Birders gather to watch a rare redwing, also known as a European thrush, at Capisic Pond Park in Portland on Jan. 30. Shawn Patrick Ouellette/Staff Photographer

In the last column, I discussed a birding phenomenon that has been termed the Patagonia Picnic Table Effect or PPTE. The effect is basically a positive feedback loop. A birder finds a rare bird, other birders come to see the rarity and find other rare birds, drawing yet more birders who find yet more rare birds.

The most famous example of the PPTE occurred in southeastern Arizona. A birder stopped at a roadside picnic area in the small town of Patagonia.

The lucky birder found a pair of rose-throated becards, a very rare species in the United States. Other birders flocked to try to find the secretive becards, found other rare birds and the snowball began.

Here in Maine, we recently had a possible example of the PPTE. On Jan. 14, Frank Paul found a black-headed grosbeak at Capisic Pond in Portland. This species is closely related to our rose-breasted grosbeak. Black-headed grosbeaks are normally found nesting broadly west of the Mississippi River and wintering in Central America. We have four accepted records for Maine and another dozen to be reviewed.

The Capisic Pond sighting drew many birders. It turned out to be fairly reliable, so Capisic Pond was getting increased coverage by birders.

On Jan. 29, Brendan McKay found a redwing at Capisic Pond. Not to be confused with red-winged blackbirds, redwings are members of the thrush family. In silhouette, a redwing looks like a robin. Redwings are found broadly throughout most of Europe and northern Asia. They colonized Greenland in 1990.

A rare redwing is perched in a tree at Capisic Pond Park in Portland on Jan. 30. Shawn Patrick Ouellette/Staff Photographer

The species is a mega-rarity in North America. Vagrants are most likely to occur in Newfoundland. The Capisic Pond redwing was only the second for Maine, the first having been seen earlier in February in Steuben on a single day.

Jan. 29 was a Friday and hordes of birders descended on Capisic Pond over the weekend to see the redwing. It was a rather furtive bird but was fairly faithful to a large patch of roses. With patience, the bird could be seen. Sometimes, it rewarded birders with killer views.

The Portland redwing stayed until at least Feb. 22, drawing birders from far and wide. In early February, birders started to report a dickcissel (perhaps the same one seen there in early January). Dickcissels are uncommon in the state but not in the same league as the grosbeak or the redwing. They are a North American species with the populations closest to us being in extreme western Pennsylvania and Ohio.

Some birders in February were able to see the black-headed grosbeak, redwing and dickcissel on the same day in the jewel of this urban park. Is this an example of the PPTE?

Some recent research out of Oregon State University disputes the existence of the PPTE. The team, led by graduate student, Jesse Haney, used 10 years of eBird data to ask if the discovery of a mega-rarity increases sightings of other rarities.

The team searched eBird for records of 81 species of mega-rarities. They determined the number of eBird records for the rarity and the range of days of observation. For instance, there were 419 eBird records for the Capisic redwing over 25 days. The authors could then calculate the intensity of birding that a rarity engendered. One would expect higher birding intensity would lead to a higher discovery rate of more rarities.

The authors looked at 271 sightings of the mega-rarities from all around the country. One example was the northern lapwing that appeared in Poland, Maine from May 3-6 in 2013.

The authors found no support for the PPTE. Rarities that drew the most eager birders did not lead to a higher rate of discovery of additional rarities than rarities that were seen by few birders. So, the PPTE is a delightful myth with no scientific support.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu