Thursday, February 26, 2015

On the Mobbing of Owls

The photos in this blog post show six owl species from five genera being mobbed by potential prey species. Although only one individual of each prey species is visible in the photo, other individuals were present during the mobbings and sometimes multiple species were present. Mobbing is when prey species, usually in groups, harass a predator using loud calls and possibly also stereotypic movement sequences (that can recruit other birds to join the mobbing party). Note that mobbing is an interaction between predator and prey and is different to a competitive interaction as may occur between an owl species and a hawk species for example. Mobbing sometimes succeeds in driving a predator away from an area, which is an obvious benefit, but it also comes with risks. These risks may be higher when mobbing certain predators. Mobbing nocturnal owl species during the daytime is probably less risky than mobbing diurnal raptors and is sometimes successful in driving the owl away from a roost site; however birds that mob owls are sometimes killed by the owl they are mobbing. Although many owls seem disturbed by mobbing, some owls might benefit. One of the several competing hypotheses as to why certain species have large “false face” markings on the back of their head is to evoke a mobbing response that subsequently provides either a prey assessment opportunity or a hunting opportunity. Mobbing is not restricted to owls of course, nor is it restricted to nocturnal owl species (one of the photos above shows a day-active Northern Hawk Owl being mobbed). Sometimes recently fledged juvenile owls get mobbed and if they are knocked from their perch during the mobbing the result can be fatal (the owlets are vulnerable on the ground).

In the case of owls, the loud mobbing calls of diurnal songbirds are often the first clue for the human observer that a nocturnal owl is roosting nearby. Therefore, if you learn to recognise the mobbing calls of bird species that are common in your area you will increase your chances of locating owls.

We really don’t know why small birds mob owls but there are many hypotheses to consider. Curio (1978) listed nine hypotheses as follows: silencing-offspring, selfish herd, confusion effect, move on, perception advertisement, alerting others, attract the mightier, cultural transmission and site avoidance. These have been grouped into three main “classes” of hypothesis: parental care, altruistic or selfish (Ostreiher 2003). At this point, we don’t have enough evidence to evaluate which of these hypotheses is correct and therefore detailed observations of mobbing behaviour are especially useful. Especially in situations where you can distinguish the age or sex of prey birds, your observations may be informative. In the very rare situation where the observer understands the kinship amongst mobbers, there is also potential for garnering new information (kin selection is a possible mechanism to consider in behaviour of this type).  

Eastern Screech-Owl (Megascops asio) being mobbed by Blue Jay (Manitoba, Canada)
The crow family are some of the most frequent and most ardent mobbers of owls and they are capable of causing an owl to flee. This male Eastern Screech-Owl retreated into his roosting cavity when mobbed by Blue Jays (the female and large nestlings remained ensconced in the nesting cavity nearby).

Great Horned Owl (Bubo virginianus) being mobbed by American Crow (Manitoba, Canada)
Another example of a crow family mobbing owls. Crows are aggressive but tangling with a Great Horned Owl is risky business (I once observed a Great Horned Owl that was apparently fleeing from a flock of mobbing crows perform a near barrel-roll manoeuvre and grab a crow in flight, then killing and eating the crow on the ground).

Northern Hawk Owl (Surnia ulula) being mobbed by Gray Jay (Manitoba, Canada)    
Mobbing a day-active owl is also risky business but this nimble Gray Jay got the owl's attention but did not encounter any problems, nor did the Northern Hawk Owl budge from the perch. Although I have seen Gray Jays mobbing Northern Hawk Owls on relatively few occasions, I have never seen them mob Great Gray Owls and I suspect that may be because Great Gray Owls are so specialised in the prey selection that they almost never take birds.

Pearl-spotted Owlet (Glaucidium perlatum) being mobbed buy Senegal Eremomela, Cameroon
Mobbing a day-active Glaucidium species (pygmy-owls) is probably the most risky of all. These tiny owls are lethal predators and they strike with surprising speed. Some authors have hypothesised that the false-face on the back of the head of some pygmy-owls incites mobbing and that the owls use this to their advantage.

Peruvian Pygmy-Owl (Glaucidium peruanum) being mobbed by Amazilia Hummingbird, Peru 
Owls are mobbed by more than just songbirds as this Amazilia Hummingbird demonstrates. This is another case of one of those highly–feared pygmy-owls being mobbed by nervous prey species.

Tengmalm’s Owl, also known as Boreal Owl (Aegolius funereus) being mobbed by Red-breasted Nuthatch (Manitoba, Canada 
In North America, chickadees and nuthatches are amongst the most frequent of mobbers and they seem to understand each other’s alarm calls and readily mob owls together.
Of course, these are only a few examples.

Thursday, January 29, 2015

Subtlety and beauty

On a recent visit to the high Andes of Ecuador, I was absolutely delighted to find a truly extraordinary shorebird of the high elevation bogs and alpine tundra, The Rufous-bellied Seedsnipe (Attagis gayi). The seedsnipe are a small family (Thinocoridae) of only four species of rather stocky shorebirds, found only in South America, whose diet is largely vegetarian (seeds and plant material) unlike the majority of shorebirds.

