The effect of visibility on space use by territorial red-capped cardinals

One of the factors that may influence an animal's use of space is visibility, which in territorial species can determine how readily an individual can monitor its territory for conspecific intruders. We hypothesized that territorial red-capped cardinals (Paroaria gularis) would prefer locations that provided them with good views of their territories. Red-capped cardinals defend territories along lakes and rivers in the Amazon Basin, and visibility can vary widely within these territories. We defined visibility as the percentage of a territory that could be seen from a particular location. Visibility was measured by dividing each territory into 20 m segments and calculating the proportion of the territory that was visible from each segment. Cardinals did not distribute their time evenly within their territories: focal observations revealed that cardinals spent more time in the segments of their territory that afforded them the best view of their territory. This preference is unlikely to be due to differences in food availability, as the abundance of arthropod prey did not vary between high-visibility sites (peninsulas) and low-visibility sites (bays). Cardinals probably benefited from their disproportionate use of sires with high visibility because conspecific intruders were more likely to be detected from those sites.