Shiny Cowbirds At Carib Grackle's Nest

Female Shiny Cowbird

Shiny Cowbirds (Molothrus bonariensis) are brood parasites, which means the female lays her eggs in the nests of other birds,  relying on them to incubate her eggs and rear her chicks.  While some birds reject the eggs straight off, many species do not.  This can harm the population of the host bird and can be even more devastating when the host is a threatened species.  This appears not to be the case here on the island as the main host of these birds, Carib Grackles (Quiscalus lugubris), since unaffected.

A Barbados Bullfinch parenting a 
Cowbird's chick

What is worrying, though,  are the few sightings of Barbados Bullfinch (Loxigilla barbadensis), our lone endemic,  and Golden Warbler, our subspecies of Yellow Warbler (Setophaga petechial) parenting Cowbird chicks. If this becomes the norm rather than the exception, it can spell trouble for these important bird species, especially as the latter is making a spirited comeback to the Barbados landscape.  In the British Ornithological Society Checklist Series #24 - The Birds of Barbados (Buckley, Massiah, et al) said that "the decline in breeding of the Barbados Golden Warblers [I like that name] in the mid-1950s has been attributed to this species [Shiny Cowbird] but without supporting data" I personally think the reason for the decline was habitat loss, but that too is without supporting data.  

Even with the reputation these birds have I am still intrigued by them. Having on occasions seen them at Grackle colonies doing their thing I must say the process is quick and efficient. On Tuesday, March 23rd, I saw something I have never noticed before and it led to me writing this post.  Outside my bedroom window is a Bajan Cherry Tree (Malpighia emarginata) which was my birding oasis during COVID-19 lockdowns 1 and 2. (see here also here).  This year for lockdown number 2, three species were nesting or in the process of building a nest in the tree.  It was surprising that one of them was a  Carib Grackle (Quiscalus lugubris) because they are colonial nesters.   

Here is what happened that Tuesday morning: As I was passing the bedroom window I noticed three female Shiny Cowbirds inspecting the nest...  

Two of the three female Shiny Cowbirds

The first Shiny Cowbird entered the nest...

Shiny Cowbird entering the Carib Grackle's nest

Sat on it for a short while, most likely laying an egg, I thought... 

Then exited the nest and another cowbird took his place...

The conveyer-belt, assembly line-like process was quick, orderly, and efficient, taking just seconds for each bird to sit on the nest.  Seeing how methodically these cowbirds worked on this single nest was scary as I could only imagine what happens in a grackles breeding colony.  The number of Shiny Cowbirds across the island "appears" to be at an all-time high.  It is now customary to see large flocks going to and from roost, a sight that is relatively new to me.  This event caused me to question my original anecdotal thinking – Is the Carib Grackle (Quiscalus lugubris) population really sustaining this brood parasitic attack or is it slowly eroding one of our native species?  We, the local birders, must be on the watch to ensure our endemic  Barbados Bullfinch, or our sub-species of Yellow Warbler are not being adversely affected.  It emphasizes the need for a closer look to be taken at the effect of Shiny Cowbirds on our avianscape.  There is documented evidence of brood parasitic species* wreaking havoc on an ecosystem, but there are also examples where their coexists.  Lets hope our is the latter.

*Ecology and Management of Shiny Cowbirds (Molothrus bonariensis) and Endangered Yellow-Shouldered Blackbirds (Agelaius xanthomus) in Puerto Rico by Cruz, López-Ortiz, et al