These birds are exquisitely plumaged such that their camouflage is exceptional but when seen well the feather detail is also truly breathtaking. I will try to let the photos speak for themselves. The first photo shows two Rufous-bellied Seedsnipe in their habitat. Can you spot them? If not, take a look at the second photo for help...

If you spotted those birds, congratulations! Now here are two photos with a telephoto lens to show off their exceptional beauty up close and personal. 

I hope you enjoy looking at these photos as much as I enjoyed finding these magnificent birds!

Saturday, January 17, 2015

Tanager dreamin’ on such a winter’s day!

Just because we don’t have any tanagers in Canada anymore (the Piranga genus that includes species like Scarlet Tanager is now considered to belong to the family Cardinalidae) and because it is -30°C and blanketed in snow, I decided to share a splash on colour with some of the MANY tanagers (family Thraupidae) that have brightened my Neotropical haunts. I tried to pick 50 species that represented the diversity of the family (currently 374 species in 97 genera), although that wasn’t easy! I hope you are sitting down—this could be overload so I will add photos approximately 10 at a time over the course of this week!

Beryl-spangled Tanager (Tangara nigroviridis)
The seemingly electric colour of the genus Tanagara tanagers, such as this Beryl-spangled Tanager, have bedazzled many an observer. A good mixed-species flock of these gems feels like a kaleidoscope exploding in your binoculars!

Burnished-buff Tanager (Tangara cayana)
Possibly the best named tanager, this male Burnished-buff Tanager sure flexes some bicep colour!

Paradise Tanager (Tangara chilensis)
I missed the bright red rump in this photo but even still you can’t miss the Paradise Tanager!

Brassy-breasted Tanager (Tangara desmaresti)
This beauty is a Brazilian endemic from the southeastern Atlantic forest.

Golden Tanager (Tangara arthus)
Not so mellow yellow!

Saffron-crowned Tanager (Tangara xanthocephala)
Sheer elegance!

Bay-headed Tanager (Tangara gyrola)
The Santa Marta endemic subspecies is very green (other subspecies have blue underparts).

Scrub Tanager (Tangara vitriolina)
There’s no vitriol when you see this tanager! The Latin species name describes the colour, deriving from “vitriolium referring to suphates such as copper sulphate (>“vitrium” (glass)).  Note how this tanager is feeding on an insect/bug.

Blue-necked Tanager (Tangara cyanicollis)
A gem in the rain, here demonstrating the genus’s frugivorous side.

Green-headed Tanager (Tangara seledon)
These tanagers are sometimes attracted to fruit feeders allowing one to gawk... 

Swallow Tanager (Tersina viridis)
The unique Swallow Tanager is in a genus all of their own (this is a male, females are a delightful green).

Grass-green Tanager (Chlorornis riefferii)
Personally, I’d go with “canopy-green”!

Multicolored Tanager (Chlorochrysa nitidissima)
For such a colourful bird, the Multicolored Tanager is hard to find in the canopy. This range-restricted Colombian endemic is considered globally Vulnerable..

Orange-eared Tanager (Chlorochrysa calliparaea)
Another amazingly brilliant Chlorochrysa that lights up the forest (Alas, I have never managed to photograph them well).

Black-capped Hemispingus (Hemispingus atropileus)
A little less vibrant but very classy, the Hemispingus are also tanagers.

Rufous-chested Tanager (Thlypopsis ornata) 
May look like a warbler but this little gem is a high elevation tanager!

Hooded Mountain Tanager (Buthraupis montana)
The so-called “mountain tanagers” make high elevation birding a treat. Here is the large and beautiful Hooded Mountain Tanager of the high Andes.

Lacrimose Mountain Tanager (Anisognathus lacrymosus)
So named for the “teardrop” marking behind the eye, the Lacrimose Mountain Tanager has a rather unique orange colour in my opinion.

Scarlet-bellied Mountain Tanager (Anisognathus igniventris)
Studies have shown that nine out of ten birders run out of adjectives before they lower their binoculars on a Scarlet-bellied Mountain Tanager!

Blue-winged Mountain Tanager (Anisognathus somptuosus)
The Blue-winged Mountain Tanager needs no introduction as a very common species of the montane mixed-species flocks but “somptuosus” is an understatement!

Black-chinned Mountain Tanager (Anisognathus notabilis)
The range restricted Chocó endemic Black-chinned Mountain Tanager is a lot harder to find than the former species (though similar in appearance).

Golden-collared Tanager (Iridosornis jelskii)
Another truly extraordinary beauty of the high Andes! These tanagers seem to hit you where it hurts when you are already low on oxygen!

Blue-capped Tanager (Thraupis cyanocephala)
The Blue-capped Tanager is another tanager of mid to high elevations.

Blue-grey Tanager (Thraupis episcopus)
Perhaps one of the most widespread and common tanager species.

Golden-chevroned Tanager (Thraupis ornata)
Another endemic of Brazil’s Atlantic forest.

Brazilian Tanager (Ramphocelus bresilius)
Switching from blue to red… the Ramphocelus tanagers, like this Brazilian Tanager, have some extraordinarily brilliant shades of reds. This male also impressed me with his twinkle toes.

