Urbanization has opposite effects on the territory size of two passerine birds

Background:Urban expansion has been identified as one of the leading drivers of biodiversity change or loss.For birds,urbanization is specifically related to survival,breeding success,and territory size.Understanding how different birds adjust territory size in response to urbanization is essential for their conservation in urban environments and to better understand why some species are lost and others persist under this condition.We evaluated the effect of urbanization on the territory size of an urban avoider species,White-eared Ground-Sparrow(Melozone leucotis),and an urban adapter species,House Wren(Troglodytes aedon),at five Costa Rican sites.Methods:We measured the size of 30 ground-sparrow and 28 wren territories using a total of 296 h of observation.We followed each individual for at least 1 h per day for at least 2 days of two consecutive years,and geo-referenced their locations.Territory size was estimated using the minimum convex polygon method.We measured the urban surfaces(roads,buildings,any other paved area,soccer fields,lawns,and gardens with short grass)within territories.Results:Ground-sparrow territories were larger at the highly urbanized site than at the non-urbanized site.Wren territories were larger at the low urbanized site than at the highly urbanized site.We found a positive relationship between urban surface and territory size for the ground-sparrow,but not for the wren.Conclusions:Our results showed that not all birds adjust territory size in the same way in response to urbaniza-tion.We showed that urban avoiders probably need to defend larger territories in urban environments to find all the resources required to survive because urban environments may provide insufficient resources such as food or shelter.Urban adapters on the other hand defend smaller territories in urban environments because even small territories may provide sufficient resources.These results suggest specific behavioral adaptations developed by Neotropical birds inhabiting urban environments.

Intruder familiarity and not duet similarity influences the territory defense in a year-round territorial bird species

To avoid unnecessary energy expenditures in territorial defense,many species(e.g.,insects,amphibians,birds,and mammals)have developed the capability to distinguish between different intruder types using visual,acoustic,and/or chemical signals.Determining the mechanism used for intruder recognition is key to understanding the dynamics of territorial behaviors.In birds that use vocalizations for territorial defense,the frequency and duration of vocalizations or the familiarity with the intruder may be the main mechanism used for intruder recognition.Here,we conducted a playback experiment with territorial White-eared Ground-sparrows(Melozone leucotis),to analyze if territorial pairs recognize intruders using frequency and duration characteristics(a bird’s response is based on how structurally similar the intruders’duets are to their own)or by familiarity with the intruders(neighbors vs.non-neighbors).We focused on duets because this species uses duets exclusively for territorial defense.We broadcasted a duet from a territorial neighbor,two duets from non-neighbors(with different frequency and duration characteristics),and a duet from a control species in 39 territorial pair from three populations.During playback we measured five behavioral responses:latency of the first vocalization,latency of the approach to the speaker,time spent close the speaker(within 5 m),number of individuals that approached the speaker,and the number of vocalizations.We found that territorial White-eared Ground-sparrow pairs responded stronger to neighbors than non-neighbors,and this response is not influenced by duet duration or frequency characteristics.This result suggests that neighbors represent a greater territorial threat for White-eared Ground-sparrows than non-neighbors.Further work is necessary to understand how common this observation is for tropical species that defend small territories year-round.

Local predation pressure predicts the strength of mobbing responses in tropical birds

Many birds join cooperative mobbing aggregations and collectively harass predators. Individuals participating in these ephemeral associations benefit by deterring the predator, but also incur energetic costs and increased risk of predation. Ex- plaining the evolution of mobbing is challenging because individuals could prevail by selfishly seeking safety while allowing others to mob. An important step in understanding the evolution of mobbing is to identify factors affecting its expression. The ecological constraints model suggests that animals are more likely to cooperate under adverse environmental conditions, such as when local predation pressure is high. We tested this prediction by comparing the mobbing responses of several species of birds to the local abundance of their primary predator, the ferruginous pygmy-owl Glaucidium brasilianum. We used acoustic playback to elicit mobbing responses in environments where owls were common, uncommon, or rare. Stimuli were either the song of a fer- ruginous pygmy-owl or the mobbing calls of three of the owl＇s common prey species. During each playback, we characterized mobbing responses by noting the number of species and individuals that approached the loudspeaker, as well as the closest ap- proach by any bird. Mobbing responses to both stimuli were strong in locations where Ferruginous Pygmy-owls were common, intermediate where owls were uncommon, and weak where they were rare. This pattern persisted even after controlling for dif- ferences in species richness and composition among the three environments. Results support the ecological constraints model and provide strong evidence that intense predation pressure increases the expression of cooperative mobbing in tropical birds [Cur- rent Zoology 58 （5）： 781-790, 2012].