Martin L. Cody

A general theory of clutch size

It is possible to think of organisms as having a certain limited amount of time or energy available for expenditure, and of natural selection as that force which oper- ates in the allocation of this time or energy in a way which maximizes the contribution of a genotype to following generations. This manner of treatment of problems con- cerning the adaptation of phenotypes is called the Principle of Allocation (Levins and MacArthur, unpublished), and one of its applications might be the formulation of a general theory to account for clutch size in birds. At this stage we will assume that clutch size is a hereditary phenotypic characteristic which can be affected to a greater or lesser extent by the prevailing environmental conditions and which ex- hibits the normal variability of such char- acteristics. Lack (1954) discusses the validity of several hypotheses which at- tempt to account for clutch size and its variation under different circumstances and conditions, all of which were rejected in favor of his now widely accepted theory that clutch size is adapted to a limited food supply. This paper is an attempt to show that this and other existing hypotheses when taken singly are inadequate in some respect to account for all the data, that each holds for some particular set of con- ditions, and that each is but a part of the complete explanation. The theories will be dealt with individually and it will be shown that as environment varies so will the fac- tors which determine clutch size. PRESENTATION OF THE THEORY

SHORT-TERM EVOLUTION OF REDUCED DISPERSAL IN ISLAND PLANT POPULATIONS

1 Dramatic reductions in dispersal potential are characteristic of many diverse taxa, both plants and animals, on oceanic islands. This paper documents the same trend of reduced dispersal ability over the course of just a few generations in some weedy, short-lived and wind-dispersed plants of inshore islands in British Columbia, Canada. 2 We measured dispersal-related morphological characteristics of diaspores from island populations of known ages, and from mainland populations. In two of three species with sufficiently large sample sizes, older island populations show increasingly reduced dispersal potential relative to mainland populations or to young island populations. 3 These and other morphological differences are consistent with results expected from strong selection for reduced dispersal potential, and may be striking examples of short-term evolution in small and isolated natural populations.

Competitive release in island Song Sparrow populations

Abstract Song Sparrow territory sizes at 30 mainland and island sites in the Pacific Northwest and one site in Wyoming were measured, and mean values were obtained for each site. Differences in territory sizes were found even though the food supply at each site was similar or varied independently. Song Sparrow territory size was also independent of vegetation structure. A direct correlation was found between Song Sparrow territory size and the number of co-occurring potential competitor species. As the number of competitors decreased, the Song Sparrow territory size decreased. The mechanism through which this decrease occurs is that of competitive release, as realized niche approaches fundamental niche with reduced competitor diversity. Theoretical predictions of Song Sparrow territory size utilizing various forms of α-values were made; results supported the conclusion that competitive release had occurred and that Song Sparrows had expanded ecologically into niches vacated by absent competitors.

Chilean Bird Distribution

The results of bird censuses from 13 Chilean habitats are presented. Any one habitat supports a slighly higher bird species diversity than structurally similar habitats in North America and Australia. The bird species turnover between habitats in the same locale, in contrast to other temperate areas, is very low–an island characteristic. Turnover between geographically distinct habitats within Chile is comparable to that between adjacent habitats in North America. Chilean birds exhibit wide habitat tolerance in narrow geographic areas, whcih is attributed to a slow development rate of this bird fauna. Character convergence, where similarities in appearance have evolved to faciliate interspecific aggression and aggregation, is a byproduct of this type of distribution. See full-text article at JSTOR

THE DISTRIBUTION OF CACTI ALONG ENVIRONMENTAL GRADIENTS IN THE SONORAN AND MOHAVE DESERTS

SUMMARY (1) The distribution of species of cacti along gradients of aspect and angle of slope is described and compared for a species-rich community in the Sonoran Desert and for a species-poor community in the Mohave Desert. (2) No differences were found between the two sites in the ecological ranges of cactus species along these gradients. (3) In the Sonoran Desert there were larger overlaps in the distribution of species along the aspect-gradient than in the Mohave Desert; however, no differences in overlap were found between the two areas along the gradient of angle of slope. (4) More species of cacti are packed into the Sonoran Desert study area, either by utilizing portions of the environmental gradients unavailable in the Mohave Desert, or by addition of species with different growth-form and maximum size. (5) The cylindropuntias (the group with the largest number of species in the two areas) illustrate the latter effect.

