In order to successfully colonise the urban environment, birds must be able tolerate pedestrian and vehicle proximity, because exhibiting a strong fleeing response each time that a harmless traffic stimulus is encountered would be maladaptive. We examined whether a native 'urban adapter', the Australian magpie (Cracticus tibicen), varied in its tolerance of human approach and a simulated vehicular traffic sound in urban Melbourne, Australia as a function of pedestrian or vehicular traffic volume in the local area. The flight initiation distance (FID) paradigm and presentation of a potentially startling sound stimulus (SSS) were used to measure the magpies' boldness in areas with differing traffic volumes. Magpies in areas with relatively high pedestrian traffic volumes exhibited a shorter FID to, and retreated a shorter distance from, an approaching human than conspecifics experiencing relatively low volumes of pedestrian traffic. Magpies in areas with a relatively high vehicular traffic volume were less likely to flee from a SSS than conspecifics in areas with little or no vehicular traffic. Habituation to human disturbance and/or adjustment of anti-predator effort seem the most likely explanations for these disparities and suggest that magpies have considerable behavioural flexibility. The ability to tolerate high volumes of traffic and the associated noise allows magpies to use: (a) ecologically suitable habitat adjoining city roads with heavy vehicular traffic and (b) urban areas with many pedestrians and consequently much human food waste, which the birds exploit.

Nest predation has a large impact on reproductive success in many taxa. Defending offspring from would-be predators can also be energetically and physiologically costly for parents. Thus, to maximize their reproductive payoffs, individuals should adjust their reproductive behaviors in relation to the presence of nest predators. However, effects of nest predator presence on parental behaviors across multiple reproductive contexts remain poorly understood, particularly in non-avian taxa. We ran a series of experiments to test how the presence of an egg predator, the invasive rockpool shrimp, Palaemon elegans, influences male reproductive decisions and egg survival in a species of fish with exclusive paternal care, the three-spined stickleback, Gasterosteus aculeatus. We found that, in the presence of shrimp, male sticklebacks were less likely to build a nest, invested less in territory defense against an intruder, and tended to fan eggs in their nest less and in shorter bouts, but did not alter their investment in courtship behavior. The predator’s presence also did not affect egg survival rates, suggesting that males effectively defended their brood from the shrimp. These results show that reproducing individuals can be highly responsive to the presence of nest predators and adjust their behavioral decisions accordingly across a suite of reproductive contexts.

It is widely assumed that caring for young limits the motivation of parents to seek additional mating opportunities. However, in situations where parental care does not involve direct provisioning of the offspring, but rather activities directed at the brood as a whole (e.g. guarding), it may be more efficient for parents to care for large numbers of young at once. This may be especially true for species with exclusive paternal care, with fathers that have recently acquired a brood of young potentially benefitting from vigorously courting prospective mates, so as to maximise their chances of attaining a large number of young to rear together. We experimentally tested this hypothesis in the three‐spined stickleback (Gasterosteus aculeatus), a fish with male only care. Contrary to our predictions, we found no evidence of any differences in courtship between recently spawned egg‐tending fathers and males that had not spawned. However, males that were permitted to spawn, but then had their eggs taken from them, courted less vigorously. Together, the results of our study suggest that the potential benefits of vigorous courtship in terms of acquiring additional young may be offset by additional costs faced by parental males.