SHORT COMMUNICATIONS 
639 
Kites had a bi-modal distribution in time of 
passage with 31% (n = 1,747) flying by between 
0900 and 1000 hrs and 37% (n = 2,080) between 
1400 and 1500 hrs (Fig. 1B>. 
DISCUSSION 
My observations, together with those of Olivo 
(2001. 2004. 2005, 2007a, b). demonstrate that 
significant movements of soaring raptors occur at 
Concepcion, where 23 migratory species converge 
during both austral autumn and spring. These 
studies show a continentally important migration 
site exists, but migration timing and number of 
migrants recorded at Concepcion are not well 
understood, especially during autumn. Concepcion 
ranks as one of the lop areas to study seasonal 
timing and abundance of migrating raptors in the 
southern cone of South America (Juhant 2011). 
The three species of vultures recorded at 
Concepcion are considered sedentary in the 
southern part of their distribution (Ferguson-Fees 
and Christie 2001). However, these species were 
previously counted on migration at the site by 
Olivo (2001, 2005. 2007a). The Black Vulture is 
the most common vulture recorded with an 
average of 810 birds counted in autumn (Olivo 
2007a, this study), and 895 birds in spring (Olivo 
2005, 2007a). Satellite data of breeding Black 
Vultures from the Argentine Pampas reveals they 
migrate short-distances within the country (www. 
vulturemovements.org/hms/hms_tv.htm). 1 he 
number of Black Vultures, together with the low' 
number of King Vultures, may indicate local 
migration movements within the lowlands ot 
Bolivia. Breeding Turkey Vultures I mm the 
Argentine Pampas migrate northward east cl the 
Andean foothills of Bolivia anti Brazil to 
overwinter (www.vulturemovements.org/hms/hms_ 
tv.htm). Thus, vultures counted at Concepcion 
could originate from southern latitudes. 
The migration route of Snail Kites throughout 
its range is largely unknown, but the species has 
been recorded at many watch sites in South 
America (Juhant 2011). The migration time of 
Snail Kite in the lowlands of Bolivia is between 
mid-February and carly-April (Zallcs and Bild- 
stein 2000, Dobbs and Huizinga 2005, Olivo 
2007a. this study), and between late-September 
and late-November (Zalles and Bildstein 2000, 
Olivo 2001. 2005, 2007a). It is not known if kites 
recorded at Concepcion belong to the Austral- 
Neotropic and Neotropic-1 ntra-tropical migration 
systems or both. Study of migration of Snail Kites 
in the Neotropics remains a fertile ground for 
future research. 
Mississippi Kites comprised nearly 80% of the 
migrating raptors. The high numbers of kites 
recorded in this study coincide with previous 
counts at the site during austral autumn and spring 
(Olivo 2001. 2004, 2007a). The average number 
of kites counted in autumn is 4.710 (Olivo 2007b, 
this study), whereas the average number of kites 
counted in spring is 132,080 birds (Olivo 2004. 
2007b). The difference in kite numbers recorded 
between seasons may be caused by seasonal 
factors. First, kites may migrate northward pri¬ 
marily in late Austral summer. A single flock of 
10,000 kites was outlined on 20 February, 1.000 km 
south of Concepcion Watch Site migrating north¬ 
ward in Fuerte Esporanza town (25 11' S. 61 55' W) 
in the Chaco lowlands of Argentina (Areta and 
Seipke 2006). Second, kites may fly very high to 
avoid storms or fly within the clouds in the autumn 
(wet season), making it difficult to detect them 
from the ground on migration (Smith 1985). 
Finally, they may have an elliptical northward 
migration route or may fly north in a more 
dispersed (broader) front, avoiding this site. The 
migration time of Mississippi Kites at Concepcion 
is through March (Olivo 2007b, this study) and 
between late-Scptember and late-November (Olivo 
2004, 2007b). 
The migration route and wintering distribution 
of Mississippi Kites throughout South America is 
largely unknown (M. A. Juhant and J. I. Areta, 
unpubl. data), and the species has been recorded 
on migration principally at Concepcion, which is 
the only bottleneck area known for the species 
south of the Panamanian isthmus (Juhant 2011). 
Mississippi Kites have been reported in several 
countries of South America but. despite the 
growing number ot reports, it is considered 
scarce, rare or transient in every country where 
it is found during the boreal winter. 
More research is needed to completely describe 
timing of the raptor migration in the austral autumn 
at Concepcion. An annual season-long count 
by experienced observers from mid-February to 
niid-Mav (northbound migration) and from 
mid-September to late-November (southbound 
migration) is recommended to gain a better under¬ 
standing of migration timing and composition at 
this important site. A full-time annual count could 
potentially provide critical long-term monitoring 
data for several species of Nearctic and Neotropic 
raptors, as done at many North American sites 
