[obol] Re: Cattle Egrets

[Mike Patterson]

If you check the _Birds of North America_ account for Cattle
Egret, it turns out that the general trend in Cattle Egret 
distribution that we see in Oregon occurred continent wide and is 
fairly standard for invasive species.

In a nutshell, juvenile birds (1st year) are far more likely to
disperse longer distances than older birds so, out of range occurrences
should theoretically occur more often in high productivity years.
Dispersal in asymetric with the majority going south, so one
would expect fewer occurrences north of breeding range.  There
was a nationwide spike of Cattle Egret occurrence in the 60's
and 70's, followed by general decrease and stabilization in best
habitats.

I quote the relevant sections of the BNA:
"Range expansion of Cattle Egret is complex, dynamic, and continuing.  
Kushlan and Hafner (2000) and Kushlan and Hancock (2005) provide 
worldwide historical population information at the continental or 
subcontinental scale. Studies worldwide show that establishment of 
Cattle Egret populations (just prior to reaching a stable population 
level) requires about 11-39 yr (mean = 26 yr, median = 26 yr) during 
which the exponential phase of population growth increases about 11-45% 
annually (mean = 25%, median = 25%).

"According to Breeding Bird Survey (BBS) data (U.S. Fish & Wildlife 
Service [USFWS]), trend in North America, as a whole: 1966–2004 
(0.1% annual increase, n = 543 BBS routes); however, the initial 
increase phase (13 yr) was followed by a decrease (24 yr): 1966-1979 
(3.7% annual increase, n  = 202 BBS routes); 1980-2004 (-1.3% annual 
decrease (n = 512 BBS routes). This trend agrees with population 
analyses of U.S. (Larson 1982, Bock and Lepthien 1976) and Texas 
populations (1983, 1993, Telfair et al. 2000a). By 1971, the increase 
in Cattle Egret populations in the U.S. had reached its upper asymptote 
and begun to stabilize, thus demonstrating classic logistic model 
for population dynamics (see analysis of Texas population dynamics 
in Telfair et al. 2000). Early rapid increase in numbers can be 
accounted for only by an annual influx of immigrant birds, i.e., 
not solely by recruitment of young produced in U.S. colonies 
(Telfair 1993)."

And relative to dispersal:
"In North America, long breeding season during which juveniles 
and adults disperse; some birds associate in premigratory staging 
groups, others disperse immediately, traveling long distances in a 
short time (see Timing and routes of migration). Most dispersal is 
southward and coastal, some southwest to westward; little evidence 
of northward movement (Byrd 1978, Telfair 1993).  Juveniles join 
adults in foraging areas by the age of 2 mo.; most (77%) reach their 
winter range in Mexico between early Oct - late Dec; movement can 
be rapid, e.g., two band recoveries of nestlings banded in east 
Texas came from the Trans-Pecos of west Texas (Aug 30) and Baja, 
California (late Dec) and a color-marked juvenile was seen at Cocos I., 
Costa Rica (Nov 16). In Australia, McKilligan et al. (1993) analyzed 
the non-sequential geographic pattern of new colonies and suggested 
that establishment of new breeding colonies is by migrants rather 
than by shifts of more sedentary birds to the nearest suitable site 
from their natal colony. Also, in Australia (Maddock and Geering 1993, 
1994) found that soon after fledging, many juveniles and adults 
dispersed throughout feeding areas surrounding the breeding colony, 
formed night roosts, and established foraging home ranges. Movement 
in feeding areas was apparently random, birds using a number of night 
roosts over a relatively short time; however, some juveniles and 
adults dispersed far more widely. In India (Gopakumar 1996), there 
is a distinct seasonal shift between breeding sites and non-breeding 
roosts.



-- 
Mike Patterson               
Astoria, OR                    
celata at pacifier.com  
 
I'm not jealous or 
Why I've never Seen Black-throated Blue Warbler in Oregon
http://www.surfbirds.com/blogs/mbalame/archives/004174.html