Mallard portrait

Although I used this photo in my other blog, I thought it is fitting here too. A drake Mallard in the park that didn't flinch and rested while I took his photo.

There is more to the eyes of the Robin

We are used to see Robins all year round, and indeed, although is a large resident population in the UK, they are migratory birds in much of their range, with northern European birds migrating every winter to Southern Europe and North Africa. Many of these migratory Robins pass through or winter in the UK. Robins migrate at night, and much research has been devoted to find out how they manage to navigate during migration. Human navigation uses a 'map and compass' system, to find out where we are and what direction to go. Recent discoveries on bird navigation have shown that they rely on an internal magnetic sense. Indeed, although many migratory birds are known to have a magnetic sense, the first demonstration of a magnetic sense in animals came from experiments on caged Robins. Experiments to document migration orientation usually involve using what is called an Emlen funnel, a round cage in the shape of a funnel covered on blotting paper on the sides, a clear top, with an inkpad at the bottom, something like this:

Emlen funnel 

 Caged migrant birds become restless at night during the migration season and jump more often in the direction where they intend to migrate, making little ink prints on the paper. Caged robins changed their heading accordingly when the magnetic field was artificially altered using strong magnets. Migratory birds magnetic sense is extraordinary as it relies on light as detected by the eyes - in particular the right eye -, which is interpreted in a magnetic way. Photons exciting a particular molecule (called 'cryptochrome') in the retinal cells form radical pairs sensitive to the magnetic field. The information reaches the brain through the optic nerve and the bird is able to form a map of intensity of the magnetic field overimposed to their visual field. The intensity of the magnetic field is region specific, helping the bird to navigate even in low light conditions. Recently, Katrin Stapput and collaborators found out that this magnetic sense depends of the bird having a clear image on their right eye as Robins that had their left eye covered with a frosted goggle - which prevents them from forming a clear image -could navigate, while if the right eye did not form a clear image they got disorientated (see their result below).

Robin wearing goggles (from Stapput et al 2010)

Orientation of European Robins during Spring Migration with the Local Geomagnetic Field as the Only Cue. (A) In the binocular control tests without goggles (Bi), the birds preferred their normal northerly migratory direction. (B) Birds tested binocularly in a radio frequency field (BiRF) were disoriented, indicating that the orientation was based on radical pair processes. (C) When the right eye was covered with a clear foil and the left eye covered with a frosted one (Rclear), the birds were oriented in their normal migratory direction. (D) When the left eye was covered with a clear foil and the right eye covered with a frosted one (Lclear), the birds were no longer oriented. The triangles at the periphery of the circles indicate mean headings calculated from three tests of the individual birds; the arrows originating from the center represent the grand mean vector drawn proportional to the radius of the circle = 1. The two inner circles are the 5% (dashed circle) and 1% (solid circle) significance borders of the Rayleigh test. (From Stapput et al 2010).  

 Next time you see the lovely bright eyes of the Robin spare a thought for the amazing things they can do with them.

More information
Wiltschko, R., & Wiltschko, W. (2006). Magnetoreception BioEssays, 28 (2), 157-168 DOI: 10.1002/bies.20363
Stapput, K., Güntürkün, O., Hoffmann, K., Wiltschko, R., & Wiltschko, W. (2010). Magnetoreception of Directional Information in Birds Requires Nondegraded Vision Current Biology, 20 (14), 1259-1262 DOI: 10.1016/j.cub.2010.05.070

Breaking sticks

The nesting instinct is starting to stir in the Crows. Early last month, the pair above examined a nest from last year, one individual getting into the nest and doing some rearrangements. I am not aware of Carrion Crows reusing old nests, so maybe they were just suddenly experiencing an urge to sit on a nest again. I haven't seen the birds visiting the old nest again.
 Today, I noticed a crow struggling on a tree. It was trying to break a twig using all its might, flapping its wings and leaning back. Unlike other birds, which collect sticks found on the ground, Carrion Crows, Rooks and Jays use fresh sticks, that the break from trees as the material for the main framework of their nests. Then they will look for bark, hair and grass as lining lining on the ground. After a couple of unsuccessful attempts, the crow moved to another part of the tree, and this time it was successful first try, cleanly snapping the stick. It then rearranged the position of the stick to pick it up securely balanced from the middle, and flew away. The crow nesting season has already started.

