Rook picnic

People were not the only ones enjoying the sunny and warm Sunday. A Rook walked around the picnic tables, intently looking around and underneath in search of scraps. A few people threw scraps to the rook and she filled her gular pouch with the tidbits. Its fledgling, precariously perched on a nearby tree, called. The Rook went back and forth taking food to its offspring.

House Sparrow communal courtship

 House Sparrows are quite vocal birds. In spring, a house sparrow colony is hard to miss, with males advertising their chosen nest sites to potential males by chirping and posturing, but mainly by their peculiar communal courtship. Communal courtships start when a fertile female not guarded by her mate flies by an unmated male, the male then will pursue her while chirping and displaying persistently soliciting copulation. More males usually join in the female chase and courtship in a cacophony of strident chirps. The female is far from passive to the male's attentions, and often lunges and pecks at the males (photo below), which seem quite unfazed. Forced copulations - and also accepted copulations - occur during these communal courtship displays.
 Male sparrows with large black chest badges are more dominant in the sparrow colony. They guard their mates during the fertile period - around the period when egg-laying is taking place - and copulate with her frequently. But males with large, in addition, join communal courtships more often than males with small bibs, so these dominant males may achieve paternity both though their mate's offspring, and through extra-pair copulations.
 Today, in a local farm, I managed to get these shots of communal courtship in House Sparrows. Two or three males were harassing a female, and she managed to peck one of the males (below).

ouch!

More information

Moller, A. (1990). Sexual behavior is related to badge size in the house sparrow Passer domesticus Behavioral Ecology and Sociobiology, 27 (1) DOI: 10.1007/BF00183309

Nectar feeding Blackcap

Yesterday I heard a male Blackcap singing on my way to work. Although it was high on a tree, luckily, I had a small pair of binoculars with me and I found him on a Sycamore in bloom. While I watched it, the Blackcap alternated singing and clearly drinking from the Sycamore flowers, in one occasion clinging upside down from a branch like a tit to reach them. It is well documented that several European birds, especially warblers, regularly drink nectar (I have posted on Blue Tits feeding on Mahonia nectar before in this blog). I could only take some distant shots of this behaviour, and when I got back, I did some research on the nectar feeding habits of the Blackcap.
 The Blackcap is actually one of the most nectarivorous European warblers. Nectar feeding in warblers is most common during spring migration, and has been documented in many stopover sites. Blackcaps often have stained faces due to their flower feeding habits (you can see some examples here and here. This can result on the formation of a 'pollen horn' a matted mass of nectar, pollen and feathers around their bills. Given that their long, thin bill leaves flowers intact after feeding, that they visit many flowers and the presence of pollen on their faces indicating regular contact with the flower's anthers, it is not surprising that Blackcaps can also act as pollinators.
 Experiments on birds trapped during migration indicates that migrating warblers seem to prefer nectar to mealworms when both are offered simultaneously. Nectar feeding is not a last food resort for emaciated birds, but a valuable source of easily digested carbohydrates when birds have reduced digestive systems and high energy demands.
 Blackcaps feed on nectar from a large diversity of flowers, both native and introduced. From the references below, I have compiled the following list of visited plants, which also includes some Mediterranean plants that they encounter during migration, and the Sycamore too. I note the native status in the UK.

1. Goat Willow, Salix capraea. Native.
2. Sycamore, Acer pseudoplatanus. Native.
3. Oregon Grape. Mahonia sp. Ornamental.
4. Citrus trees, Citrus. Mediterranean.
5. Hawthorn. Native
6. Damsons. Here. Native.
7. Almond blossom. here. Mediterranean.
8. Scrophularia. Here. Four large flowered species. Mediterranean, Canaries.
9. Aloe arborescens This female has so much orange pollen on its chest that almost looks like a robin. South African plant widely introduced in the Mediterranean.
10. Anagyris foetida. The only documented native European plant that is exclusively pollinated by birds. Mediterranean.
11. Giant fennel Ferula communis. Mediterranean.
12. Brassica oleracea-group.

More information

Ortega-Olivencia, A., Rodríguez-Riaño, T., Valtueña, F., López, J., & Devesa, J. (2005). First confirmation of a native bird-pollinated plant in Europe Oikos, 110 (3), 578-590 DOI: 10.1111/j.0030-1299.2005.13877.x

Ford, H. (1985). Nectarivory and Pollination by Birds in Southern Australia and Europe Oikos, 44 (1) DOI: 10.2307/3544053

Schwilch, R., Mantovani, R., Spina, F., & Jenni, L. (2001). Nectar consumption of warblers after long-distance flights during spring migration Ibis, 143 (1), 24-32 DOI: 10.1111/j.1474-919X.2001.tb04166.x

Jacopo G.Cecere, Costanza Matricardi, Beatrice Frank, Simona Imperio, Fernando Spina, Gabriel Gargallo, Christos Barboutis & Luigi Boitani. (2010). Nectar exploitation by songbirds at Mediterranean stopover sites. Ardeola, 57(1), 143-157.

Ortega-Olivencia, A., Rodriguez-Riano, T., Perez-Bote, J., Lopez, J., Mayo, C., Valtuena, F., & Navarro-Perez, M. (2011). Insects, birds and lizards as pollinators of the largest-flowered Scrophularia of Europe and Macaronesia Annals of Botany, 109 (1), 153-167 DOI: 10.1093/aob/mcr255

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