CHARACTER CONVERGENCE IN MEXICAN FINCHES

The natural history literature contains many striking examples of sympatric species more similar in appearance than common ancestry or chance warrant. Recent papers by Moynihan (1968) and Cody (1969) summarized the relevant literature, termed this phenomenon character convergence, and emphasized the mutual advantages to convergently similar species in gregarious and aggressive interactions respectively. In particular Cody (1969) suggested that convergence in appearance or voice may evolve as a response to selection for interspecific territoriality between ecologically similar species, as territories are defended by vocalization and/or visual displays. The examples to support this hypothesis were few, because in most apparent cases of convergence attributable to this phenomenon the necessary field observations on interspecific territorial interactions were lacking. Species showing an apparent convergence where their ranges overlapped became, ex hypothesi, good candidates for interspecific territoriality. It will be shown that, in one such instance at least, the speculation was justified.

Structural design principles of complex bird songs: a network-based approach.

Bird songs are acoustic communication signals primarily used in male-male aggression and in male-female attraction. These are often monotonous patterns composed of a few phrases, yet some birds have extremely complex songs with a large phrase repertoire, organized in non-random fashion with discernible patterns. Since structure is typically associated with function, the structures of complex bird songs provide important clues to the evolution of animal communication systems. Here we propose an efficient network-based approach to explore structural design principles of complex bird songs, in which the song networks–transition relationships among different phrases and the related structural measures–are employed. We demonstrate how this approach works with an example using California Thrasher songs, which are sequences of highly varied phrases delivered in succession over several minutes. These songs display two distinct features: a large phrase repertoire with a ‘small-world’ architecture, in which subsets of phrases are highly grouped and linked with a short average path length; and a balanced transition diversity amongst phrases, in which deterministic and non-deterministic transition patterns are moderately mixed. We explore the robustness of this approach with variations in sample size and the amount of noise. Our approach enables a more quantitative study of global and local structural properties of complex bird songs than has been possible to date.

POPULATIONS IN A SEASONAL ENVIRONMENT

Most organisms live in a seasonal environment. During their life cycles, some species face seasons of cold and heat, aridity and abundant rainfall, migration and stable residence, breeding and nonbreeding. Populations grow and decline as supplies of materials essential to their survival wax and wane. Such qualitative truths as these flow obviously from field observations. In this original monograph, Stephen Fretwell analyzes the highly complex interaction between a population and a regularly varying environment in an attempt to define and measure seasonality as a critical parameter in the general theory of population regulation. Concerned primarily with the size and the habitat distribution of populations, Professor Fretwell develops simple models that, when applied to specific populations, usually of birds, demonstrate the effect of seasonal variations on the regulation of populations. He maintains that seasonality, as a concept, is essential to a full understanding of environmental interaction. During the course of his exposition, the author offers several new hypotheses, including theories affecting the breeding, numbers, distribution, and diversity of wintering birds, and a theory affecting the body size of sparrows.

A sparse representation-based classifier for in-set bird phrase verification and classification with limited training data

The performance of a sparse representation-based (SR) classifier for in-set bird phrase verification and classification is studied. The database contains phrases segmented from songs of the Cassins Vireo (Vireo cassinii). Each test phrase belongs to one of 33 phrase classes - 32 in-set categories, and 1 collective out-of-set category. Only in-set phrases are used for training. From each phrase segment, spectrographic features were extracted, followed by dimension reduction using PCA. A threshold is applied on the sparsity concentration index (SCI) computed by the SR classifier, for in-set bird phrase verification using a limited number of training tokens (3 - 7) per phrase class. When evaluated against the nearest subspace (NS) and support vector machine (SVM) classifiers using the same framework, the SR classifier has the highest classification accuracy, due to its good performances in both the verification and classification tasks.

Structure, syntax and “small-world” organization in the complex songs of California Thrashers (Toxostoma redivivum)

Abstract We describe songs of the California Thrasher (Toxostoma redivivum), a territorial, monogamous species whose complex songs are composed of extended sequences of phonetically diverse phrases. We take a network approach, so that network nodes represent specific phrases, and links or transitions between nodes describe a subgroup structure that reveals the syntax of phrases within the songs. We found that individual birds have large and largely distinct repertoires, with limited phrase sharing between neighbours and repertoire similarity decaying between individuals with distance apart, decaying also over time within individuals. During song sequences, only a limited number of phrases (ca. 15–20) were found to be actually “in play” at any given time; these phrases can be grouped into themes within which transitions are much more common than among them, a feature contributing to a small-world structure. It appears that such “small-world themes” arise abruptly, while old themes are abandoned more gradually during extended song sequences; most individual thrashers switch among 3–4 themes over the course of several successive songs, and some small-world themes appear to have specific roles in starting or ending thrasher songs.