Boston’s “Other Emerald Necklace”?

In June and July and August, volunteers reported four species of herons at Fresh Pond’s Black’s Nook: Great Blue Heron, Great Egret, Green Heron (at least six individuals), and Black-crowned Night Heron (at least four, including two juveniles). Green Herons have been present since May, and with the late-summer appearance of juveniles we seem to have collected good evidence that they have probably nested successfully at the site or nearby. Black-crowned Night Herons were reliably present in July, but may well have arrived from another nesting colony.

It’s interesting to look at this cluster of herons at Fresh Pond in a larger geographic context. 

The following maps show sightings from the eBird database of all four species in 2012 (as of mid July) and over the past 10 years or so.  Here’s one species as an example:

Black-crowned Night Heron, 2012 (as of mid July)

Black-crowned Night Heron, all years in eBird (mainly last ten):

Within the mapped area (approximately 10 x 15 miles) and outside the banks of the Charles and Mystic Rivers, Black-crowned Night Herons seem to be concentrated along a long north-south corridor of green, wet spaces, including Black’s Nook.  The corridor extends from the Middlesex Fells through Upper and Lower Mystic Lakes, Arlington, Alewife, Fresh Pond, and Mt. Auburn Cemetery to the Charles River.  (Cont.)

Of course, before we jump to the conclusion that these herons are probably not present in areas that are empty on the map, we need to confirm that people are in fact looking for birds at all in those other locations, too.  Consider the following maps of the same region for other common birds, both resident and migrant, and associated with water and not (click for larger view):   

You can see that the area to the west of Fresh Pond, for instance, is pretty well surveyed by eBirders, and filled with sightings of other species. 

Now compare those maps to Black-crowned Night Heron, Green Heron and Great Egret sightings, and we get at least a qualitative impression of the relatively rarity of these herons in this region, as well as of the way they are concentrated in that north-south corridor.  (Again click for larger versions of the maps. All 2012 maps are as of mid-July.)

    

    Great Egret, 2012                                     Great Egret, all years

Great Egret is reported pretty rarely between Concord and this corridor, especially in the summer.  In fact, the Black’s Nook project recorded the first Great Egrets for Fresh Pond outside of August and September, despite some 900 checklists for Fresh Pond Reservation. When Great Egrets are seen in the several miles around Cambridge, they pick out that north-south corridor quite uniquely.  On the other hand, these birds are transients, and don’t nest here.  (Which isn’t to say providing habitat for transients isn’t important.)

         

  Green Heron, 2012                               Green Heron, all years

Green Herons, on the other hand, are a nesting species in this local area.  And there seems to be good evidence that Green Herons did nest successfully at Black’s Nook this year.  (See the spike in the number of birds seen when juveniles first start being reported at the end of July.  And see the comparison of counts this year with previous years at Fresh Pond as a whole.) The regional Green Heron pattern is much more like the Black-crowned Night Heron pattern.  We see a strong signal in this north-south corridor and only scattered sightings in areas outside it.  The corridor seems especially important to these species. 

          

Great Blue Heron, 2012                          Great Blue Heron, all years

Great Blue Heron, interestingly, has almost the same wide distribution as Mallard in the maps.  Pete Dunne’s Essential Field Guide Companion describes Great Blue’s habitat this way: “Almost any open, still, or slow-moving fresh, bracksish or coastal fish-bearing water”. A generalization borne out by this local sample.  This is the most commonly observed of these herons, and the least specific in its preferences and habitat requirements.

To draw rigorous conclusions about the true distribution of herons with respect to this north-south corridor, we’d need more data, and more analytical tools.  But maybe we can come to some tentative conclusions even from what’s available to us here as laypeople:

• A space as small as Black’s Nook (and as close to a sea of parking lots) can be suitable breeding habitat for Green Herons.  So even small urban wetland spaces can be important conservation sites.

• Green Herons, Black-Crowned Night Herons, and Great Egrets, especially, are more likely to be seen in the corridor of loosely connected protected lands identified above than in most other places in the mapped area.  We might even call this corridor greater Boston’s “Other Emerald Necklace.”  

• At least as from the point of view of many herons, there seems to be nothing like that network of parks for miles east and west.  Obviously, many of these wet sites used to be connected by continuous habitat.  Much of the area around Fresh Pond is literally landfill. The birds’ distribution perhaps gives a hint of those old connections.

• This aggregated, citizen-science bird data gives us citizens some public evidence of habitat quality in this whole string of protected lands. If these (predatory) birds are there, so are many other populations of organisms that support them.  And it gives us additional reason to protect those places.

• It might well be worth thinking of these connected spaces together as citizens and communities. While each of the spaces individually has its passionate advocates, maybe there’s room for more sharing of information and collaboration across groups. 

I would love to know what people think about some of these conclusions.   How much do you think we can draw from this citizen-science data?