Territory size is affected by distribution of resources, density of potential competitors, body size of the territory holder and its ability to detect intruders. During four consecutive years (2009–2013), we determined territory size of the serra antwren, a highly territorial bird, that occupies permanent territories in the restinga habitat. We studied seasonal variation and relationship between territory size, number of neighboring territories and floaters (mature individuals without fixed territories), and body size of the territory holders. We also estimated population density using a capture–recapture method. Territory size was smaller during the breeding season with its size affected by number of neighboring territory holders and year. Densities were estimated to be 1.6–5.8 indiv. ha^–1, which were 2 to 6 times higher than the value previously estimated by fixed-radius point counts (0.93 indiv. ha^–1). This may be due to the high number of floaters, potentially important but usually neglected in density estimates. Neighboring territory holders affected more than floaters territory size, possibly because of their similar individual qualities.