The Migrant Loons of Western Pennsylvania

Abstract

BENT (1919: 48) says of the Common Loon (Gavia immer), loons are apparently paired when they arrive on their breeding grounds, and I believe they mate for life. The implication would seem to be that they migrate as mated pairs. Todd (1940: 32) says, loon is a rather solitary bird in our region (Western Pennsylvania), and a number together have a common interest rather than a sociable disposition. In the spring, however, presumably mated pairs may sometimes be encountered and on one occasion (May 1, 1932) Professor Seiple saw as many as twelve together on Conneaut Lake. This might be interpreted to mean that Todd construed Seiple's dozen as comprising six mated pairs, though I do not think Todd so intended it. However, in conversation, Dr. Parkes of Carnegie Museum expressed the opinion that a considerable proportion of migrant loons may be moving in mated pairs, along with a lesser number of unmated birds. My own observations seemed to lend no support to this view, and on the contrary, tended to the opposite view, that all loons migrate as independent voyageurs, though sometimes assembling into fortuitous flocks of various sizes. It seemed advisable to submit the full evidence to such mathematical treatment as might be helpful on this point. Material.-Western Pennsylvania from the West Virginia border north for a hundred miles or more, including the Pittsburgh area, contains few bodies of water attractive to loons: it is an upland country, much of it wooded, ranging in elevation from roughly 1,000 to 2,000 feet. None the less, the loons pass over it in some numbers, but they fly high, fast, and apparently silently, and are not often observed. I have seen one or two, traveling singly at a great height. The best opportunity to observe them is when they have alighted on some body of water, which happens frequently enough to give some idea of their behavior. The materials used in the present analysis are five years' observations at Oneida Dam, Butler County, in the years 1949 to 1953, inclusive. Most of the observations are my own, but some are by Mrs. Preston and others. An account of the water birds of Oneida Dam, including all migrant species, for the first four of these years has been given elsewhere (Preston, in press), and a map of the area is there included. It is shown that in the four years in question, the peak of the loon migration came at April 22, the standard deviation (of the timing) was 11.6 days,