Animals use behavioural plasticity to cope with conflicting selective pressures. We investigated how prey availability affects the spinning of cobwebs, whose architecture influences both prey capture and defence. Fed western black widows spun cobwebs containing more silk than did fasted spiders. However, fed spiders invested relatively less silk in the sheets and sticky gumfooted threads of webs and relatively more silk in supporting threads than did fasted spiders. The material properties of silk spun by fed and fasted spiders were relatively similar, but silk threads spun by fed spiders were twice as thick as those of fasted spiders, increasing web strength by 225%. Finally, spiders fed late in the experiment did not reproduce, even though they received the same amount of food as spiders fed earlier, 43% of which reproduced. The reallocation of silk in cobwebs away from sheets and sticky gumfooted threads, which function in prey capture, to supporting threads, which surround spiders with defensive clouds of silk, is consistent with reduced foraging effort and increased predation risk avoidance by fed spiders. Furthermore, this behavioural shift occurs as spiders are reproducing, when spiders can most afford the costs of reduced prey capture and would most benefit from enhanced protection. Our results contrast with those of previous studies on orb-weaving spiders, which found reductions in the sizes of webs as spiders caught more food. Thus, the architectures of spider webs reflect many selective pressures, the effects of which may vary with physiological state.