No abstract

Alexander Skutch hypothesized that increased parental activity can increase the risk of nest predation. We tested this hypothesis using ten open-nesting bird species in Arizona, USA. Parental activity was greater during the nestling than incubation stage because parents visited the nest frequently to feed their young during the nestling stage. However, nest predation did not generally increase with parental activity between nesting stages across the ten study species. Previous investigators have found similar results. We tested whether nest site e¡ects might yield higher predation during incubation because the most obvious sites are depredated most rapidly. We conducted experiments using nest sites from the previous year to remove parental activity. Our results showed that nest sites have highly repeatable e¡ects on nest predation risk; poor nest sites incurred rapid predation and caused predation rates to be greater during the incubation than nestling stage. This pattern also was exhibited in a bird species with similar (i.e. controlled) parental activity between nesting stages. Once nest site e¡ects are taken into account, nest predation shows a strong proximate increase with parental activity during the nestling stage within and across species. Parental activity and nest sites exert antagonistic in£uences on current estimates of nest predation between nesting stages and both must be considered in order to understand current patterns of nest predation, which is an important source of natural selection.

No abstract

The evolutionary causes of small clutch sizes in tropical and Southern Hemisphere regions are poorly understood. Alexander Skutch proposed 50 years ago that higher nest predation in the south constrains the rate at which parent birds can deliver food to young and thereby constrains clutch size by limiting the number of young that parents can feed. This hypothesis for explaining differences in clutch size and parental behaviors between latitudes has remained untested. Here, a detailed study of bird species in Arizona and Argentina shows that Skutch's hypothesis explains clutch size variation within North and South America. However, neither Skutch's hypothesis nor two major alternatives explain differences between latitudes.

No abstract

Abstract.— The reasons why growth and developmental rates vary widely among species have remained unclear. Previous examinations of possible environmental influences on growth rates of birds yielded few correlations, leading to suggestions that young may be growing at maximum rates allowed within physiological constraints. However, estimations of growth rates can be confounded by variation in relative developmental stage at fledging. Here, we re‐estimate growth rates to control for developmental stage. We used these data to examine the potential covariation of growth and development with environmental variation across a sample of 115 North American passerines. Contrary to previous results, we found that growth rates of altricial nestlings were strongly positively correlated to daily nest predation rates, even after controlling for adult body mass and phylogeny. In addition, nestlings of species under stronger predation pressure remained in the nest for a shorter period, and they left the nest at lower body mass relative to adult body mass. Thus, nestlings both grew faster and left the nest at an earlier developmental stage in species with higher risk of predation. Growth patterns were also related to food, clutch size, and latitude. These results support a view that growth and developmental rates of altricial nestlings are strongly influenced by the environmental conditions experienced by species, and they generally lend support to an adaptive view of interspecific variation in growth and developmental rates.

No abstract

.— Incubation behavior is one component of reproductive effort and thus influences the evolution of life‐history strategies. We examined the relative importance of body mass, frequency of mate feeding, food, nest predation, and ambient temperature to explain interspecific variation in incubation behavior (nest attentiveness, on‐ and off‐bout durations, and nest trips per hour) using comparative analyses for North American passerines in which only females incubate. Body mass and frequency of mate feeding explained little variation in incubation behavior. We were also unable to detect any influence of food; diet and foraging strategy explained little interspecific variation in incubation behavior. However, the typical temperature encountered during reproduction explained significant variation in incubation behavior: Species breeding in colder environments take shorter bouts off the nest, which prevents eggs from cooling to temperatures below the physiological zero temperature. These species must compensate for shorter off‐bouts by taking more of them (thus shorter on‐bouts) to obtain needed energy for incubation. Nest predation also explains significant variation in incubation behavior among passerines: Species that endure high nest predation have evolved an incubation strategy (long on‐ and off‐bouts) that minimizes activity that could attract predators. Nest substrate explained additional variation in incubation behavior (cavity‐nesting birds have shorter on‐bouts and make more frequent nest trips), presumably because nest predation and/or temperature varies among nest substrates. Thus, nest predation can influence reproductive effort in a way previously not demonstrated–by placing a constraint on parental activity at the nest. Incubating birds face an ecological cost associated with reproductive effort (predation of entire brood) that should be considered in future attempts to explain avian life‐history evolution.