Birds migrate, eBirders count!

And it is again the time of the year where some impressive flocks of migrant birds can be seen in the sky on their annual southbound passage to South America through the Panama isthmus.  This is certainly true for the daylight migrants, like the raptors.  Since yesterday, some impressive flocks of both Plumbeous and Mississippi Kites have crossed Panama City's skies... although few Panamanians are aware of that.  For me, it was kind of special because I recorded for the first time a flock of Swallow-tailed Kites from my balcony.

Swallow-tailed Kites

Swallow-tailed Kite

Yes, now my balcony list stands at 118 species!  Just second before I recorded a huge flock of Plumbeous Kites that were new for my list too.  These birds approached from the southwest, took the thermal current very close to my apartment (thus I was able to see the black tails with white bars and the rufous primaries) and leave it very high flying eastward.

Plumbeous Kite

However, today was truly spectacular... at first, a little flock of 50 Plumbeous Kites flew low enough to see the same field marks I witnessed yesterday; then, a second group follow them with some Black Vultures... however, this were congeneric Mississippi Kites.  Notice the pale heads and pale secondaries of these birds.

Mississippi Kites and Black Vultures

While seeing this second flock, I noticed some "tiny spots" in the background.  After focusing it properly, I realized the these "tiny spots" were thousands of kites very high in the sky... too high to ID properly to species.  They were Ictinia kites for sure.  Immediately, I started to estimate the number of individuals in this Mississippi/Plumbeous Kites flock.  First, I quickly counted 100 individuals, got a sense of the proportion of the flock they take up and then extrapolated by hundreds the rest of the flock.  My estimation was 6000 birds.  It sounds straight forward... but it needs some practice; however, after a while you will make it automatically.  A very interesting article about counting birds can be found in the eBird main (or just click HERE).  Of course, to use this method you need a fairly uniform flock of the same species/group.  For example, this photo shows approximately one third of the flock I saw today:

All those dots are Mississippi Kites, with at least three Black Vultures mixed in (you may need to enlarge the photo)

For purely educational purposes, I divided this photo into four equal parts and counted individuals in one of these parties (which represent 1/12 of the original flock).  Do not pay attention to the size and shape of the red circles ... I only drew them for not count the same individual two or three or ten times!

533 Mississippi/Plumbeous Kites (and a Black Vulture); thus, a flock of 6396 individuals (533 x 12)

I did the same with the next picture, which I took with a larger zoom, and representing approximately one tenth of the flock:

Many Mississippi/Plumbeous Kites

578 Mississippi/Plumbeous Kites; thus, a flock of 5780 birds (578 x 10)

Using both estimates, I calculated an average: 6,088 Mississippi/Plumbeous Kites in that single flock (6396 + 5780 / 2).  That's why I wrote down that number in my eBird checklist and not my first estimate of 6000 birds... although they are pretty similar!  So what are you waiting for... it is time to practice and to look up for migrant flocks of birds!

Bird of the Month: Wedge-rumped Storm-Petrel

The Wedge-rumped Storm-Petrel (Oceanodroma tethys) is a relatively small member of the Hidrobatidae family (the Northern Storm-Petrels) that breeds on the Galapagos islands (nominate tethys) and on islets off the central coast of Peru (ssp. kelsalli).  This is the commonest storm-petrel species in Panamanian waters, present year-round, but commoner from May to November.

Wedge-rumped Storm-Petrel (Panama Gulf)

It is sooty brown overall (sooty black when fresh), with darker flight feathers and tail and pale upperwing panel and conspicuous white uppertail coverts.  There are slight differences among the two different subspecies, with kelsalli being smaller, shorter winged, shorter tailed, with smaller white "rump", slighter bill and with relatively long tail projection and more forked tail than nominate tethys.

Wedge-rumped Storm-Petrel (Panama Gulf)

Like other storm-petrels species, it have a steep forehead due to the large olfactory bulbs that facilitates a keen sense of smell, which is very important to locate food and for social interaction at its colonies.  Also like many other storm-petrels species, it can be seen pattering its feet on the waves while fluttering over the water.  This behavior is why it is call "Paíño Danzarín" in spanish (meaning "Dancing Storm-Petrel").

Wedge-rumped Storm-Petrel (off western Azuero Peninsula)

They are usually seen as small and dark little birds flying swiftly over the water among the waves, but they can be attracted by chumming to a boat, where you can have nice and prolonged views of these feathered marvels.

Wedge-rumped Storm-Petrel (Panama Gulf)

About specific ID to subspecies, more studies are needed to separate both subspecies in the field.  Presumably, all the photos in this post pertain to the kelsalli subspecies.  Notice the short arm, long tail projection beyond the white uppertail coverts, the forked tails and the small size noticed at sea.  We can not be 100% about the IDs, since both subspecies have been recorded in Panama and all these noted differences depend on wear, shape, angle of sighting, light and so on; however, some references indicate that the form found closer to shore is kelsalli, while nominate tethys seems to be more pelagic in general (more than 100 km from shore).  For these, and many other reasons, is why we chose the Wedge-rumped Storm-Petrel as our Bird of the Month!

Wedge-rumped Storm-Petrel (off western Azuero Peninsula)

Literature consulted:

1. Ridgely R, Gwynne J. A Guide to the Birds of Panama. Princeton University Press 1989.

2. Angehr G, Dean R. The Birds of Panama. A Field Guide. Zona Tropical 2010.

3. Howell SNG. Petrels, Albatrosses and Storm-Petrels of North America.  Princeton University Press 2012.

4. Carboneras, C., Jutglar, F. & Kirwan, G.M. (2014). Wedge-rumped Storm-petrel (Hydrobates tethys). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.) (2014). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from http://www.hbw.com/node/52591 on 1 September 2015).