Males of many passerine species make off-territory movements to pursue extra-pair fertilizations. Male off-territory movement investment should depend on the likelihood of paternity gain (extra-pair fertilizations) and loss (cuckoldey). Habitat fragmentation may influence fertilization success by reducing conspecific density and functional connectivity of the landscape. I studied American Redstart ( Setophaga ruticilla ) movement and fertilization success within breeding clusters in a fragmented agricultural landscape in Meanook, Alberta. Extra-pair paternity was higher in high-breeding-density years (54% of broods) than in low-breeding-density years (17% of broods). High-quality males, with many local females, in highly forested clusters invested most in movement. In the high-breeding-density years these males cuckolded low-quality, late-arriving males at the periphery of these clusters and in isolated clusters. Cuckoldry was equally likely in all clusters, yet only males in highly forested clusters with many local females achieved extra-pair fertilizations. Since males benefit highly from extra-pair fertilizations they may avoid settling in isolated clusters.