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Saturday, October 20, 2012

Migratory birds

Each year, millions of birds come to Nicaragua from the north, stay for several months, and then head northward again to nest. Their existence is treacherous, depending on excellent navigation skills, luck in weather patterns, and finding and maintaining adequate habitat at each end of the journey, year after year. Perhaps a quarter of the more than 700 bird species found in Nicaragua only spend part of their year in Nicaragua. Many of these birds spend a substantial part of their lives in Laguna de Apoyo Nature Reserve.
Tennessee Warbler
Tennessee Warbler (Oreothlypis peregrina) in Laguna de Apoyo. Photo by Pier-Olivier Beaudrault.
Some of these birds are seen as early as August, but most are first seen in late September. The forest is different with these birds present, for sure, because there are simply more birds than before. New arrivals are sometimes quite easy to detect, because the birds are tired from a long and arduous journey. The new arrivals may move sluggishly and less cautiously, and stay lower in trees while resting and recovering. Many don't survive the journey and the adjustment to the new environment with all its new threats and challenges. 

We have often wondered what the resident birds must think to find their habitat more filled than ever with new birds arriving from afar. Within weeks, the forest population seemingly doubles, with birds that probably have to be more active to survive, because they are the new kids on the block. After lots of rain, the forests are burgeoning with fresh vegetation and new flowers. The insects, nectar and fruits provide a feast for resident and migratory birds, alike. 

great crested flycatcher
Great Crested Flycatcher (Myiarchus crinitus) in Laguna de Apoyo Nature Reserve, Nicaragua. Photo by Pier-Oliver Beaudrault.
We often catch migratory birds during our studies using mist nets. Most of the catches are passerines (Order Passeriformes). Many of the passerines in North America are what we call songbirds (clade Passeri), which have vocal chords developed for unique and melodious songs. The Tennessee Warbler (Oreothlypis peregrina) is a common bird in our nets and in the forest canopy of Laguna de Apoyo Nature Reserve, too. The bird nests much further north than Tennessee, so Tennessee Warblers spend only hours or at most, very few days in that state. Perhaps it should be called the Nicaragua Warbler! Like almost all migratory songbirds, however, the Tennessee Warbler does not sing while in Nicaragua. It produces a weak, monotonous chip while in its southern home. It usually is found in a loose flock with other insectivorous birds, feeding on arthropods on the undersides of leaves in medium and tall trees.

The Great Crested Flycatcher (Myiarchus crinitus) is a migratory bird from the other major passerine division, the flycatchers and similar birds (clade Tyranni). Whereas more songbirds found in Laguna de Apoyo Nature Reserve are migratory than resident, the majority of the flycatchers are residents, and several other very similar flycatchers of the same genus, Myiarchus, are found in Laguna de Apoyo Nature Reserve.
migratory birds
The phenomenon of seasonal migration in birds is amazing, more so in the tiny Ruby-throated Hummingbird (Archilochus colubris). Photo by Pier-Oliver Beaudrault.
Hummingbirds are always everyone's favorite birds. Most of North America is inhabited with only a single hummingbird species, whereas ten can be found in Laguna de Apoyo Nature Reserve. There are plenty of hummingbirds year round for the birdwatcher, but sighting a Ruby-throated Hummingbird (Archilochus colubris) is always comforting. Such a small bird making such a long trip, twice each year, seems improbable, but we see them each year, and many are captured during mist netting.
migratory birds
The characteristic tail pattern in the Ruby-throated Hummingbird (Archilochus colubris) helps identify it in the field. Note the clipped leftmost rectrice (tail feather), to demonstrate the bird has been previously captured. The mark should last until the bird's next molt. Photo by Pier-Oliver Beaudrault.
The great northern bird migration coincides with flowering of many plants in Mesoamerica, and this is not a coincidence. By January, many flowers can be found in forests and fields in Nicaragua. Most migratory birds are not spending their entire winter season in a single spot, but rather, following the flowers and fruits as each tree peaks. Migratory birds tend to flock in their southern range and their distribution is patchy, concentrated near trees with food for them or for the insects upon which they feed.
migratory birds
Female Indigo Bunting (Passerina cyanea). Photo by Pier-Oliver Beaudrault.
Not all migratory birds eat insects and fruit. Lots of birds which inhabit our seas and shores migrate latitudinally. Other land birds consume small seeds, especially the sparrow-like birds. The Indigo Bunting (Passerina cyanea) is far less common than the Painted Bunting (Passerina ciris) in Laguna de Apoyo, perhaps partly because we are closer to the southern end of its southern range. We also have noted that the Indigo Buntings we have sighted and captured are almost all female. In many species, males and females migrate differently and reunite once on their northern territories. 
migratory birds
Swainson's Thrushes (Catharus ustulatus) are deceptively common in Laguna de Apoyo Nature Reserve, turning up often in mist nets during their southern season. Photo by Pier-Oliver Beaudrault.
The resident Clay-colored Thrush (Turdus grayi) is abundant in Laguna de Apoyo Nature Reserve, but other thrushes may be found here during migration. The ebullient resident birds tend to distract the birdwatcher from the migratory Swainson's Thrush (Catharus ustulatus). We capture them far more than we expected, based on birdwatching observations. Like many migratory birds, the Swainson's Thrush does not sing most of its time in the south, although a beautiful song might be heard as they begin their northward journeys.
migratory birds
The most abundant migratory bird in Laguna de Apoyo Nature Reserve is the Yellow Warbler (Dendroica petechia). Photo by Joe Taylor.
The most common and typical of the migratory passerine birds in Laguna de Apoyo Nature Reserve is the Yellow Warbler (Setophaga petechia). The tropical dry forest ecosystem of western Nicaragua is prime southern habitat for this bird. It occupies the outer portions of trees from high to low, moves rapidly, and repeatedly emits a loud, sharp chip. Other birds seem to flock around it. Where they are found, other, less obvious birds might be lurking, so a birdwatcher always likes to seen them.
migratory birds
The Louisiana Waterthrush is uncommon and local to wet areas. Photo by Pier-Oliver Beaudrault.
A lurking warbler which acts more thrush-like is the Louisiana Waterthrush (Parkesia motacilla). Like many more species, it is present in low numbers, so it is not seen just any day in any place. It is not so easy to distinguish it from its sister species, the Northern Waterthrush (Parkesia noveboracensis). This species is thought to be in decline due to elimination of habitat through channelization of streams and draining of swampy areas.
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