parks now actively discourage animal feeding. In Canadian national parks, it is illegal to disturb or feed wildlife, and Parks Canada advises visitors not to leave out "food attractants" such as dirty dishes. Ironically, the "it is unlawful to feed animals" signs may themselves become food attractants for porcupines. Road salt and roadkill may also act as food attractants, and removing roadkill is considered good park management. Marine parks Tourism operators often provide food to attract marine wildlife such as sharks to areas where they can be more easily viewed. Such a practice is controversial, however, because it can create a dependency on artificial feeding, habituate animals to feeding locations, increase inter-species and intra-species aggression, and increase the spread of disease. In Australia's Great Barrier Reef Marine Park, shark feeding is prohibited. In Hawaiian waters, shark feeding is permitted only in connection with traditional Hawaiian cultural or religious activities. The feeding of wild dolphins for tourist purposes is also controversial, and is prohibited in the US because it can alter natural hunting behaviour, disrupt social interaction, encourage the dolphins to approach or ingest dangerous objects, and endanger the person doing the feeding. At Monkey Mia in Western Australia, dolphin feeding is permitted under Department of Environment and Conservation supervision. Backyards Similar issues to those in national and state parks also apply in suburban and rural backyards. Artificial feeding of coyotes, deer, and other wildlife is discouraged. Feeding deer, for example, may contribute to the spread of bovine tuberculosis. The feeding of birds with bird feeders is an exception, at least in the US, even though it can sometimes contribute to spreading disease. In Australia, artificial bird feeding is viewed more negatively. Instead, growing native plants that can act as a natural food source for birds is recommended. Similar suggestions have been made in the US. Public spaces Feral pigeons are often found in urban public spaces. They are often considered environmental pests, and can transmit diseases such as psittacosis. Deliberate feeding of feral pigeons, though popular, contributes to these problems. Ducks are also commonly fed in public spaces. In an early 1970s US study, 67% of people visiting urban parks did so to feed ducks. However, such feeding may contribute to water pollution and to over-population of the birds, as well as delaying winter migration to an extent that may be dangerous for the birds. Feeding foods such as white