Gordon H. Orians

On the Evolution of Mating Systems in Birds and Mammals

Predictions from a theory assuming mate selection on the part of females, which maximizes reproductive success of individuals, are found to accord closely, though not completely, with known mating patterns. These predictions are that (1) polyandry should be rare, (2) polygyny should be more common among mammals than among birds, (3) polygyny should be more prevalent among precocial than among altricial birds, (4) conditions for polygyny should be met in marshes more regularly than among terrestrial environments, (5) polygyny should be more prevalent among species of early successional habitats, (6) polygyny should be more prevalent among species in which feeding areas are widespread but nesting sites are restricted, and (7) polygyny should evolve more readily among species in which clutch size is strongly influenced by factors other than the ability of the adults to provide food for the young. Most cases of polygyny in birds, a group in which monogamy is the most common mating pattern, can be explained on the basis of the model, and those cases not apparently fitting into the predictions are clearly indicated. Thus, there is no need at present to invoke more complicated and restrictive mechanisms to explain the mating patterns known to exist.

Spatial and Temporal Scales in Habitat Selection

Female yellow-headed blackbirds in eastern Washington State settle to nest at higher densities on marshes with higher emergence rates of odonates, the most important prey delivered to nestlings. However, settling densities of females were not correlated with odonate emergence rates on individual territories or on individual territories plus adjacent ones. Apparently, females assessed production of insects on breeding marshes at the time they settled, and they used this information when making settling decisions. However, they selected nest sites on the basis of vegetation density rather than food availability. The complexity of decision making by female yellowheads would not have been detected had our analysis been restricted to one spatial scale. Because interpretations of habitat selection behavior are scale-dependent, careful attention to scale and performing analyses on more than one spatial scale are essential in studies of habitat selection.

Sucessional Status and the Palatability of Plants to Generalized Herbivores

Theoretical considerations and evidence from the literature suggest that early successional plant species should make a lesser commitment of resources to defense against herbivores, and should then provide better food sources for generalized herbivores than later successional and climax plants. Commitment to defense by plants is estimated by determining short-term palatabilities to two slug species; one native to western Washington, Ariolimax columbianus, and one introduced from Europe, Arion ater. Results of tests with 100 plant species of three growth forms and from different seral stages indicated that early successional annuals were significantly more palatable than early successional perennials which were sig- nificantly more palatable than later successional species. No correlation was found between palatability and evolutionary association of the herbivores with the plant species.

Interspecific Territories of Birds

Territories of birds, usually defended against conspecific individuals, are sometimes defended against individuals of other species. Since such behavior is demanding both of time and energy, natural selection should favor ecological should favor ecological divergence, the establishment of overlapping territories, and the reduction of aggression. Lack of divergence in modes of exploitation could mean that insufficient time has elapsed for the changes to be completed or that the environment imposes some limitation preventing the evolution of the required degree of divergence. Such environmental limitation can be predicted in (a) structurally simple environments, (b) when feeding sites are strongly stratified in structurally complex vegetation, or (c) when the presence of other species in the environment prevents divergence in certain directions. The known cases of interspecific territoriality in birds are analyzed and shown to be largely in accordance with these predictions, although several cases of overlapping territories in situations where interspecific territoriality has been predicted provide relationships worthy of further study. We suggest that Darwinian selection at the level of the individual permits an understanding of the known structure of avian communities and that there is no need at present to invoke new selective mechanisms at the level of the community or ecosystem.

A COST-INCOME MODEL OF LEAVES AND ROOTS WITH SPECIAL REFERENCE TO ARID AND SEMIARID AREAS

The inevitable close association between water loss and entrance of carbon dioxide through stomates, together with cell-morphological and physiological traits that affect these exchanges, prevent plants capable of high rates of photosynthesis per unit leaf surface area when the soil is moist from being able to extract moisture from dry soils, and vice versa. Hypothetical photosynthesis curves for leaves of mesophytic and xerophytic plants are inferred from these associations and shown to be similar to those obtained from desert shrubs. Because they are cheaper to build and maintain per unit surface area, mesophytic leaves are amortized quickly and yield profits at faster rates than xerophytic leaves, provided soil water potential is low. Which type of leaf is the most advantageous to a plant depends on the length of time during the year that soil moisture is high and on the extent of periods of high soil water potential during otherwise wet periods that necessitate either closing stomates or dropping leaves. Costs of root construction and maintenance are poorly known, but it is probably more expensive to maintain roots under conditions of high soil and plant water potential. The optimal root/shoot ratio of a plant with mesophytic leaves should be close to that providing sufficient moisture for maximum photosynthesis during the wet season, while plants with xerophytic leaves should have an R/S ratio well below the optimal for maximizing photosynthesis. The model provides a reasonable explanation for the large numbers of desert plants with deciduous, mesophytic leaves and for the great variations reported in R/S ratios in desert plants. It also can explain why dry desert flats are dominated by evergreen sclerophyllous plants, while closer to the washes there is a zone of drought-deciduous perennials, replaced again by evergreens in the washes.