Territorial Great Crested Grebes

The breeding season of the Great Crested Grebes is approaching. I have never seen their spectacular courtship so I thought I would pay them a visit in a local park where they breed regularly. I had almost given up on finding them when, from the bridge, I located a winter plumage individual at the far end of the lake. When I got there, I saw that there were two winter plumage individuals, and then an individual with full breeding plumage emerged from the water quite close to me (above). Breeding plumage individuals have a long dark crest and orange and black 'tippets' on the sides on the head, while these are lacking in the winter plumage. I sat down and watched the interactions between the breeding plumage and one of the winter plumage ones. The breeding plumage individual watched the other one, and approached slowly, nonchalantly, occasionally preening its back, or shaking its head, but the winter plumage one appeared to take offence, and swam in the aggressive posture of this species, with head low and stretched forward over the water, and then started to fly, while paddling on the water, towards the breeding plumage one and kept it away. Occasionally it would try to approach diving (see video below). This behaviour went on for quite a while. Both winter plumage individuals appreared tolerant to each other and at some point they stayed close together, so I wonder if I they were displaying early territorial behaviour, and the breeding plumage individual was trying to find a new mate.
 Great Crested Grebes show some variability in their head moult and in February a range of plumages from full winter to full breeding can be seen, so the three individuals were likely adults in different stages of their moult.

Changes in the plumage of the head. The plumage types of individual grebes in relation to date are

given (males = filled and females = open symbols). Dots refer to scores based on examining photographs of grebe-heads, whereas the triangles indicate the results of examining museum-skins. (from Piersma, 1988)

Winter plumage individual on agressive posture,  a few crest feathers and the dark tips of the tippets have already emerged.

 Breeding plumage individual preening its back feathers

It appeared to try and grab the attention from the winter plumage individual, and... 

...got very close. 

But it got the wrong sort of attention, as the winter plumage individual threatened it

the summer plumage individual flies away. 

The two winter plumaged individuals close to each other, watched by the breeding plumage one

A couple of short videos on the interactions between the two individuals.

More information

Theunis Piersma (1988). The annual molt cycle of Great Crested Grebes. Ardea, 76, 82-95

The beak of a grazer

This male Greylag approached me nervously in the park today. It hissed regularly while walking towards me, probably expecting handouts. I crouched down and tried to get a close shot of its beak as it hissed. Geese - and the duck family, Anatidae, in general - have very distinctive beaks with a rounded, 'nail' at the top tip (young Greylags have a black nail) and serrated edges alongside both sides of the upper and lower bill. In the case of the geese, these are quite hard and sharp, remarkably teeth-like (they can draw blood if they get to bite your finger). When looking at the photo I noticed even the tongue has serrated projections. In geese, these help cut grass and plant material when they are grazing. In some ducks these edges have wafer-thin lamellar forming something akin to a whale's baleens, helping them filter out small planktonic organisms from the water.

Crow roost at the large poplar

For over 15 years, there has been a Carrion Crow roost at University. In winter, I used to leave work when the crows started to gather on a large poplar tree to spend the night. Each arrival being greeted with raucous calls. I found the atmosphere amazing as their vocal repertoire is so diverse: gurgling, croaking, trumpeting, beak drumming, almost quacking, compounded by the often different tone of voice of different individuals. Tonight, I happened to be by the roost at the right time again. At least 50 crows were already assembled on the top branches, leaving respectful spaces between them, members of pairs closer to each other, and, as I was approaching, pairs and bands of crows flew over me to settle on the tree.
 Crows can fly long distances to roost in their traditional trees (I have read up to 50 miles for American Crow). It can be well past sunset when they settle and they leave first light in the morning. When the nesting season starts, around March, territory holders stop roosting communally, while non breeding birds might carry on sleeping on the traditional roost.
I took this little video of tonights' crow night-time assembly.

City scavengers

We humans can make a mess. Bins left open or toppled over, the remains of takeaways eaten on the street thrown to the pavement. It is not a pretty sight, but it means extra feeding opportunities for some. Birds take advantage of their surroundings: they eat the berries, buds, and flowers of the trees we plant in our gardens, they do reward our love for them feeding on delicacies on the bird table, but they will also eat our rubbish. Today, a Carrion Crow (top shot) searched for tidbits under a rubbish bag using that lovely crow technique of delicately picking 'things' and moving them aside. They do the same when feeding on leaf litter,  and it is quite different to the energetic, almost full body technique for the same effect used by Blackbirds. The Carrion Crows and Blackbirds often feed at the back of a row of flats, where bins are often left open and bin bags on the ground. My favourite sighting of a scavenging crow was one strutting on a pavement with a large sausage held across its bill, a pity I was driving at the time! Seagulls are supreme city scavengers too, with their bouncy, manoeuvrable flight, Black-headed Gulls are able to avoid the passing traffic - as they will avoid waves out at sea - to deftly dip in the middle of the road to pick items from in between cars.

Snow bonanza for a male blackbird...

...and a female too

Even Robins enjoy takeaway pizza

Black-Headed Gull on the lookout for feeding opportunities