Where are all the migrating birds?

Ever since reading about fruit "strategies" in the Peterson Field Guide to Eastern Forests (John C. Kricher & Roger Tory Peterson, 1998), I have been preparing to watch for the coming of migrating birds.  I searched for the species that bear the fat-rich fruit they prize: flowering dogwood, Virginia creeper, spicebush, sassafras.   A casual census of the neighborhood revealed a pretty big population of flowering dogwood--a popular small, yard tree--if no other high-quality fruit-bearers.  Flowering dogwoods it is, then.  

I watched in anticipatory delight as a large crop of berries ripened to blood-red.  I and the neighborhood dogwoods all waited for the thrushes, catbirds and waxwings that--catching sight of the "foliar flags" of their bronze leaves--would descend on us in search of the high-fat fruit that would fuel their southward migration.  We waited some more.  By late October the bronze leaves--signals to birds-in-the-know that HERE they could eat their fill of the very best to be had--had almost all fallen.  The trees, still bearing laden platters, were forlornly waiting for guests who would never arrive.  One nearby tree in particular I looked in on often and hopefully, as it gradually lost berries, but then I discovered many of them in the grass beneath, untouched. 

 Spicebush (Lindera benzoin) is not found much nearer than the Blue Hill Reservation,

in Milton/Canton.  And it doesn't seem to be fruiting here this year.

 

Virginia creeper (Parthenocissus quinquefolia) is more common, but I haven't seen any with fruit.

Flowering dogwood (Cornus florida) was already turning on September 20th.

 Rich pickin's!  Come and get it!

All dressed up, and no one's coming.

From my journal of October 8th:

"Flowering dogwood in front of red house losing fruit, but a good deal of this is on the ground.  Another Cornus florida on corner of N Belcher has a lot of bare seeds on ground underneath--not exactly dispersal, but better than just falling off.  This same promised to show me one of the culprits: moving leaves & twigs and a harsh call sent me carefully adjusting my camera sight angles [in hope of catching sight of a migratory bird]--only to discover a squirrel calling for all the world like a blue jay.  (I even saw him move his head as he began each call.0   This went on for some time."

I don't think the problem is fruit scarcity, since I can't imagine there would be a much greater density of flowering dogwood in most forests.  My only hypothesis at the moment is that migrating birds simply avoid landing here in the city for any of a variety of reasons.  There may be more limitations to studying nature here than I'd imagined.  

Fruit "Strategies"

One of the amazing things I learned from my encounter with the Peterson Field Guide to Eastern Forests concerns fruiting strategies.  From the plant's point of view, fruit is the way to give the kids (inside the seeds) the best chance possible for survival.  Among those plants which produce edible fruit in order to recruit animals to help with this, there are at least three different strategies in operation. 

The first is to get out front with a sweet fruit to attract resident birds and mammals, who will seek out this fruit for its sweetness, poop out the seeds probably some distance from the parent plant, and thereby "plant" the babies with a helpful accompaniment of fertilizer.  These fruits attract us, as well, so you can probably name a few: blackberries and raspberries (Rubus), and cherries and their relatives (Prunus).

 

Black raspberry (Rubus occidentalis): a sweet, early season fruit.

A second strategy is to produce a fruit high in fat that will be valued by migrating birds because of its high energy density.  Instead of sweet, these fruits are typically sour or pungent.  Such "high quality fruit" fruit is more expensive for the plant to produce, but recruiting migratory birds insures that, whatever the future holds, your children will at least be well-traveled!  Because migrating birds will often be in unfamiliar territory, many of these "high quality fruit" bearers have leaves that turn earlier in the fall, providing visual "foliar flags" that announce, "here's the good stuff!" to all comers.  Most of the "comers" are thrushes of one sort or another, catbirds and waxwings.  Around here, the high quality fruit-producers are spicebush, flowering and gray dogwoods, Virginia creeper, sassafras, and magnolia. 

 

I didn't think to take a closer look at the dogwood above at the time, so can't say what it is.

The one below is flowering dogwood (Cornus florida), which bears a high-fat fruit.

 Virginia creeper (Parthenocissus quinquefolia) 

is displaying its foliar flag, but has no fruit to advertise.

A third strategy is a miserly one that relies on animals' need for food in the lean months of winter and early spring, when better food is scarce.  This fruit is neither sweet nor high in fat, so is cheaper for the plants to produce.  Low quality fruit is the last chosen, often remaining on the plant into the spring.  Besides starving residents, these fruits will help migrating birds on their way back north.  Around here, the low-quality fruits are hawthorns, sumacs, chokecherry, greenbriars, roses, maple-leaved viburnum, foxgrape, poison ivy, hollies, redcedar, and bayberry.  

 The rose hips of this multiflora rose are low in boh sugar and fat,

and so may be hanging around for a loonng time.

Since reading the Field Guide to Eastern Forests, I have waited for fall to see if I could discern the high-fat fruit strategy, in particular.*  But I'm afraid migratory birds aren't keen on landing here in the urb: the flowering dogwood I've been able to reach has seen the fruit falling uneaten, or fattening squirrels.  I will have to visit Ponkapoag and its nice spice bush and sassafras populations and see if they're getting much traffic.

*Yes, strategy is a confusing word here, since it makes it seem that plants can make plans!  In reality, what looks like conscious planning is really the cumulative effect of variation and differential reproductive success: these modes of life diverged from common ancestors by mutations, and developed and still exist because the plants that have them succeeded and are still in the game.