Thursday, February 25, 2010

Feeding Strategies

During short bursts of better weather between the hideous storms we've been getting of late (I currently can hear winds gusting over 50mph through our battle-weary Red Maple as I type), I've been noticing some of the more interesting feeding strategies that various bird species adopt which helps get them through the cold weather. Most obvious to our human eyes, of course, are the birds that visit our garden feeders - and they've certainly been descending in great numbers of late. Indeed, something of a competition developed amongst us local birders for the most unexpected feeder visitor, with not all of them being rarities. My personal best at our home feeder was the Eastern Meadowlark mentioned in an earlier post, while the zaniest at work was the party of up to 10 Mallards that wandered across from nearby Lily Lake, taking great care to walk single file up the front steps and around the side path to the backyard feeder! Many insect-eating birds adapt to prevailing conditions in one of two ways - they either migrate south for the winter, where they can continue to eat insects in the tropics or the summer months of the southern hemisphere, or they adapt to eating berries.

Yellow-rumped Warblers are common to abundant around Cape May in winter, which may seem strange for an insectivore....

....but as can be seen here, they survive readily by turning to juniper berries for food. By late January, most of these berries have dropped to the ground (if not already eaten!) and Yellow-rumps (or Sunny-bums!) can be seen easily on the open grass under juniper trees at the state park. Clearly however, a good snow fall can be a serious problem for this species, as many of the berries will be covered and unavailable as food.

More interesting than the backyard birds though, were the birds going about more natural ways of catching food, so here's some species showing behaviour that perhaps strays from what we may think of when we consider how birds go about finding a meal.

The concrete ship, stranded on a sand bar off Sunset Beach and gradually decaying into the salty waters of Delaware Bay, acts as a baffle in the face of outgoing tides, resulting in a lot of water churning. Several species of birds take advantage of this as the resultant upwellings bring potential food to the surface where the birds can reach it. During the winter, hardy flocks of dainty Bonaparte's Gulls feed here, but I discovered that they were employing another feeding method nearby, off the west end of the Cape May Canal.

Bonaparte's Gulls would fly in to an area just off the south jetty at the west end of the Cape May Canal. This picture shows the black bill and pale underwing that help to distinguish the species from Black-headed Gull.

Eventually a nice group of 26 birds was swimming on the water and I was able to get close to them by crouching among the granite rocks of the jetty. Here the pale pink legs can be seen well (as opposed to the red legs of a Black-headed Gull).

The gulls swam around for a while, peering intently into the water and I assumed they would be gleaning small morsels from the surface - but I was to be surprised!! Birds would periodically, quite suddenly leap up out of the water....

....and take a short flight, which terminated in them suddenly swerving back into the water....

...and coming up with a nice wholesome fish!! This party had found a nice, surface-feeding shoal of small fish, so had cut out the middle man and gone for the fish that were probably feeding on the titbits that they themselves would normally be making do with - smart move!

Inland to the woods, but still with a watery theme; I mentioned the Rusty Blackbirds in an earlier post and spent several lunch times watching these interesting birds. In fact, the feeding strategy they adopted was typical for the species, but did seem rather unexpected for a passerine and more what one would expect from a wading bird.

There's nothing that odd really about the Rusty Blackbird's feeding strategy, but it was interesting to see just how intensely they peered into the water of a slow-running stream. For it was here that they found their lunch. The birds specialise in hunting out the larvae of invertebrates that spend the early part of their lives in aquatic habitats. This shows a sensible exploitation of a potential food source that would otherwise be left untouched. What is interesting, however, is that - unlike dippers - these birds don't appear to show any special adaptations to this feeding style....

....they just work meticulously along the water's edge and peer in with their beady eyes agog!

Males and females all mix in together and stare and stare...

....and every now and then, one grabs a cracking mouthful! But what really makes this feeding strategy fun to watch.... the amazing lengths they'll go to for a good meal!!! Just look at the foot position of this bird and imagine recovering your place on the branch from here!

My final example of winter feeding strategies involves the assortment of ducks on Lily Lake. With ice covering the margins of the lake, only the deeper waters towards the centre of the lake remained open, potentially denying the dabbling (ie non-diving) ducks of tasty waterweeds growing below the surface of the water. But the Gadwall and American Wigeon on the lake have solved this problem....

Even the most casual scan of the ducks on Lily Lake would reveal something odd going on. Rather than being peacefully and evenly spread across the lake, the Gadwall and American Wigeon were clearly gathered in tight, boisterous gangs.

A closer look may at first reveal little more than that some are getting water kicked in their faces!!

But just now and again, an apparent interloper appears on the scene, in the form of a Ring-necked Duck (left).

And there laid the answer. What was happening was that the dabbling ducks were tagging along behind the Ring-necked Ducks - a diving species. When the duck dived and worked around under water, its movements created upwellings that brought up broken and disturbed pieces of waterweed. These the Gadwall were quick to snap up. Actually, though I didn't capture it on camera, some of the ducks took a more straightforward approach too and simply tried to steal food direct from the Ring-necks as soon as they surfaced. There's always a few thugs in every society!