May 3rd – Warblers At Last – The Fox Den Trail Racks Them Up

Ink and watercolour sketch of the Purple Martin boxes by Jason Currier.

Sometimes banding numbers don’t come close to describing what is actually happening. Today was like that. We banded a modest 38 birds suggesting that there wasn’t much happening but…..there was a massive movement going on – and was picked up by other aspects of our migration monitoring protocol. Banding is just one aspect of each day’s “workload”. We also keep track of the numbers of each species we encounter while doing net rounds and we do a formal census along a set route, counting each bird we see or hear. So, at the end of the day, we use all of these numbers to come up with an “Estimated Total” for each species encountered – this is simply an educated guess, based on these numbers, of how many birds we think were in the Ruthven banding area during the time we were there. We do this each day.

Two Blue-winged Warblers caught at the same time in the same net – probably chasing each other in a territorial skirmage.

We suspected something was up when Common Loons started going over right on cue: shortly after sunrise (at 6:17) the first one flew over and by 7:00, when they stopped, 21 had gone by, our biggest single day count for them of the Spring. We got a further hint when “firsts” for the year began to show up in the nets: Yellow Warblers, Ovenbird, Common Yellowthroat. But it wasn’t until we were closing that we were really “hit”. Matt thought he heard a Cerulean Warbler (he didn’t) so we all set off to try to find it. This took us along the Fox Den Trail. This trail, maintained by volunteer Anne Klaus, is a magical place anyway but today it was really special as there was a wave of warblers “dripping” from the branches above it: Blue-winged Warblers, Nashville Warblers, Yellow Warblers, Chestnut-sided Warblers, Black-throated Blue Warblers, Myrtle Warblers, Black-throated Green Warblers, Blackburnian Warblers (4 in one tree!), Western Palm Warblers, Black and White Warblers, and Northern Waterthrush.

When we were finally done for the day and I tallied up the numbers I discovered that we had encountered 71 species(!) including 8 that were new for the season. This was by far our biggest “diversity day” of the year so far. It was magical.

Here’s a little primer on telling how old some songibrds are. All birds molt their feathers. These molts are named after the plumage they result in. What used to be called a bird’s “winter plumage” is now referred to as its “basic plumage” and the molt that results in this plumage is called the “prebasic molt”. After the nesting season, there are two groups of birds: those that just bred (adults) and those that are the result of that breeding (juveniles). Most adult passerines or songibirds go through a “complete” prebasic molt; i.e., they molt ALL of their feathers – head, body and flight feathers. On the other hand, most juveniles go through a “partial” prebasic molt; they tend to molt their head and body feathers and, on their wings, their secondary coverts (the feathers covering the secondary flight feathers) but not their primary coverts (feathers covering their primary flight feathers) or their flight feathers: secondaries, primaries and tail feathers (rectrices). The next Spring these two generations of feathers can be readily seen in many species with a marked contrast between the age and colour of the primary coverts vs the secondary coverts. Below are a couple of clear examples (and give you an idea of what we’re looking for when we’re trying to tell the age of a bird we’re holding in our hand). [The important thing is to learn all the exceptions to this pattern!]

An older (ASY) male Rose-breasted Grosbeak – note all the wing feathers are the same colour. It went through a “complete” pre-basic molt after last year’s nesting season.

This young male Rose-breasted Grosbeak clearly shows a partial molt: black=adult feathers; brown=juvenile feathers.

Mixed generation of tail feathers (black=adult; brown=juvenile) on this male Rose-breasted Grosbeak.

Young (SY) male Yellow-rumped Warbler. Note the dull brown, enedged primary coverts (juvenile feathers) vs the black, white-edged secondary coverts (adult feathers) indicating a partial prebasic molt.

An adult (ASY) Yellow Warbler – Note the yellow edging to the primary coverts and alula. This would be lacking in a young (SY) bird.

Note that these “fault bars” on the tail of this White-throated Sparrow form a straight line suggesting that the rectrices were all growing at the same time – a sign that this is a young (SY) bird. Adults (ASY) molt their tail feathers a couple at a time resulting in “staggered” fault bars.

We also have a chance to show you the two different colour “morphs” of the White-throated Sparrow. There is a “tan morph”:

Dull tan morph White-throated Sparrow.

and there is a “white morph”:

Bright white morph White-throated Sparrow. {The one on the header is evern brighter!)

Research has shown that white morphs tend to be more aggressive and good at establishing and maintaining a territory. Tan morphs aren’t so aggressive but are much better at providing for their young. It was found that a pair that contained one of each was the most successful. Two white morphs spent too much time defending their territory and not enough provisioning their young and two tan morphs couldn’t maintain a territory.

Banded 38:
1 Mourning Dove
1 Tree Swallow
1 House Wren
3 Ruby-crowned Kinglets
1 Blue-winged Warbler
1 Nashville Warbler
2 Yellow Warblers
1 Myrtle Warbler
1 Ovenbird
1 Common Yellowthroat
3 Rose-breasted Grosbeaks
8 Chipping Sparrows
2 Field Sparrows
1 Swamp Sparrow
2 White-throated Sparrows
1 Eastern White-crowned Sparrow
1 Red-winged Blackbird
1 Baltimore Oriole
6 American Goldfinches