To us, home is an area where we are at ease, because we are familiar with neighbors, with local peculiarities of food and shelter, and we recognize friends and potential enemies. I suspect it is similar with ravens. Alan Gussow (in A Sense of Place, 1971) says, “A place is a piece of the whole environment that has been claimed by feelings.” Feelings of comfort with familiarity could motivate bird migrants to sustain their long arduous journeys, bringing them back over thousands of miles, year after year, to the very same bush. Do these animals yearn for home as we do? We do not know what they feel. We know only what they do. We also know only some of the adaptive reasons for our feelings, which make us do what we do. Yet, until shown otherwise, we can assume that the same powerful mechanisms driving behavior in one species apply in another.
Ecologists have generally called the area where a raven settles its “territory,” as though ravens behave like other birds, defending boundaries to maintain a specific spacing between themselves and neighbors. Territoriality is considered an adaptive response for monopolizing resources, especially those needed for rearing young. However, there is no reason to suppose that a territorial bird “knows” what it is doing. It just does it, and the behavior, even if unconscious, has served survival and reproduction. Home, or a place an animal stays because it is locally adapted, has a different connotation, but it is not exclusive of territoriality. Nevertheless, nest and home are entirely different. A bird’s nest is not its dwelling place. It is a temporary shelter or receptacle for rearing generally only one brood of young.
There are several ways to get insights into how ravens might locate and settle in a home area and whether they use it exclusively. One is to try to follow individual birds, the other is to see the result, determining where the birds end up. We have little information available on the first question, and very little on the second. Thomas Grünkorn, who was doing an extensive marking study of ravens in northern Germany, was trying to get answers by using both approaches.
I had never met him, and we had made no prior arrangement to identify ourselves at the Hamburg airport. After coming through customs, I saw a tall blond man hold a black feather aloft. I went over and smelled it. It had the distinctive musky raven smell, and so we identified ourselves.
Grünkorn had three years earlier finished his Diplomarbeit, similar to a U.S. master’s thesis, at the University of Kiel on a study of population trends and habitat preferences of ravens over a 7,200-square-kilometer area of Schleswig-Holstein, a northern state abutting Denmark. He had documented 344 of an estimated 400 to 450 active nests for the entire region. A crew of volunteer assistants included Volker Looft, a teacher, Hans-Dieter Martens, a policeman, Jörg Reimers, a computer programmer, and other dedicated unpaid amateurs. They had laid the groundwork over many years, and continued to help on nest surveys and the annual banding of the young.
Volker Looft had for thirty years monitored basically all of the breeding ravens and goshawks in one of the eight survey areas. He had shown Thomas most of the raven territories in his study, and he helped with banding the young ravens in April. Thomas reciprocated in Volker’s goshawk project, banding their young in June. Volker’s 2,300-square-kilometer area contained fifty to sixty pairs of goshawks. It also contained about the same number of breeding ravens, which usually nested near the hawks (because of similar habitat?). Volker and Thomas had banded close to 2,000 young ravens, and they were ready to band the current 1994 cohort in late April when I stopped by to visit. The study had already revealed that the raven population had rebounded nicely from a big decline in the 1970s.
We immediately made plans for the day following my arrival. Thomas had taken a week’s vacation from his job at an environmental assessment project, and planned to be out all day every day, climbing raven nest trees and banding the young. We convened at Thomas’s parents’ house in the village of Selk in Holstein. Before we left, I got a brief glance at their marked-up Schleswig-Holstein map. It showed a quiltwork of small green patches signifying woodlands, and many of these small patches had black dots on them. The dots represented the locations of the raven nests. The distribution of these nests was fascinating. They were not in a random pattern. If they had been strictly random, some would be close together, others far apart. These, however, were regularly spaced, as though each had some repellent force that radiated in all directions to other nests. The pattern of nests showed that in this part of Germany, the habitat was full of breeding ravens, each pair occupying about 43 square kilometers. If another raven pair were to fit in here or there, it would have to do so where a gap still existed.
The repellent force that created the regular nest spacing was not due to a feature of the landscape, such as the unavailability of nest trees. The probable force was intolerance—pairs of ravens repel one another. Although this was part of the explanation, it could not be the only one. In some other areas, raven nests are also regularly spaced, but they are more densely packed. It was unlikely that ravens of northern Germany were more aggressive and intolerant than other ravens, such as the Italian ravens on the island of Stromboli, where ten pairs nest in an area of only 12 square kilometers. To try to understand what provides the repellent force between nests, and what increases and reduces that force, requires a close look at the behavior of ravens subjected to a variety of conditions.
