Seasonal Distribution of Bird Populations at the Patuxent Research Refuge

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Abstract
A detailed study of seasonal changes in bird populations was made at the Patuxent Research Refuge, during 1936-1949. The history of the Refuge is reviewed and its physical and biological characteristics summarized. The phenology of the Refuge is described in considerable detail throughout the year, with special attention given to major fluctuations in bird populations as correlated with climatic changes and with seasonal aspection of the vegetation. The component spp. of birds in the more important migration waves are listed. Refuge breeding and wintering populations are estimated, while indices representing the relative abundance of bird populations, based on figures from the 2-yr. seasonal population study, were obtained for the entire year. The ecological affinities of the bird populations differed greatly from one season to another. Species characteristic of edge habitats were much more numerous in winter, while forest species were predominant in summer. Insectivorous species comprised 40-60% of the total population during the warmer months, but were of minor importance in winter. The greatest number of spp. of birds occurred during the population peaks of insectivorous species, while the largest number of individuals was found during the population peaks of omnivorous and herbivorous spp. The population peaks of insectivorous spp. were found to occur much later in spring and considerably earlier in fall than the corresponding peaks of omnivorous and herbivorous species. The Fringillidae contributed the greatest number of individuals in winter, while the Parulidae was numerically the most important family in summer. Water birds and marsh birds were relatively unimportant throughout the yr., due to the scarcity of suitable habitats. Permanent resident species were found to vary from about 20% to slightly less than 50% of the total population throughout the year, although many individuals of these sp. were either transients or part-time residents. Summer and winter residents were more abundant than permanent residents during their respective periods of occurrence. During the greater part of the migration period, transient spp. were found to comprise only 10 to 20% of the total population. However, transient individuals of all spp. would account for a much larger proportion of the population at this time. Yearly variations in seasonal population changes are described and the causative factors indicated, when known. Of these, food supply and weather conditions were generally the most important. The seasonal distribution of the population of each of the 229 spp. of birds that have been recorded on the Refuge is described in detail. This includes data on migration periods such as earliest dates of arrival, latest dates of departure, median dates of arrival and departure, and migration peaks. Population indices are also listed for any of the more common spp. throughout their periods of occurrence, and the numerical status of wintering and breeding populations is indicated as well. In many cases, the highest numbers of individuals observed in one day during the migration periods are given and the size of flocks at different seasons described.

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