Ground-nesting seabirds are vulnerable to human intrusion which can potentially impair their breeding success. We examined whether tourist traffic on a boardwalk through a Silver Gull Larus novaehollandiae breeding colony affected the birds' behaviour sufficiently to potentially decrease reproductive success. Nest density was positively correlated with distance from the boardwalk. Gulls nesting ≤5 m from the boardwalk were more vigilant towards a stationary investigator than those further away. Tourists walking past breeding gulls stimulated a large increase in intra-specific aggression; stopping alongside parents with mobile chicks increased their intra-specific aggression even more. Larger tourist groups (≥3) stimulated more intra-specific aggression than smaller ones. An investigator walking past breeding gulls similarly stimulated an increase in intra-specific aggression, but the investigator's clothing colour, noise emission level and walking speed had no effect on the birds. An ambulatory investigator in another colony infrequently visited by people had a similar effect on gulls' intra-specific aggression, but these birds generally reacted more strongly to human intrusion. Silver Gulls' behavioural responses to tourist traffic could potentially decrease their reproductive success. These responses could be reduced by regulating traffic volume and flow and tourists' group size and behaviour, but the species' public image currently probably mitigates against such regulation occurring.

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