We hopped into a station wagon and drove over almost entirely flat countryside to the location indicated by the first black dot on the map. Broad fields were either recently plowed or just sprouting fresh new grass or fall-planted winter grain. The fields were broken up by “knicks”—long straight berms of earth and rock discarded from the fields covered with oak shrub, small white-flowering hawthorne and plum trees, hazelnuts, elderberry, ash, and other bushes. Except for a few trees left standing, these knicks were cut back regularly to make the brush sprout more profusely; a tangle of bushes and blackberry brambles then engulfed the berms. Hedgehogs, hares and cottontails, thrushes, nightingales, and other wildlife lived in these knicks and undoubtedly found them convenient corridors between the patches of forest that covered a mere 5.5 percent of the land. Country roads led to and through all of the forest patches, and almost all nests we would see would be within sight of, or a short walking distance to, our car.
We stopped to visit the local head forester, Christian von Buchwaldt, who led us to a recent new raven nest in a stand of mighty 190- to 200-year-old beeches. Buchwaldt told us these would not be cut down. I had never before heard a forester make such a claim for a managed forest. The beeches and oaks, trees some three to five feet thick, towered above us as we walked through the forest on the spring’s blooming carpet of white anemones and golden buttercups. In the wetter places grew giant alders, ash, carpinus trees, and birch. Patches of old forest trees alternated with clearings of planted young spruces and low brush, where wild boars had dug in the sod and elk and roe deer had browsed.
The trees had not yet unfurled their leaves, although throughout this warm day the beeches were uncurling their long brown buds, unfolding the tightly packed leaves like pale green butterfly wings. Sunshine dabbled the limbs and trunks of green-algae-tinged beech trees and the brown layer of last year’s leaves and beechnut hulls on the ground. For the moment, it was still possible to scan far into the forest for the nests of ravens, mouse buzzards, and goshawks.
Buchwaldt proudly showed us a rare black stork. Its red bill and red face were visible as the bird sat on a large, white-spattered nest in an oak. The black stork was at the time represented by only five breeding pairs in the whole region of Schleswig-Holstein.
Everywhere, the forest rang with the songs of chaffinches, coal and blue tits, nuthatches, winter wren, starlings, and thrushes. The bright, flashing yellow wings of a citron butterfly, flying quickly and erratically, caught my eye.
The sweetest songs and sights were, of course, the ravens. As soon as we came near a nest, the pair rose into the air and flew high above the forest, calling continuously. Unlike other birds
under similar circumstances, the pairs flew in tight formation, sometimes with their wing-tips almost touching each other. In New England, the birds at each nest make a great variety of calls, but these ravens made krrok, krrok calls and almost nothing else.
The beech and oak trees were giants compared to those in Maine. Smooth round boles reached limbless for at least eighty feet into the sky, with another ten to thirty feet of thick forks and branches into which the seemingly inaccessible raven nests were set. As we readied to climb the first one, I looked up, doubting the tree could be climbed.
Jörg threaded the end of a thin monofilament fishing line through the hole drilled into the end of an arrow. Volker then walked away with the arrow while Thomas unwound the spool of monofilament line. When 100 to 150 feet of line had been unwound, Volker came back while Thomas pulled the line back hand-over-hand to lay it in loose long loops onto a blanket spread out at his feet. With the line in a heap of loose coils on a smooth surface where it would not get entangled, he picked up his bow and shot the arrow with the monofilament line up and over one of the hefty branches in the tree’s crown.
The arrow proceeded along the intended trajectory, then fell back toward earth on the other side of the limb, pulling the monofilament line behind it. Thomas motioned for me to remove the arrow and attach a thicker nylon line to the monofilament line. Then he pulled the monofilament line along the path where the arrow had flown, to be replaced with the stronger line that would have been too heavy to shoot across. That line would now be strong enough to pull up a halfinch braided hemp rope. After Jörg pulled the thick rope across, Thomas attached a rope ladder to it, and we pulled the ladder up. Since the sixty-foot ladder was not long enough to reach the limbs, we attached a second rope ladder of equal length onto the bottom of the first. Pulling-up continued until the top of the first ladder reached the limb over which the rope was looped, and Jörg then pulled the rope taut and tied it securely around a tree trunk. The rope ladder in place, either Thomas or Volker climbing up, carrying small bags containing rings and ringing pliers on their belts.
