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By James G. Dickson

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The relation between time of day and detectability reverses (Fig. 6 ) . The magnitude of this effect is also a function of species and delay between samples. t. (Shields, 1977). In winter, in this region detectability increases with increasing temperature. Avian Census Techniques 47 The interaction between temperature and detectability is more complex than implied by the data from either season alone. It appears that warmer temperatures in summer, and colder in winter, can both have a depressive effect on detectability.

Valid estimates of numbers can only be made if the relation between detectability and density is known, or in some way controlled. , mark-recapture, territory mapping, see Emlen, 1971), or detectability can be measured, and eq. 2 used to estimate density from contact frequency, C = N/d. In either case, sampling areas must be measured (and therefore limited in size), and the investigator must estimate distances in the field. Of the many methods available for estimating bird density, variations of the transect (strip) census have recently received much attention (iïlerikallio, 1958; Emlen, 1971; 1977; Järvinen and Väisänen, 1975; Järvinen, 1976).

There are four larval instars. The first two remain in the phloem region and the last two migrate into the outer corky bark where pupation and adult emergence occur. Development can be completed in as few as 35 days, with six to eight generations per year occurring in some regions of the South. The host tree must be killed before successful colonization occurs. The process of tree death is initiated by the successful introduction of the blue staining fungi (Geratocystis minor Hedge and Hunt) and by girdling brought Robert Coulsonef al.

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