Abstract. Food limitation is generally thought to underlie much of the variation in life history traits of birds. I examined variation and covariation of life history traits of 123 North American Passeriformes and Piciformes in relation to nest sites, nest predation, and foraging sites to examine the possible roles of these ecological factors in life history evolution of birds. Annual fecundity was strongly inversely related to adult survival, even when phylogenetic effects were controlled. Only a little of the variation in fecundity and survival was related to foraging sites, whereas these traits varied strongly among nest sites. Interspecific differences in nest predation were correlated with much of the variation in life history traits among nest sites, although energy trade-offs with covarying traits also may account for some variation. For example, increased nest predation is associated with a shortened nestling period and both are associated with more broods per year, but number of broods is inversely correlated with clutch size, possibly due to an energy trade-off. Number of broods was much more strongly correlated with annual fecundity and adult survival among species than was clutch size, suggesting that clutch size may not be the primary fecundity trait on which selection is acting. Ultimately, food limitation may cause trade-offs between annual fecundity and adult survival, but differences among species in fecundity and adult survival may not be explained by differences in food abundance and instead represent differing tactics for partitioning similar levels of food limitation. Variation in fecundity and adult survival is more clearly organized by nest sites and more closely correlated with nest predation; species that use nest sites with greater nest predation have shorter nestling periods and more broods, yielding higher fecundity, which in turn is associated with reduced adult survival.Fecundity also varied with migratory tendencies; short-distance migrants had more broods and greater fecundity than did neotropical migrants and residents using similar nest sites. However, migratory tendencies and habitat use were confounded, making separation of these two effects difficult. Nonetheless, the conventional view that neotropical migrants have fewer broods than residents was not supported when nest site effects were controlled.

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Life history theory predicts that parents should value their own survival over that of their offspring in species with a higher probability of adult survival and fewer offspring. We report that Southern Hemisphere birds have higher adult survival and smaller clutch sizes than Northern Hemisphere birds. We subsequently manipulated predation risk to adults versus offspring in 10 species that were paired between North and South America on the basis of phylogeny and ecology. As predicted, southern parents responded more strongly to reduce mortality risk to themselves even at a cost to their offspring, whereas northern parents responded more strongly to reduce risk to their offspring even at greater risk to themselves.

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Avian life history theory has long assumed that nest predation plays a minor role in shaping reproductive strategies. Yet, this assumption remains conspicuously untested by broad experiments that alter environmental risk of nest predation, despite the fact that nest predation is a major source of reproductive failure. Here, we examined whether parents can assess experimentally reduced nest predation risk and alter their reproductive strategies. We experimentally reduced nest predation risk and show that in safer environments parents increased investment in young through increased egg size, clutch mass, and the rate they fed nestlings. Parents also increased investment in female condition by increasing the rates that males fed incubating females at the nest, and decreasing the time that females spent incubating. These results demonstrate that birds can assess nest predation risk at large and that nest predation plays a key role in the expression of avian reproductive strategies.

Viewing life-history evolution in birds based on an age-specific mortality framework can explain broad life-history patterns, including the long incubation periods in southern latitudes documented here. I show that incubation periods of species that are matched phylogenetically and ecologically between Argentina and Arizona are longer in Argentina. Long incubation periods have mystified scientists because they increase the accumulated risk of time-dependent mortality to young without providing a clear benefit. I hypothesize that parents of species with low adult mortality accept increased risk of mortality to their young from longer incubation if this allows reduced risk of mortality to themselves. During incubation, songbird parents can reduce risk of mortality to themselves by reducing nest attentiveness (percentage of time on the nest). Here I show that parents of species with lower adult mortality exhibit reduced nest attentiveness and that lower attentiveness is associated with longer incubation periods. However, the incubation period is also modified by juvenile mortality. Clutch size variation is also strongly correlated with age-specific mortality. Ultimately, adult and juvenile mortality explain variation in incubation and other life-history traits better than the historical paradigm.

Greater nest predation rates on ground-nesting birds than on off-ground-nesting birds have long been assumed and used as an explanation for patterns such as greater cryptic and monomorphic coloration of ground-nesting birds and for area sensitivity and population decline of many Neotropical migrant species. I use three independent data sets to show that this assumption is not true in forest habitats, where nest predation is instead least on ground-nesting birds. Larger clutch sizes and longer nestling periods of ground-nesting species in forest habitats are indirect evidence that ground-nesting species in forest habitats have suffered lower nest predation over evolutionary time. In contrast, ground-nesting birds seem to suffer greater predation than off-ground-nesting species in shrub and grassland habitats, but evaluation of predation is complicated by habitat disturbance in many studies. Nesting mortality in general appears to be greater in shrub and grassland habitats, and species in these habitats are showing some of the most consistent long-term population declines. Additional examination of nesting mortality of coexisting species in various ecological conditions is needed to uncover patterns that may influence evolution of life-history traits and population demographies.

Audubon' s Hermit Thrushes (Cutharus guttatus auduboni) in central Arizona have a low nesting success (7 to 20%) due almost exclusively to nest predation. We examine the sites chosen for nesting and compare them to nonuse sites randomly selected within the vegetation types associated with nests. Hermit Thrush nest sites differ from nonuse sites primarily in that nest sites have more small (l-to 3-m tall) white firs (Abies concolor) in the patch (5-m radius circle) surrounding the nest. Hermit Thrushes nest almost exclusively in small white firs and they do not forage in or near them. Hermit Thrushes may select nest sites that have a large number of other potential nest sites (i.e., small white firs) near the nest because predation risk is thereby reduced. Indeed, nests with a high probability of predation were surrounded by a lower density of small white firs than more successful nests. However, low predation nests also were more concealed than high predation nests. Nestsite selection appears to be a function of characteristics in the immediate vicinity of the nest (concealment, overhead cover, nest orientation), but also on a larger scale surrounding the nest. Consideration ofnest-site selection on this larger scale may cast light on the question of whether nest sites limit territory and habitat selection by birds.