Altitudinal migrations
Although it is known that some birds do not undergo seasonal movements, the altitudinal migration is thought to play an important role in the seasonal distribution of tropical birds over an elevational gradient (Stiles 1988, Powell and Bjork 1995, Winker et al. 1997). The role and cause of these movements remain a little studied and poorly understood phenomenon. Movements by vertebrates may occur in response to changes in microclimatic conditions (e.g. relative humidity), regional weather patterns (rainfall, temperature), and spatial and temporal variation in food abundance or reproductive sources (Stiles 1980, Terborgh 1985). Essential ignorance is particularly true for small passerines, where nearly all knowledge has been derived from seasonal fluctuations in observations and capture-rates at different elevations (Stiles 1985a, Dasilva 1993, Winker et al. 1997, Burgess and Mlingwa 2000, Wright 2005, Brown 2006) and occasional band recoveries (Loiselle and Blake 1991, Burgess and Mlingwa 2000).
Stable isotope analysis is relatively new method and may provide a means by which altitudinal migration could be detected without prior capture of individuals (Hobson et al. 2003). Recent studies show a highly significant correlation between δD, δ13C, and δ13N in feathers and claws (Bearhop et al. 2003, Hobson et al. 2006), and studies using both tissues have assumed that keratinous tissues grown in the same locale share δD values (Bearhop et al. 2005). Thus, comparisons of δD values in various tissues may provide clues about the prior movement of organisms.
Globally, hydrogen isotopic values in meteoric waters (δDp) show a depletion trend of about −2.2 per mil per 100 m elevation (Poage and Chamberlain 2001, Hobson et al. 2003). Hobson et al. (2003) found strong correlation between hydrogen isotopes values in feathers (δDk) collected along an elevational gradient with expected capture site values based on annual mean precipitation. Fraser et al. (2008) tested possibilities fo this technique based on hydrogen and summarized that (1) comparisons of claw and tail δD values from individual birds can be used to detect altitudinal migration, and (2) this approach may reveal movement not detected by other methods.
Mainly frugivores and nectarivores are thought to migrate altitudinally as they track seasonal fluctuations in food availability (Levey and Stiles 1992). One of the most recent works confirmed (Loiselle and Blake 1991) this hypothesis as they found that partial-frugivores examined were altitudinal migrants, while nectarivores seemed to be more sedentary. Much remains to be learned in this field. Missing are especially information for different guilds from multiyear studies.
Stable isotope analysis is relatively new method and may provide a means by which altitudinal migration could be detected without prior capture of individuals (Hobson et al. 2003). Recent studies show a highly significant correlation between δD, δ13C, and δ13N in feathers and claws (Bearhop et al. 2003, Hobson et al. 2006), and studies using both tissues have assumed that keratinous tissues grown in the same locale share δD values (Bearhop et al. 2005). Thus, comparisons of δD values in various tissues may provide clues about the prior movement of organisms.
Globally, hydrogen isotopic values in meteoric waters (δDp) show a depletion trend of about −2.2 per mil per 100 m elevation (Poage and Chamberlain 2001, Hobson et al. 2003). Hobson et al. (2003) found strong correlation between hydrogen isotopes values in feathers (δDk) collected along an elevational gradient with expected capture site values based on annual mean precipitation. Fraser et al. (2008) tested possibilities fo this technique based on hydrogen and summarized that (1) comparisons of claw and tail δD values from individual birds can be used to detect altitudinal migration, and (2) this approach may reveal movement not detected by other methods.
Mainly frugivores and nectarivores are thought to migrate altitudinally as they track seasonal fluctuations in food availability (Levey and Stiles 1992). One of the most recent works confirmed (Loiselle and Blake 1991) this hypothesis as they found that partial-frugivores examined were altitudinal migrants, while nectarivores seemed to be more sedentary. Much remains to be learned in this field. Missing are especially information for different guilds from multiyear studies.