Crimson-collared Tanager (Ramphocelus sanguinolentus)
The Crimson-collared Tanager is another brilliant Ramphocelus tanager but with a little less red...  just don’t mistake them for a bishop or a cardinal!

Flame-rumped Tanager (Ramphocelus flammigerus)
In the Flame-rumped Tanager and a few other Ramphocelus the brilliant splash of colour is confined to the rump.

Lemon-rumped Tanager (Ramphocelus icteronotus)
The Lemon-rumped Tanager replaces red with yellow and was formerly considered conspecific with Flame-rumped Tanager.

Ruby-crowned Tanager (Tachyphonus coronatus)
Unless you see the red crown spot of the Ruby-crowned Tanager you could easily be forgiven for mistaking them for an icterid.

Flame-crested Tanager (Tachyphonus cristatus)
The Flame-crested Tanager replaces a red crown with a red crest!

Fawn-breasted Tanager (Pipraeidea melanonota)
The Fawn-breasted Tanager is now placed in their own genus. Tanagers also eat invertebrates as you can see here!

Magpie Tanager (Cissopis leverianus)
Who needs colour when you can be this classy in black and white!

Brown Tanager (Orchesticus abeillei)
The unique (monotypic) Brown Tanager showing the family’s more sombre side.

Rufous-browed Conebill (Conirostrum rufum)
The tanager family is actually remarkable large and diverse (374 species in 97 genera by current IOC taxonomy). Many tanagers go by other names and their diet can be primarily frugivorous, insectivorous or granivorous. Two largely insectivorous genera of tanager are referred to as conebills for obvious reasons. This Rufous-browed Conebill demonstrates how the cone-shaped bill is used.

Giant Conebill (Oreomanes fraseri)
The unique Giant Conebill (monotypic genus) is a denizen of the high elevation Polylepis forests and is listed as Near-threatened because of the high degree of habitat loss and fragmentation in this unique woodland type. This photo also gives a little glimpse at the gnarled beauty of Polylepis.

Bay-chested Warbling-Finch (Poospiza thoracica)
The warbling-finches are tanagers that feed on both invertebrates and fruit.

Cuban Grassquit (Tiaris canorus)
Some readers may be surprised to learn that there are some tanagers that are seed eaters with very finch-like bills (may also east small fruits). This includes the Tiaris grassquits like this magnificent Cuban endemic.

Plain-coloured Seedeater (Catamenia inornata)
They may look more like finches but the “seedeaters” are tanagers too! (yes, I know it is hard to believe)

Saffron Finch (Sicalis flaveola)
No, I am not kidding, these so-called “finches” (and certainly they are finch look-alikes) are really tanagers! If you’re just wild about saffron, this is a bird you’ve got to see!

Chestnut-bellied Seed-finch (Oryzoborus angolensis), female. 
No prizes for guessing what this tanager eats!

Wedge-tailed Grass Finch (Emberizoides herbicola) and Great Pampa Finch (Embernagra platensis)
Just in case you though all tanagers were forest birds, here are two seed-eating tanagers of grassland and shrubland in a single photo.

Red Pileated Finch (Coryphospingus cucullatus)
Also called the Red-crested Finch, this is yet another “unexpected” tanager in a genus with only two species. This is a poor photo but I felt this species should be included in this overview.

Red-capped Cardinal (Paroaria gularis)
Ironically (especially since the genus Piranga which goes by the common English name of tanager, as in Scarlet Tanager and Summer Tanager and others, has now been determined to belong to the family Cardinalidae), some real tanagers in the genus Paroaria go by the common English name of “Cardinal”. These recent taxonomic changes have certainly made the nomenclature very confusing. This Red-capped Cardinal is trying to pick a fight with their arch-nemesis!

Black Flowerpiercer (Diglossa humeralis)
Some tanagers are nectar specialists and have a hooked bill tip that enables them to pierce flowers and extract nectar. This Black Flowerpiercer demonstrates the unusual bill shape.

Masked Flowerpiercer (Diglossa cyanea)
This Masked Flowerpiercer shows how the bill tip is used in foraging. This method is less likely to pollinate plants than the more “straightforward” feeding style of hummingbirds but the flowerpiercers may still transport some pollen.

Red-legged Honeycreeper (Cyanerpes cyaneus)
As their name suggests, the honeycreepers are tanagers with a penchant for nectar. They also pack a lot of colour. This is a male Red-legged Honeycreeper.

Green Honeycreeper (Chlorophanes spiza)
A male Green Honeycreeper—widespread and unmistakable!
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Yellow-bellied Dacnis (Dacnis flaviventer)
Dacnis are a genus of small and stunning canopy-dwelling tanagers, mostly from lower elevations. When someone calls “Dacnis!” they will not be ignored (by other birders at least)!

Western Spindalis (Spindalis zena)
Spindalis is a Caribbean endemic genus that packs a lot of punch! This is a male Western Spindalis from Cuba.

I hope you enjoyed this glimpse into this incredible family of songbirds.
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