Interference from conspecifics can have dramatic impacts on animals’ reproduction. Territory defense consumes energy that individuals would otherwise use to provision their young, and an increase in interference can therefore decrease productivity. Thus, interference can be a key factor in density dependent population regulation (Fretwell and Lucas 1970). We studied territory defense in an expanding population of gray hawks (Buteo plagiatus) along the upper San Pedro River in Arizona to determine whether interference influenced productivity and nest-spacing. Gray hawks will alarm call when they encounter an intruder near their nest, and we examined this defense behavior in relation to their distance to nearest neighbor. We hypothesized that nests with closest nearest neighbors would have higher levels of interference and lower productivity. We used remote microphones (Song Meter SM4, Wildlife Acoustics) to record vocalizations at twelve nests for nine weeks. This interval encompassed the incubation period (10 days before chicks hatched) through fledging (35+ days post hatch). Recorders were set up less than five meters from the nest tree. We quantified the number of alarm calls recorded between 07:00 – 08:00 because gray hawks are most active in the morning. We used the audio analysis program RAVEN (Cornell Lab of Ornithology) to create a spectrogram of the recording and counted the number of alarm calls during this 1-hour period. In addition, we found all nests in the vicinity of the recorder nest, and used ArcGIS to calculate each site’s distance to its nearest neighbor. We then ran a linear mixed model to determine the relationship between distance to nearest neighbor and number of alarm calls per hour.
Results/Conclusions
Preliminary results suggest that there is no significant increase in the rate of alarm calls as nests density increases. This could be due to the dear enemy effect, where pairs recognize individuals that nest nearby and reduce their level of aggression towards them (Hardouin et al. 2008) . Recognizing their neighbors may allow gray hawks to budget their energy for larger, less predictable threats.