Spiders alter web architecture in response to many environmental factors. Yet, the functional implications of this behavioural plasticity are poorly understood, especially for spiders that spin long-lasting webs such as cobwebs. The western black widow spins a three-dimensional web that captures terrestrial prey and can persist for many days. However, the shape of the cobweb depends upon changes in the spider's body condition. Starved spiders invest more silk in sticky gumfooted threads, whereas sated spiders invest more silk in supporting threads. We tested the hypothesis that the increased investment in sticky gumfooted threads by starved spiders functions to improve the prey capture of their webs. Regardless of intrinsic foraging motivation, both fed and starved spiders captured more prey, more quickly and more efficiently, when hunting on a web that was spun by a starved spider. Our study shows that cobweb spiders actively manipulate web-spinning behaviours in ways that alter the functions of their webs and that even spiders spinning relatively long-lasting webs may use behavioural plasticity to respond to changing selective pressures in their environment.