A citadel of herons

As the man swings his bucket, scattering silvery fish through the air, an extraordinary balletic display commences. Gangling, prancing birds, one metre high, jostle in competition. Spiky black crests spring up and vast grey wings are spread like capes. Strong orange bills grasp their catch. Long, sinuous necks bulge as it is swallowed.

The scene is Barcelona Zoo, and the birds putting on the show are grey herons (Ardea cinerea). A common enough species but rarely seen in the centre of a city, especially in such quantities and proximity. There are over 125 pairs in this corner of Ciutadella Park, nesting high in the treetops, which makes it the largest urban heronry in Europe.

The herons are sometimes mistaken by visitors as another exhibit, a decorative extra thrown in for the price of the ticket, like the peacocks who have the run of the place. But they are wild birds who have chosen to live in close proximity to man. When hunting for food outside the zoo grounds, they revert to extreme wariness, fleeing at the slightest human intrusion. Inside, other rules apply, providing a tremendous opportunity to observe them close-up.

Walking through the zoo on a mild spring morning, I spotted a heron in a palm tree snaking its neck to pluck some nesting material. Another had alighted in a plane tree to present a long twig to its mate. The pair raised their crests in greeting and exchanged raucous “ruarks”. Over by the penguin pool, herons kept guard, glassy eyes giving nothing away as they waited for feeding time.

Gradually, the zoo geared into action. A train laden with visitors wound along the paths, bell ringing. Flaming orange Cuban flamingos were in display mode, trumpeting in formation. Peacocks were screaming. Children on a school trip were yelling “Baloo! Baloo!” at a pair of slumbering bears. Adding to the congestion were lines of wide-eyed tiny tots hanging onto long ropes. Dodging all these obstacles were the zoo staff, mounted on bikes.

The heronry adds a few more decibels to the general cacophony. By the primate and pelican enclosures, a sign advises visitors to look up. But my attention had already been drawn to the treetops by a strange nattering sound.

Overhead, the towering plane trees were filled by large untidy nests. Scanning the boughs, I saw that several were occupied by chicks, the source of the babble. Goggle-eyed, spiky and ravenous, they were leaning out precariously, wobbling their throats and demanding food.

Though still weeks away from acquiring the sleek elegance of their parents, some of the chicks were already quite grown, a sign of the colony’s success. Mild Barcelona weather encourages early breeding. A record was set in 2007, after a particularly balmy winter, when the first chick hatched on January 8th. With such favourable circumstances, some herons undertake two broods a year.

Ornithologist Josep García, who has studied Barcelona’s herons for more than 20 years, told me that “this is the oldest heronry in Catalunya. It was founded more than 30 years ago, in 1974, by a pair of captive birds with clipped wings. Their offspring were free to fly away, but surprised everyone by returning to the zoo to breed.”

As Barcelona is on a major bird migratory route, other herons soon spotted the nests and came down to investigate. They obviously found the conditions in the zoo to their liking and moved in.

A benign climate is not the only advantage of the location. Surrounded by a densely urban environment, the herons have few predators to worry about. The principal danger for the chicks is falling from the nest or being pushed out by a rival sibling. In the Llobregat Delta, by Barcelona’s airport, where herons nest low down among the reeds, conditions are much harsher. García explained that the small Llobregat colony “suffers intense predatory pressure from the introduced American mink, as well as boars and birds of prey like the marsh harrier and Bonelli’s eagle.” In the zoo, their main enemy is the nest-raiding black rat.

Spring is a busy time as García monitors the entire heron population of Catalunya, wading unsteadily through lagoons and climbing shaky ladders to look at nests. In the zoo, he has the amenities of a city at hand, and glides smoothly upwards in a tree pruner’s lift. I ask him about his chick-ringing strategy.

“It’s very important to know the exact moment of hatching. The ringing is done when they’re about three weeks old. Leave it any later and they’re able to escape by hopping onto another branch. I normally throw a towel over them so they stay calm, and I can ring them and take samples by just uncovering a leg.”

I wondered if the large concentration of nests bothers the visitors of the zoo, there being a certain risk factor when walking underneath.

“The chance of being hit by a bird dropping is quite small. Sure, it’s never pleasant, but the people who come to the zoo have some kind of interest in the animal world, and when they discover the sheer size of the offender and the number of nests above their heads, they are entranced rather than annoyed.

“And the danger of falling nests is extremely low,” he continued. “Do you remember the gales that hit Barcelona a couple of years back, ripping up roofs, traffic lights, trees . . . well, not one heron nest fell!”

Just as my neck was starting to ache from so much treetop gazing, a keeper approached the pelican enclosure and herons began parachuting down. I felt the turbulence generated by a 1.85 metre wingspan as one settled on the railing by my side. Ignoring me, it only had eyes for the bloke with the bucket.

Most of the colony in fact disperses to feed, mainly to nearby rivers. Watching herons fish in the Llobregat Delta, you’re struck by their patience and stealth. Often remaining motionless for long periods, they will suddenly snatch their prey out of the water. But those who take their chances in the zoo, hoping to steal some of the fish destined for the captive population, need to move fast in the face of fierce competition.

Electrified herons, with their most decorative plumes rising like the hackles of a cat, were soon engaged in tugs of war over spoils. One managed to fly off with a large trout, only to let it fall slapping to the ground. Another opportunist engulfed the fish on the spot, neck swollen like a recently-fed python. One heron dared to take an offering directly from the keeper’s hand, maintaining him safely at neck’s length.

Fish are an important but not the only part of the grey heron’s varied diet. They will take amphibians and insects, as well as the eggs and chicks of other birds if available. Josep García has observed surprising interactions between herons and their neighbours in the canopy, Monk parakeets. Relentless twig-collectors, the Monks construct huge communal nests, an irresistible source of material for the herons’ own large structures. In their raids, the herons have discovered the edible contents of the multi-chambers within, much to the helpless agitation of the parents.

The flamboyant herons have found a unique niche in Barcelona, which they’re exploiting with great success. They have transformed the zoo with their presence, as well as the city’s sky. Look up at any time of day and you might see their distinctive silhouettes, neck folded, broad wings beating steadily as they cross from breeding to hunting grounds. In the quiet of early morning, before the traffic roar builds up, their wild harsh cries carry far.

FACTS AND FIGURES

• The zoo heronry was founded in 1974.

• Until 1992 it was the only stable heron breeding colony in Catalunya.

• In 1997 the herons were joined by two other members of the ardeidae family: the Cattle egret and Little egret.

• Landmark years in the colony’s growth were 1997, when 21 new pairs swelled the number of nests to 62, and 2003, when they reached 106.

• The breeding period in the zoo can last from the end of December until the end of July or beginning of August. In 2007, the first egg hatched on January 8th.

A grey heron’s vital statistics:

Length—94 cm

Wingspan—185 cm

Weight (male and female)—1.5 kg