Date of this Version
Applied Animal Behaviour Science 185 (2016) 113–120
Grasslands represent 39%–50% of U.S. airport properties, and a recent management framework recommended exploiting both antipredator behaviours and food resources in airport grasslands to curb use by birds considered hazardous to aviation safety. We evaluated framework predictions empirically by exposing unsated and sated brown-headed cowbirds (Molothrus ater) to visually obstructive (∼13-cm vegetation height; tall), higher-risk plots versus unobstructive (height; short) plots, and relative to prey resources. We predicted that 1) unsated birds (unfed since the previous day) would be present in greater numbers and forage more in short than tall vegetation plots 24 h post-mowing because of invertebrate flush resulting from mowing; 2) unsated birds would show increasing numbers and foraging in tall plots >24 h post-mowing because of decreasing food abundance and availability in short plots; and 3) sated birds would be present in greater numbers and forage more in short vegetation overall, because vigilance needs would exceed that of food needs. We evaluated effects of visual obstruction (a metric correlated with both vegetation height and insect density) on behaviours within plots via generalized linear mixed models. Unsated cowbirds showed nearly equal numbers in tall and short plots (X [SE] individuals using tall plots: 9.5 [5.1]; short plots: 9.8 [5.1], P = 1.00, Wilcoxon Signed Ranks Test), and foraged nearly equally in both plots 24 h post-mowing (tall plots: 6.9 [4.7] individuals; short plots: 6.6 [4.1] individuals, P = 0.94). Prey availability was likely enhanced within short plots within 24 h of mowing, but possibly in adjacent tall plots as well. Over the course of the experiments (8–9 days) unsated cowbirds showed no difference in numbers between plots (tall plots: 8.2 [4.9] individuals; short plots: 11.4 [4.9] individuals, P = 0.13), but foraged more in short plots (tall plots: 4.4 [3.8] individuals; short plots: 7.8 [4.2] individuals, P = 0.01); visual obstruction was significantly and negatively correlated with foraging in tall plots. Sated cowbirds selected for short plots (use of tall plots: 5.9 [4.2] individuals; short plots: 11.7 [4.6] individuals, P < 0.01; foraging in tall plots: 4.1 [3.3] individuals; short plots: 8.2 [4.6] individuals, P < 0.01). Our findings support recommendations for use of visually obstructive vegetation in combination with proactive control of food resources to reduce use of airport grasslands by birds that select against visually obstructive cover.