Effects of Urban Lighting at Night on Daytime Songbird Aggression

Many animals require sleep for daily functions. In spring, songbirds expend a great deal of energy early in the day by singing to attract mates and aggressively protecting territory. Some evidence suggests that artificial lighting at night (ALAN) in the environment of wild birds increases their nighttime activity that, in turn, contributes to sleep loss. This study investigated how ALAN affects northern cardinal (Cardinalis cardinalis) response to experimental aggressive intrusion by conspecifics. The hypothesis tested is that high ALAN near cardinal roosting spots would cause those individuals to have less energy for daytime aggressive behaviors. After selecting known dark and bright urban roosting locations, ALAN was measured at specific sites with active cardinal territories during nighttime hours. Observers returned in the morning to present cardinal territorial songs using standardized call playback protocols. Aggressive behaviors of responding cardinals were recorded and used to quantify aggression intensity. Standard aggression measures included response latency, distance from bird to speaker, number of songs, hops, and flip flops. An aggression index was obtained using factor analysis which confirmed that aggressive responses featured short latency and approach distances, few chips, and many songs, flip flops, and hops. Analysis of variance revealed that cardinals roosting in high ALAN engaged in significantly more aggressive morning responses; directly contradicting predictions and previously published findings. One possible explanation for this result is that high ALAN might improve sleep in cardinals. To test this, autonomous recording units (ARUs) were placed in study locations to record nighttime calling behaviors in high and low ALAN. Insufficient data were obtained to clarify whether nighttime activity was reduced significantly by ALAN exposure. However, results showed that nighttime activity by cardinals is strongly influenced by moonlight, and that determination of the influence of ALAN on nighttime and daytime avian behavior must include measures of lunar illumination.