This is the so-called “ blue banana”, or dorsale européenne (European backbone), identified by French geographer Roger Brunet in 1989, and it is home to more than 110m people.īut we can get further clarity still by honing in on “built-up” density, which takes into account only those 1km² areas with people living in them. For example, we can see an area of high population density extending in a rough arc from north-west England down to Milan, with a little break in the Alps. (2010) do not compensate for problems with distance sampling found in this study.This bird’s eye view helps us to understand the wider context. Multiple-observer and time-of-detection approaches as applied by Stanislav et al.

For example, the protocol for the mourning dove Zenaida macroura call-count survey does not permit counting of other species to avoid distractions ( Sanders 2009). Recent research has begun targeting one or a suite of species by using point-count protocol to increase efficiency ( Alldredge et al. Distractions from vocalizations of other bird species can bias counts of a particular species. 1998) and are one of the two most important sources of bias in Breeding Bird Survey routes ( DeMaso et al. Point counts are dependent on observer experience ( Rappole et al. (2008) found the assumption that observers could accurately map locations of aural detections was not met. Older observers have poorer hearing acuity than do younger observers, and the result is missed vocalizations of some species and missed vocalizations at an observer-specific critical distance. Differences in observers related to hearing ability are well documented ( Emlen 1984 DeJong and Emlen 1985 Farnsworth et al. These biases include, but are not limited to, observer biases caused by hearing acuity, ability to estimate distance, expertise with bird vocalizations, distractions due to other species, attitude and condition of observers, and other observer-related factors ( Faanes and Bystrak 1981 Scott et al.

It is difficult for observers to accurately judge distances, especially beyond 100 m.īecause avian conservation planning strives to track populations of multiple bird species, additional biases are introduced into the point count and distance sampling process. An observer simply cannot accurately place a bird within the proper distance band even if the bird is stationary. This has a profound effect upon an observer's ability to determine the distance from which a whistle is occurring in an environment where there are many competing sounds and environmental factors that can further modulate or filter sounds ( Ramsey and Scott 1981 Simons et al. Thus, for humans, a loud whistle may be close or far depending on the singing bird's projection and position relative to the observer or its response to the observer. Whistling males continually change position and direction of vocalization and when approached by other male(s) will “whisper” the “bobwhite” whistle ( Stokes 1967). Bobwhites have auditory-localization ability, meaning that they can detect and thus project location into their vocalizations ( Gatehouse and Shelton 1978 Gatehouse and Bailey 1986). Northern bobwhite detection is difficult because bobwhite whistling, like all avian vocalization, is modulated in frequency, amplitude, or both ( Valleru 2006). Distance sampling is based on the concept that the probability to detect a bird decreases as the distance from the point being surveyed increases, providing a framework for fitting empirical data that represent this inverse relationship to known distributions. Point counts for avian species may be used in conjunction with distance sampling theory ( Buckland et al. Thus, managers are looking to distance-based point counts to provide reliable measures of population density for northern bobwhite and other birds as part of a coordinated monitoring system ( Somershoe et al. These indices alone are not robust and cannot be used as surrogates for density estimates ( Norton et al.

Geological Survey, Patuxent Wildlife Research Center and Environment Canada, Canadian Wildlife Service, 2001) routes to develop trends in northern bobwhite populations ( Rosene 1957 Robbins et al. Historically, managers use indices such as morning whistle counts or North American Breeding Bird Survey ( U.S.

Us bird population density map