Date of this Version
Johnsgard, "An Analysis of Migration Schedules of Non-Passerine Birds in Nebraska," from Nebraska Bird Review (June 1980) 48(3).
AN ANALYSIS OF MIGRATION SCHEDULES OF NON-PASSERINE BIRDS IN NEBRASKA
One of the major components of the Nebraska Bird Review since its inception has been the annual spring migration and occurrence report, and in more recent years the fall occurrence report has become equally important. Yet, other than an uncompleted effort by W.F. Rapp, Jr. to summarize the first 25 years of spring migration data, these records apparently have not been analyzed by anyone. Part of the problem is the sheer mass of data to be assimilated, and a second issue is the question of how to classify certain species, for the analysis of a winter visitor required different consideration from that of a spring and fall migrant. Further confusing the question is the fact that many Nebraska species migrate completely out of the state in some years but variably overwinter at other times. Even more difficult are the species that are summer residents in one part of the state, but may be winter visitors elsewhere, such as Townsend's Solitaires. Finally there are the problems of several species that are so little known that is it presently impossible to classify them as summer residents, permanent residents, or migrants. Nevertheless, these are a very small proportion of the total, and it thus seemed worthwhile to made an attempt at an understanding of the migration patterns of Nebraska's avifauna by the present analysis and perhaps pinpoint if not resolve some of these questions. This first report will deal only with the non-passerine species; a later one will concern the passerines.
For the species known to breed regularly in the state I have generally estimated only spring arrivals (initial spring sightings or records) and fall departures (final fall sightings or records). For species that I consider winter visitors I have estimated fall arrival and spring departure periods. For species that are largely or entirely migrants in the state I have estimated spring arrivals, spring departures, fall arrivals, and fall departures. In the case of a few very rare species I have simply summarized all available spring and fall records (literature or specimen) or sightings. All gallinaceous birds, some owls, and most woodpeckers were considered permanent residents and were not analyzed. Median dates have been determined for all samples of ten or more available dates; for smaller samples arithmetic means have been calculated.
Except in the cases of the rarer species, I have not attempted to include all of the available published data*, but rather have carried the analysis backward in time far enough to satisfy myself that the migration pattern was sufficiently clear to terminate the sample. In some cases this meant that as few as about 50 dates were seemingly adequate, while in others more than 100 were considered necessary. The reader is cautioned against relating the size of the sample to the abundance of the species, there is no necessary relationship. As a matter of convention, the spring migration period was considered to begin on 1 January (as in the Review), and terminated on 10 June. The fall migration period was considered to begin on 20 July and terminated on 31 December. In a few instances (such as some shorebirds and gulls) where fall migrants sometimes appear prior to 20 July, this date has normally been used as the initial fall occurrence. However, for rare species actual earlier fall dates are indicated. Additionally, although the initial spring or fall sightings are usually greater in number than are the final ones, in some cases the reverse is true, such as when part of the population over-summered or overwintered. In the case of more common migrants, data were utilized only when such species were reported present over a period of several days, in order to make the arrival and departure estimates more meaningful. Yet, in frequent cases the latest initial sightings may actually be later than the usual departure period. Such anomalies make the extreme dates cited far less significant than the mean or median dates. Further, whenever there was a definite clustering of dates I have determined the period within which at least half of the total migration sightings occurred; the spread of such records provides a useful estimate of the relative predictability of each species' migration.