Sunday, January 29, 2012

Red-breasted Satellites

While hiking at the MacCready Reserve in southern Jackson County a few weeks ago, I heard what I was pretty sure was a White-winged Crossbill fly over me twice, but subsequent attempts to find the bird (or the flock that it was hanging out with) were unsuccessful.  This Friday, Don and Robyn Henise found a small flock of crossbills at MacCready, so this morning I went out to try and make up for my missed opportunity to record crossbills last weekend in Washtenaw County.

To make a long and disappointing story short, I found no crossbills, though Craig Robson and Heidi Doman did have a single bird fly over just prior to my arrival.  We also found evidence of recent crossbill activity, such as bits of tamarack cones on top of the snow (indicating very recent activity, given the snow accumulation today) and mangled Norway Spruce cones.

After parting ways with Heidi and Craig, I stirred up a mixed flock of birds in a transitional area between a small spruce stand and the red pines that dominate the majority of the reserve.  I actually got the attention of the birds by playing a White-winged Crossbill recording, which prompted a single Red-breasted Nuthatch to call.

Instantly, the calling of the nuthatch prompted a mob of chickadees, a few Blue Jays, a couple of White-breasted Nuthatches, and a Downy Woodpecker to swoop down and begin mobbing me.  I had hoped to record the entire succession of the encounter to illustrate some of the dynamics of mixed species flocks.  Unfortunately, in addition to having a dead camera battery, my recorder was having problems with the memory card, allowing me to make only brief bits of recordings as the scene unfolded.  I would later find a single Ruby-crowned Kinglet in this flock, which really got me down about my camera being MIA, given the rarity of this species this far north during the winter.

During the winter in temperate North America, chickadees, nuthatches, kinglets, woodpeckers and a few other species (e.g. Brown Creepers) aggregate in small flocks that travel and forage together.  This is thought to be a strategy that is selected for based on the benefits individual flock members can accrue, such as increased foraging efficiency, reduced risk of predation, and reduced vigilance that is required from each individual to detect predators (Sullivan 1984, Dolby and Grubb Jr. 1998).

What I find particularly interesting is the fact that the two groups of birds that are almost always found in these flocks (Parids and Sittids) have distinct functional roles within the flock during a predatory encounter -- roles that reflect the differences in the ecology of the two groups.  Nuthatches are primarily bark-foragers that are usually found high in trees, while chickadees and titmice tend to forage lower in the canopy and can be found foraging in shrubs, on tree branches, the ground, and even the bark of trees.  These differences are maintained in mixed flocks and have given rise to the terms satellite and nuclear species, respectively, to describe the spatial relations of these two groups.  This concept is not restricted to temperate North America, and species in a variety of ecosystems at (almost) all latitudes engage in similar relationships.

Depending on where a potential predator is approaching a mixed flock from, either group might be more likely to alert other members of the flock with an alarm call.  I've often noticed that when a hawk flies over or I approach a flock of birds from a distance in an open area, nuthatches and woodpeckers are usually the ones to expose my whereabouts.  Conversely, many a time I've watched Snapper wander into a patch of thick bushes, rousing the suspicions of chickadees and titmice.  This fits well with the available literature, which indicates that nuclear species are more likely to detect terrestrial predators in low-visibility situations, whereas satellite species are likely to detect aerial predators approaching the flock from a distance.

I actually did a little project on mixed flocks for an independent study last year.  Specifically, I was interested in finding out if there are any key parameters of alarm calls that birds might hone-in on to assess the level of a threat.  In order to investigate this, I altered different parameters of a White-breasted Nuthatch alarm call that I recorded at Dahlem last winter and conducted playback experiments to birds in the field.  In short, I found that increasing the amount of time between notes (by 4/10 sec.) and increasing the length of the notes themselves (by 5/100 sec.) significantly altered antipredatory responses of birds.  I really have no intentions of publishing any of these data, so below are barplots depicting these results.

The responses on the x-axis are categorical responses, meaning there is no 'order' as you increase or decrease the value.  "0" denotes no apparent antipredatory response, "1" denotes a "freezing" behavior that I usually observed when birds were already in an area with sufficient cover, and "2" denotes a bird immediately fleeing to bushes or other forms of cover.  The trend for the second plot is at least intuitively easy to explain (insert multiple caveats), but I'm still a bit puzzled by the results from the first.  Much to my delight, Ellis (2007) found very similar results when investigating key parameters of alarm calls in White-throated Magpie-Jays, which just might spur me to redesign my study and have another go at publishing the results.

In highly unrelated news, I think it's worth mentioning that the trails at MacCready today were absolutely covered with Snow Fleas (Collembola), which seemed unusual given the relatively cool temperatures and overcast skies this morning.  My only guess is that they might have emerged in large numbers yesterday during the sunny afternoon.  I wonder if the extreme day-to-day variation in weather that we're seeing this winter will have any effect on these hardy little beasts.  Oh, and I also saw quite a few Skunk Cabbage peaking through the snow!

Dolby, A.S., Grubb, jr T.C. 1998. Benefits to Satellite Members in Mixed-Species Foraging Groups: An Experimental Analysis. Animal Behavior. 56(2): 501-509.
Ellis, J.M.S. 2007. Which Call Parameters Signal Threat to Conspecifics in White-Throated Magpie-Jay Mobbing Calls? Ethology. 114(2): 154-163.
Sullivan, K.A. 1984. Information Exploitation by Downy Woodpeckers in Mixed-Species Flocks. Behaviour. 91(4): 294-311.

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