The world is urbanising rapidly, and it is predicted that by 2050, 66% of the global human population will be living in urban areas. Urbanisation is characterised by land-use changes such as increased residential housing, business development and transport infrastructure, resulting in habitat loss and fragmentation. Over the past two decades, interest has grown in how urbanisation influences fundamental aspects of avian biology such as life-history strategies, survival, breeding performance, behaviour and individual health. Here, we review current knowledge on how urbanisation influences the nesting biology of birds, which determines important fitness-associated processes such as nest predation and community assembly. We identify three major research areas: (i) nest sites of birds in urban areas, (ii) the composition of their nests, and (iii) how these aspects of their nesting biology influence their persistence (and therefore conservation efforts) in urban areas. We show that birds inhabiting urban areas nest in a wide variety of locations, some beneficial through exploitation of otherwise relatively empty avian ecological niches, but others detrimental when birds breed in ecological traps. We describe urban-associated changes in nesting materials such as plastic and cigarette butts, and discuss several functional hypotheses that propose the adaptive value and potential costs of this new nesting strategy. Urban areas provide a relatively new habitat in which to conserve birds, and we show that nestboxes and other artificial nest sites can be used successfully to conserve some, but not all, bird species. Finally, we identify those subject areas that warrant further research attention in the hope of advancing our understanding of the nesting biology of birds in urban areas.

Intentional feeding of wild birds in gardens or backyards is one of the most popular forms of human-wildlife interactions in the developed world, especially in urban environments. Scale and intensity of bird feeding are enormous with mainly birdseed consumed daily by a range of species. This represents a subsidy to natural diets of birds attracted to the feeders and typically involves novel dietary components. Yet, relatively little is known about how it influences the behavior and ecology of the species visiting feeders. In part, research has been hampered by logistical difficulties of working in urban areas but studies have demonstrated powerful influences on behavior and phenology of avian breeding, the spread of disease, and the structure of avian communities. Here, we compare bird feeding between Northern and Southern Hemispheres as a means of exploring how similarities and differences in avian responses might inform knowledge of this global urban phenomenon. We start by tracing its origins to north-western Europe and how its expansion has occurred before considering how geographical differences in feeding practices and attitudes map onto bird feeding ‘on the ground’. We explore some of the major emerging themes of recent interest, including why citizens are motivated to feed birds, whether birds become fully dependent on food supplements, the role of feeding in avian disease transmission, and how feeding changes urban bird communities. By proposing that scientists work in collaboration with the public providing food to birds, we pose key research questions that need to be answered urgently and suggest accompanying experimental approaches to do so. These approaches are essential if we are to improve our understanding of how bird feeding shapes the behavior, ecology, movements, and community structure of urban birds. Our hope is that through such citizen science we will be able to provide advice as to location-relevant practices that should maximise benefits to both urban biodiversity and human wellbeing, and minimise potential adverse impacts. We demonstrate that bird feeding is important for urban biodiversity conservation, community engagement, and in establishing personal connections with nature and their associated benefits.

Calcium is an essential nutrient for avian reproduction. Calcium‐rich foods are consumed by breeding birds for production of eggshells and for provisioning chicks that are mineralizing skeletal tissues. A number of studies have documented calcium‐limited reproduction, and calcium supplementation has been employed over the last decade to demonstrate degrees, causes and consequences of calcium limitation. However, supplementation studies have produced equivocal findings resulting from an absence of calcium limitation in the study species, a poorly designed supplementation procedure or both. Prior to effective calcium supplementation, many factors need to be considered. Calcium‐limited breeding in birds can only be detected by monitoring breeding attempts for more than one year and by ensuring that the measured breeding parameters are sensitive to calcium availability. Natural calcium availability needs to be estimated, and daily calcium budgets for the appropriate reproductive stages determined for the study species. Most crucially, if calcium limitation of breeding is caused by secondary calcium limitation (e.g. through heavy metal toxicity), calcium supplementation will probably be ineffective. Effective calcium supplementation will then be achieved through careful planning – a study over several years using appropriate supplements (i.e. naturally occurring ones used by breeding birds), applied at the appropriate time of year (i.e. prelaying and/or chick‐rearing phases) and using a response variable that is highly sensitive to calcium availability. If properly planned and performed, calcium supplementation is a cost‐effective and potent tool for the study of bird breeding biology.