Monday, July 6, 2009

"Odes" and Purple Martins

If you watch the Purple Martin colony at Cape May Point State Park, you'll notice that, contrary to their biting-fly-and-mosquito-eating reputation, the birds feed their nestlings many "odes" - i.e. dragonflies and damselflies. Sometimes, Laughing Gulls even pirate the big bugs from the martins.

Tony Leukering sent me some of these amazing images and offered to write up what he carefully observed last week. Thanks, Tony! Be sure to click to enlarge these photos.

Purple Martins and Odonata
Text and photographs (all copyrighted) by Tony Leukering

Cape May is well-known for a number of reasons. The one that probably accounts for the most widespread knowledge is the area's beaches and summer resort status. Among birders, of course, it is best known for magical fall days with thousands or, even, millions of birds on or passing over the point. However, there are more than birds among the huge swarms of winged migrants and many of the areas's birders have turned their eyes, and binoculars, onto the insect group, Odonata: the dragonflies and damselflies. The same cold fronts with following northwest winds that bring the birds also, in appropriate seasons, bring the dragonflies.

As northwest winds at virtually any season bring vultures and/or raptors to the Point, so do these winds (in summer and fall) bring huge influxes of dragonflies. The two days of such winds 27-28 June brought such a flight, with literally thousands of Spot-winged (Pantala hymenaea) and Wandering (P. flavescens) gliders arriving along with, probably, an influx of Black Saddlebags (Tramea lacerata) and Common Green Darners (Anax junius). All of these species are well-known as migrants, at least at Cape May, though all also breed here. They are also fairly larger and are particularly good fliers, often found far from water and fairly high up in the air column, up there where the Purple Martins fly.

For a while on the 28th, I attempted photographing Purple Martins around the colonies near the headquarters of Cape May Point State Park, noting that quite a few of the martins were bringing dragonflies to their nestlings. Also noting some red eyes (both glider species have red eyes) on some of the dragonflies, I figured that the martins were taking advantage of this new surfeit of food to feed their hungry youngsters. Indeed, one of the first martins that I photographed had a Wandering Glider.

[Adult female Purple Martin with Wandering Glider, Cape May Point State Park, 28 June 2009. Click to enlarge this and all photos below.]

Thus, when I found myself at the Park on 2 July with my camera and some time to kill, I filled up an 8 gb memory card with pictures of Purple Martins ferrying food. I went into the effort with two suppositions that could conflict with each other: 1) with these most aerial of dragonflies occupying the upper airspace often frequented by Purple Martins, these species would comprise a hefty percentage of the dragonflies brought to the nest and 2) these large dragonflies would be poor options for small nestlings, as such nestlings would not be able to swallow them.

While I did photograph martins with Common Green Darner...

[Adult male Purple Martin with Common Green Darner, Cape May Point State Park, 2 July 2009]

and Black Saddlebags...

[Adult female Purple Martin with Black Saddlebags, Cape May Point State Park, 2 July 2009]

... the vast majority of dragonflies were smaller, less-aerial species common at Bunker Pond in the Park. The species that bore the brunt of martin predation was Blue Dasher (Pachydiplax longipennis), with Eastern Pondhawk (Erythemis simplicicollis) also getting the worst of the martin-dragonfly relationship.

[Adult male Purple Martin with adult male Blue Dasher, Cape May Point State Park, 2 July 2009]

[Adult male Purple Martin with immature male Eastern Pondhawk, Cape May Point State Park, 2 July 2009 (Though I was more interested in identifying the dragonflies, I sure wish that I could have kept this bird entirely within the frame!)]

[Purple Martin (immature male?) with adult male Eastern Pondhawk, Cape May Point State Park, 2 July 2009 (Note that this bird is color-banded. The nestlings at the Park colony are banded each year, but I do not know whether the researchers conducting that effort are also color-banding.)]

Other species that I identified in the beaks of Purple Martins were Common Whitetail (Libellula lydia)...

[Immature female Purple Martin with female Common Whitetail, Cape May Point State Park, 2 July 2009]

Needham's Skimmer (Libellula needhami)...

[Immature female Purple Martin with Needham's Skimmer, Cape May Point State Park, 2 July 2009 ]

Eastern Amberwing (Perithemis tenera)...

[Immature female Purple Martin with female Eastern Amberwing, Cape May Point State Park, 2 July 2009]

Four-spotted Pennant (Brachymesia gravida)...

[Adult male Purple Martin with female Four-spotted Pennant, Cape May Point State Park, 2 July 2009]

and Halloween Pennant (Celithemis eponina).

[Immature female Purple Martin with Halloween Pennant, Cape May Point State Park, 2 July 2009]

I also photographed one martin with a damselfly that was one of the bluets and was probably - by sheer abundance if for no other reason - Familiar Bluet (Enallagma civile).

This, then, means that most of the foraging for dragonflies by Purple Martins - at least at Cape May Point State Park - may take place low over the water or vegetation where these species typically occur. And, as is so typical of biology, I could not answer a simple question with my simple efforts at obtaining data. Knowing the size of the nestlings being fed has the potential to solving the problem. Do Purple Martins preferentially take less-aerial dragonfly species? Are these highly-aerial dragonfly species more adept at avoiding martin predation? Or, are martins equally skilled at capturing darner, gliders, and saddlebags as they are at catching the smaller species, but that, like terns, they scale their catch to the size of the nestlings they are feeding?

I am honor-bound to note one additional confounding factor: the number of large, aerial dragonflies was noticeably smaller on the 2nd compared to that on the 28th, though most dragonflies in photographed martin beaks were Blue Dashers on that date, too.

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