Diversity, stability and maturity in natural ecosystems

The belief that natural ecosystems become more diverse and, hence, more stable with time after a disturbance is widely accepted and regularly repeated in ecology textbooks (Clements & Shelford, 1939; Colinvaux, 1973; Collier et al., 1973; Odum, 1953). There are suggestions on empirical and theoretical grounds of quantitative relationships between diversity and some measure of stability (Hairston et al., 1968; Hurd et al., 1971; Goel et al., 1971; Leigh, 1965; May, 1972, 1973b; Murdoch et al., 1972; Paine, 1969; Patten, 1963; Pimentel, 1961; Volterra, 1937; Watt, 1964) but the correlations, not to mention causations, are still obscure. In any case, the popularity of the notions that succession generates diversity and that diversity enhances stability predates empirical or theoretical justification. Also, the concepts are normally discussed with poorly defined terms, reflecting an uncertainty about what concept(s) of stability are useful in ecology and, even more important, what we wish to understand about natural ecosystems.

EVOLUTION OF BROOD PARASITISM IN ALTRICIAL BIRDS

habits but also why certain close relatives have not. Brood parasitism has evolved independently a number of times in birds. Best known are the cuckoos, with a complex of Old World species (and three less specialized New World species) highly specialized in the parasitic habit (Friedmann, 1933, 1948). Also, all the African honey guides (Indicatoridae) whose breeding biology is known are brood parasites (Friedmann, 1955). Among passerines brood parasitism has evolved independently in African weaver birds (Ploceidae) and in several species of blackbirds (Icteridae). The only obligate parasite in precocial species is the South American Black-headed Duck (Heteronetta atricapilla), but the North American Redhead (Aythya americana) has populations at various stages of parasitism, from completely independent to complete brood parasitism (Weller, 1959). The independent evolution of brood parasitism in these diverse phyletic lines is clear. But brood parasitism has not necessarily evolved only once in each group. The polyphyletic origin of such a specialized habit within a group might seem quite improbable, but the conditions which might lead to the development of such a habit sug

Age and hunting success in the brown pelican (Pelecanus occidentalis)

Abstract Adult brown pelicans (Pelecanus occidentalis) capture prey on a larger percentage of their dives than immatures hunting under the same conditions. This difference provides at least a partial explanation of delayed maturation and smaller clutch sizes of younger birds in species using skilled foraging techniques.

Internal heterogeneity of gaps and species richness in Costa Rican tropical wet forest

In Costa Rican tropical wet forest the distribution of seedlings of most tree species colonizing 51 tree-fall gaps is clumped, probably because of differences in the proximity of gaps to fruiting trees. Different gap zones (root, bole and crown) are more similar to one another, in terms of their species composition, than they are to other zones in the same gap. This nonrandom pattern is established soon after gap formation, indicating that mortality of young seedlings is both high and related to gap zones. Some tree species are strongly positively associated with one of the gap zones, whereas others are associated with the age of the gap at the time the census was taken or the species of tree whose fall caused the gap. Root zones are dominated by fewer species than are bole or crown zones. Results are consistent with the assumption that an initial random distribution of seedlings is quickly changed to a strongly non-random pattern by selective mortality of seedlings of different tree species in the different gap zones. The internal heterogeneity of gaps is probably one of the factors helping to maintain the high tree species richness characteristic of tropical wet forests.

Territoriality among male red-winged blackbirds

SummaryWe tested two hypotheses to explain territorial dominance in male birds. Male red-winged blackbirds were removed from their territories for 7 d and then released after replacement owners had held their territories 2 to 7 d. Original owners regained territories from short-term replacements, but could not defeat 6 to 7 d replacements. This outcome suggests that replacement males relinquished their territories to persistent original owners after 2 to 3 d of ownership because the territory lacked sufficient value to them, but not after 7 d, when its value was greater. This result supports the Value Asymmetry Hypothesis of territorial dominance and provides strong evidence in birds that differences in the extent of knowledge of or investment in an area and, hence, willingness to escalate contests, contribute to territorial dominance.