When Good Animals Love Bad Habitats

If you'd like to read a bit more about the idea of source and sink habitats then there's a nice review article in Conservation Biology:

When Good Animals Love Bad Habitats: Ecological Traps and the Conservation of Animal Populations
They expand the idea of a sink by dividing it into poor quality habitat that is preferentially avoided by the species (a sink) and poor quality habitat that is preferentially selected by the species (a trap). They also give a number of nice examples.

Perhaps the best-documented trap is the case of Cooper's Hawks (Accipiter cooperii) nesting in the city of Tucson, Arizona (Boal 1997; Boal & Mannan 1999). The hawks occur at much higher densities in the city than in exurban areas. Nesting begins earlier in the city, and clutches are larger than in outlying areas. The trap is sprung after the eggs hatch: nestling mortality is much higher in the city (>50%) than in exurban areas (<5%). The primary cause of mortality is trichomoniasis, a disease carried by pigeons and doves, which make up 84% of the diet of urban hawks. Based on demographic analysis, the urban population should experience significant declines (Boal 1997), but it appears to be stable or increasing, suggesting that birds are immigrating from outside the city.