The dynamics of the organization of a community of six species of migrant predatory shorebirds (Least Sandpiper, Semipalmated Sandpiper, Dunlin, Short—billed Dowitcher, Lesser Yellowlegs, and Semipalmated Plover) was appraised by studying foraging behavior and habitat utilization under winter conditions in southern Florida and under summer conditions in the eastern Canadian Arctic. Eight foraging methods, defined primarily on the basis of how the bill is used and the pattern of locomotion, constitute the behavioral repertoire of the species. Each foraging method is correlated with a particular rate of locomotion (distance/time) and rate of feeding (pecking or probing/time). Feeding and locomotion rates seem independent of air temperature, number of conspecifics, and total number of shorebirds foraging nearby. Instead, the seasonal changes in these rates are probably related to food density. On the basis of these findings and of differences in rates of feeding and locomotion between species the most reasonable hypothesis is that food density is higher in summer on the breeding grounds and foraging is more selective. Each foraging method is combination with a particular microhabitat defines a statistically different resource. During the winter, on intertidal habitats of southern Florida, shorebirds on the whole exhibit a low behavioral and microhabitat diversity and low resource overlap between species. The small niche breadth in winter is probably a response to food limitation, and each species exists in its exclusive niche where it is optimally adapted and therefore has high foraging efficiency. In summer tundra and taiga habitats of the Arctic, shorebirds generally have a higher behavioral and microhabitat diversity (broader niche) and higher overlap between species. Exceptions to these general patterns exist among the study species. Seasonal differences in prey density, prey behavior, time available for foraging, feeding and locomotion rates, and the pattern of resource partitioning imply that shorebird populations are regulated through competitive processes occurring on their wintering habitats. Conclusions concerning coexistence mechanisms in migratory bird species and residents in seasonal environments may be erroneous if populations are studied only in the breeding season.

The plumage characteristics of male Indigo and Lazuli Buntings are distinct, but the two species can learn each other's songs. Populations comprising Indigo, Lazuli and hybrid individuals occur in the Great Plains of North America, and assortative mating has been inferred from morphometric data. We devised a laboratory assay for determining female preferences for visual and vocal characteristics of conspecific and heterospecific males and for mixtures of these characteristics, such as might be encountered in an overlap population. Females of both species gave more copulation-solicitation displays when exposed to conspecific plumage and vocalizations than when exposed to heterospecific plumage and vocalizations. Females gave intermediate and similar responses to the combinations of conspecific plumage with heterospecific vocalizations and heterospecific plumage with conspecific vocalizations. Thus, in the absence of other potentially important variables, female reproductive behavior is consistent with the hypothesis of assortative mating, based upon both vocal and visual traits of the males and caused by female choice in this semispecies pair.

Abstract. -The plumage characteristics of male Indigo and Lazuli Buntings are distinct, but the two species can learn each other's songs. Populations comprising Indigo, Lazuli and hybrid individuals occur in the Great Plains of North America, and assortative mating has been inferred from morphometric data. We devised a laboratory assay for determining female preferences for visual and vocal characteristics of conspecific and heterospecific males and for mixtures of these characteristics, such as might be encountered in an overlap population. Females of both species gave more copulation-solicitation displays when exposed to conspecific plumage and vocalizations than when exposed to heterospecific plumage and vocalizations. Females gave intermediate and similar responses to the combinations of conspecific plumage with heterospecific vocalizations and heterospecific plumage with conspecific vocalizations. Thus, in the absence of other potentially important variables, female reproductive behavior is consistent with the hypothesis of assortative mating, based upon both vocal and visual traits of the males and caused by female choice in this semispecies pair.

Recovery of cached sunflower seeds by Black-capped Chickadees (Parus atricapillus) was observed in four laboratory experiments. Results of the first experiment were consistent with the hypothesis that chickadees use spatial memory to recover seeds cached 24 h earlier. The second experiment demonstrated that individuals have a high recovery rate for their own caches and a low recovery rate for caches made by another. The third and fourth experiments demonstrated that one chickadee observing another caching seeds provided no recovery benefit to the observer in comparison to its performance when recovering seeds hidden in its absence. This result held for 2-h and for 6-min delays between observation and attempted recovery. We believe that spatial memory is used by chickadees, that the individual carrying out the caching has a large recovery advantage over a conspecific that searches the same patch, and that the perceptual and motor experience involved in the act of traveling to a cache location may be necessary for the establishment of spatial memory.

From March to October 1986, 35 house mice, Mus domesticus, were fitted with radio‐transmitters and tracked in and around a Colorado feed shed for an average of 5 (range 2 to 17) days. 5 of 36 simultaneously tracked pairs of lactating females used identical nesting sites. Genotypic similarity at 5 enzyme loci, assayed from biopsies of blood and toes, indicated that these females were more genetically alike than would be expected if pairing occurred randomly among females. We posit that female house mice recognize and preferentially form communal nests with close relatives. Aside from the communally nesting females, minimal overlap occurred among home ranges of lactating females. Only 4 of 10 adult male home ranges appreciably overlapped female home ranges. The home ranges of these 4 males overlapped less than the other adult male home ranges suggesting that males defend one or more females for access to mating. The possibility that communally nesting females are nursing each other's pups is discussed.