Thomas E. Martin

Avian Life-History Evolution in Relation to Nest Sites, Nest Predation, and Food

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 survivaI, 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 tecundity 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

Nest predation and nest-site selection of a western population of the hermit thrush.

Audubons Hermit Thrushes (Catharus 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 (1to 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 of nest-site selection on this larger scale may cast light on the question of whether nest sites limit territory and habitat selection by birds.

Nest predation among vegetation layers and habitat types: revising the dogmas.

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.

Nest predation increases with parental activity: separating nest site and parental activity effects.

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 effects 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 effects 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 effects 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 influences 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.

ARE MICROHABITAT PREFERENCES OF COEXISTING SPECIES UNDER SELECTION AND ADAPTIVE

Microhabitat preferences are assumed to be adaptive, suggesting that fitness is higher in preferred microhabitats and potentially reflecting natural selection on habitat choices. I examined microhabitat preferences and adaptiveness of preferences for seven bird species coexisting in high elevation snowmelt drainages based on study of microhabitat and survival of 1556 nests. Habitat features in the nest patch differed from both random and non-use (sites centered on the same plant species as used for the nest) sites within each bird species, indicating nonrandom nest site choice. Bird species within a nesting guild (ground, shrub) also differed from each other based on the same vegetation features that differentiated nest sites from non-use and random sites, and this vegetation feature dominated the microhabitat type that was used most frequently by each species. In short, frequency of use of dominant vegetation types, comparisons of nest vs. random and non-use sites, and comparisons among species were concordant in their indications of microhabitat preferences. The frequency in use of microhabitats was taken as an unambiguous measure of microhabitat preference within this study system: vegetation varied along a short microclimate gradient in the study system and territories of birds encompassed the entire gradient, thereby making all microhabitats available within the territory of each individual, such that use reflected a clear choice among alternatives. Microhabitat preferences differed among species and reflected differing positions on the microclimate gradient. Thus, species partitioned either microhabitat or microclimate within each nesting guild. Nest success was greater at preferred than at nonpreferred microhabitats for all seven species, indicating that preferences were adaptive. Examination of cubic spline curves and standardized directional selection differentials (s) and selection gradients (β) indicated that preferences had positive directional selection coefficients. These selection coefficients suggested that selection might be acting to favor preferences, but information on genetic bases of habitat choices is needed before selection can be ascertained. Advances in understanding evolution of habitat preferences depend on an individual-level examination of habitat choices and their fitness consequences, and also examination of the phenotypic traits and mechanisms that underlie habitat-induced variation in fitness components.

Life history evolution in tropical and south temperate birds : What do we really know ?

ity in driving life history evolution (e.g., Cole 1954, Murphy 1968, Charlesworth 1980, Reznick and Bryga 1987, Curio 1989, Reznick et al. 1990). Yet, food is thought to be more important in birds (Lack 1948, 1968, Murphy and Haukioja 1986, Martin 1987). Nest predation is a theoretical alternative to food limitation (Slagsvold 1982, Lima 1987, Martin 1992) and recent evidence suggests that nest predation may exert a greater influence on life history evolution than previously thought (Slagsvold 1982, Martin 1993a, b, 1995, Martin and Clobert 1996, McCleery et al. 1996). However, nest predation may often interact with food limitation to influence life history traits (Lima 1987, Martin 1992, 1995). These contrasting and interacting roles of food limitation versus mortality (nest predation) make birds an intriguing system for examining ecological and evolutionary causes of life history variation.

Interaction of Nest Predation and Food Limitation in Reproductive Strategies

Heritable reproductive strategies that yield the greatest lifetime production of surviving offspring are favored by natural selection. I consider a reproductive strategy to be the complement of behaviors and traits associated with reproduction that influence lifetime reproductive success. Trade-offs among standard life history traits (e.g., clutch size, growth rate, juvenile and adult mortality) influence success of reproductive strategies (see reviews by Stearns, 1976; Martin, 1987). In addition, energy allocation between reproduction and other activities can influence expression of traits such as clutch size (Cody 1966). However, for organisms that care for their young, success of traits is also strongly affected by the amount of energy and time allocated to care of young by parents (Trivers, 1972, 1974; Low, 1978; Clutton-Brock, 1984). In other words, allocation of energy and time among different activities within reproduction can be as important as allocation between reproduction and other activities.

Life History Traits of Open- vs. Cavity-Nesting Birds

We re-examined the longstanding dogma that cavity-nesting birds have larg- er clutch sizes than open-nesting species because of lower nest predation rates, which allow longer developmental periods. We provide data on nesting success of open-nesting species plus excavator and nonexcavator species (cavity-nesters that do vs. do not create their own cavities) of birds that coexist in the same habitat and use natural nest sites; studies were conducted in central Arizona in high-elevation forest drainages from 1987 to 1989. In comparisons among species, nest success increased in the order open-nesting < nonex- cavator < excavator species. Length of the nestling period increased, whereas number of broods decreased, with increased nest success across these nest types. Contrary to conven- tional expectation, clutch size did not increase directly with increased nest success and length of the nestling period, and these patterns were not simply a result of phylogenetic effects. A literature review of data on adult survival in land birds indicated that nonexcavators have significantly lower adult survival than the other two nest types. The lower adult survival of nonexcavators was associated with greater annual productivity than for the other two nest types. Annual productivity appeared to be associated with nest site attributes, whereas number of brood attempts was related to nest failure rate. Clutch size was a byproduct of annual productivity and number of brood attempts. Thus, the long-standing paradigm relating clutch size directly to nesting failure and developmental period was not supported.

BREEDING PRODUCTIVITY DOES NOT DECLINE WITH INCREASING FRAGMENTATION IN A WESTERN LANDSCAPE

Fragmentation of breeding habitat may cause declines in many bird populations. Our perception of the demographic effects of habitat fragmentation comes primarily from studies in the midwestern and eastern United States and Scandinavia. We know very little about the demographic effects of anthropogenically caused habitat fragmentation in habitats prone to natural disturbance, as is typical of most forest types in the western United States. We located and monitored 1916 nests on eight sites located in mostly forested landscapes and eight sites located in primarily agricultural landscapes to study the effects of landscape-level fragmentation on nest predation and brood parasitism in riparian areas in western Montana. Patterns of nest predation were opposite those documented from more eastern locales; predation rates were higher in forested landscapes than in fragmented landscapes dominated by agriculture. This pattern probably reflects the importance of forest predators in these landscapes: red squirrels (Ta...

Mortality costs of sexual dimorphism in birds

Sexually selected traits provide a mating advantage to the bearer but they should also exact a cost through natural selection. Whereas the mating benefits from such traits have been well documented, the costs have been difficult to demonstrate. In this analysis of mortality patterns across 28 North American passerine bird species, we show that sex-biased mortality (log10 male mortality – log10 female mortality) is positively correlated with both sexual size dimorphism and male plumage brightness. Male (but not female) mortality is positively correlated with sexual size dimorphism, suggesting a cost to male–male competition. Female (but not male) mortality is negatively correlated with male brightness, and we argue from this that the evolution of male brightness has been constrained by mortality costs. Thus sexual dimorphism in body size and plumage colour within bird species appears to be influenced by the opposing forces of sexual selection, acting to increase dimorphism, and adult mortality rates, which constrain the evolution of these traits. Differences in the expression of ornamental traits across species may be explained not only by variation in the mating benefits that accrue from ornaments, as is so often assumed, but also by variation in the costs of these traits.