitators in Their Native and Invaded Areas

“…that covered both rural and urban habitats in the native, e.g., [31] and invaded ranges [9,[32][33][34] of monk parakeets, finding that the species is more abundant in rural areas in its native range, but proliferates more in urban areas in its invaded ranges. Once monk parakeet nests were located, we identified the substrate on which they were placed (tree or human construction), estimated their height above ground level (visually or using a laser range-finder for heights > 10 m), and counted the number of chambers as a measure of nest size.…”

“…From 2013 to 2020, we monitored populations of monk parakeets established in different countries including the native (Argentina, Brazil, Paraguay, and Uruguay) and invaded range (Italy, Puerto Rico, and Spain) of the species (Figure 1a) in urban and rural ranges. This monitoring was conducted in different campaigns of extensive field work that covered both rural and urban habitats in the native, e.g., [31] and invaded ranges [9,[32][33][34] of monk parakeets, finding that the species is more abundant in rural areas in its native range, but proliferates more in urban areas in its invaded ranges. Once monk parakeet nests were located, we identified the substrate on which they were placed (tree or human construction), estimated their height above ground level (visually or using a laser rangefinder for heights > 10 m), and counted the number of chambers as a measure of nest size.…”

Annual Censuses and Citizen Science Data Show Rapid Population Increases and Range Expansion of Invasive Rose-Ringed and Monk Parakeets in Seville, Spain

“…Citizen science data have several biases that may explain these results. Although occurrence records from volunteers can be temporally and spatially biased when the detectability of the species is difficult or survey efforts are inadequate [66,[83][84][85], these biases are unlikely in this case, given that parakeets are loud and highly conspicuous [59,86], thus facilitating their detection by birdwatchers. However, volunteers can accidentally introduce sampling bias such as a misidentification between both parakeet species (which is rather unlikely due to their different morphologies and behaviors) or under-reporting, especially when these species become abundant, or simply because they are not interesting to observers [87,88].…”

Assessing the Relative Impacts of Roadkill and Nest Poaching on the Population Viability of the Blue-and-Yellow Macaw, Ara ararauna (Aves: Psittaciformes), in a Brazilian National Park

“…Another study shows lower values; this survey was performed in neotropical regions on unpaved and low-transit paved roads, where the density of this species ranged between 0.22 and 1.18 individuals/km 2 (mean 0.48), with a value of 0.34 for the Brazilian Cerrado. The lower density estimates are from different landscape types, varying from well-preserved areas to urban areas [51], and the higher rates are from a large protected area [50]. Therefore, we decided to use 1.18 individuals/km 2 .…”