[obol] Brood Parasitism

[Dan Gleason]

Such a match-up would be extraordinarily unlikely. Brood parasitism  
has never been reported in Green Herons, either as host or as  
parasite. The same is mostly true of crows, although Brown-headed  
Cowbirds have been reported as brood parasites (rarely) on crows. The  
attempts by cowbirds to parasitize crows were unsuccessful and may  
actually have been cowbird nestlings brought to the nest by the crows  
themselves as potential food.

The diet necessary for a young Green Heron differs from what a crow  
would provide its own young and the incubation periods also differ:  
Green Heron eggs require 19-20 days to hatch and crows hatch in 16-18  
days. Crows start incubating about the time of the third egg so some  
eggs will hatch earlier. With young crows hatching before a heron egg  
could complete it full incubation, the heron egg would likely be  
rejected if it were in a crow's nest and not rejected when first  
discovered.

Green Herons often nest a considerable distance from water. The  
youngsters get quite active the few days before fledging and are  
found out on branches near the nest and occasionally fall to the  
ground. The nest itself often becomes somewhat broken and may not  
support the young well in their final days before fledging. This is a  
critical stage where youngsters can get lost.

The most likely explanation for what you observed is that the crows  
were tending to their own chick, but harassing the Green Heron chick,  
either as a threat or as potential prey.

Dan Gleason
-------------
Dan Gleason
dan-gleason at comcast.net
541 345-0450


On Jul 21, 2008, at 10:17 AM, Jack Holley wrote:

>  Yesterday as My wife and I were walking our dog in a park in SE  
> Portland, I spotted two young birds not able to fly jet hopping  
> around on the ground. The first was a young crow, and the second  
> was a young Green Heron. There were no marshy areas close to the  
> park. Crows were  flying from one to the other birds. I was  
> wondering if anyone has heard if Green Herons lay their eggs in  
> other birds nest?
>
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> obol at oregonbirdwatch.org
> http://oregonbirdwatch.org/mailman/listinfo/obol

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