Last year, with a Danish friend, Hans Christensen, Thomas banded 400 raven nestlings in Holstein and neighboring Denmark. Every year he does about 250 trees. Volker had done over 2,000.
After watching the two men climb several trees, I was most impressed. They walked up the ladder like walking up a flight of stairs. I tried as best as I could and failed. All my previous experience made me hang on and hug the tree or pull up with my arms, which had the unnerving effect of flipping the ladder with my feet on it, to the horizontal. With great mental effort (due to the stress) to visualize cause and effect, I overcame some of my previous training effect, and put more of my weight to my feet. I got high into the tree, but when panic reactions overrode my concentration, I again tried to rely on arm strength. Volker and Thomas no longer needed to think about what they were doing. To them it was by now all automatic and unconscious.
Once up the swaying rope ladder, Volker and Thomas would swing up onto the limbs, then almost leap from limb to limb to the nest still farther up. There they straddled the large limbs that held the giant nests. Sitting at the edge of a nest, as if on some great upholstered chair, they reached in and pulled out feathered birds, then attached the plastic leg rings to each of the one to five nestlings, and ambled back down the rope ladder to enter data into a notebook for later computer logging.
The plastic leg bands that Thomas had made for the 1994 crop of young were bright orange for easy future identification in the field. The birds of any one nest were banded with rings bearing the same number or letter. The individuals of any one nest were distinguished from one another by a white stripe, solid or broken, that was either at the top or the bottom of the ring. Thus, every individual was uniquely tagged and its origin coded. To them, each bird could now be associated with a specific nest, much like to a raven the ability to have (arbitrary) sensation of taste, sight, or smell might tag discrete experiences that are useful to remember.
While either Volker or Thomas was up in the tree attaching the leg bands to the baby ravens, Jörg and I wound up the monofilament line and twine and waited, lying on our backs in the soft dry beech leaves for ten to fifteen minutes. After the ringer came back down, we untied the rope from the tree, let down the ladder, and rolled it and the rope up to be carried to the car for the drive to the next raven tree, or to search for another nest at a suspected nest site.
It was 8:55 P.M. as Volker stepped off the rope ladder and back onto the ground from the fourteenth and last tree of the day. It was getting dark, and the redbreasts sang as the full moon was coming up. After we had rolled up the ladders for the last time, Volker walked briskly back to the car in time to listen to the latest soccer reports. By dawn, he and the crew would be off again to other nests.
With rigorous statistical procedures, Thomas had analyzed the distribution pattern of the raven nests on his map and proved that nests were indeed dispersed. The raven had always been thought of as highly territorial. These results by themselves confirmed that supposition. They also suggested that the habitat was “full,” and that no new nest sites could be expected. But the pattern of nest sites had changed over the years, and had surprisingly revealed something new that ultimately relates to the raven’s perception of their world.
Previous data from amateur ornithologists who had tracked the location of raven nests since their recovery in northern Germany in the 1960s showed that the nests were already dispersed, even when there were many fewer birds, well before the present nest density of 2.3 pairs per 100 square kilometers was achieved. Absolute size of territory had changed, but the tendency for nests to be located away from neighbors was preserved. This meant that territory size of ravens is not a given. Ravens do not exclude others at a set distance from the nest. Barring a sampling problem, territory size seemed to have changed through time. Territories were large at first, then in later years they apparently had shrunk. Surely, the ravens hadn’t changed. Their reactions to each other had.
Another pattern in the black dots on the map was that very few new ones had appeared in recent years. Each nest was still producing three to five young each spring, more than doubling the whole population of these very long-lived birds every single year. Why did the number of breeders stay constant? Johannes Goethe, working in the Mecklenburg area of northeastern Germany, and Derek Ratcliffe in Britain had found the same patterns of dispersed nest spacing, with at first a steady increase and then a limit or ceiling in the number of breeding pairs. What was happening to all the birds? They seemed to vanish.
Only isolated individuals of the 1,860 nestlings that Thomas and Volker had so far banded found breeding territories within his study area. Thomas had documented five individuals that settled down to breed, but only at ages seven, five, four, four, and four years, even though ravens can breed at the age of three years. These results showed that breeding in ravens is a privilege of the select few, and that privilege is not easily acquired.
Derek Ratcliffe had drawn data from the efforts of hundreds of eager cliff climbers and amateur British ornithologists who had for decades ascended to the nests of ravens. He had at his disposal an impressive data set on the population ecology of ravens in Britain. These data allowed him to assess how raven territoriality related to different land uses, the available food supply, and to raven persecution past and present. Ratcliffe determined that the key to nest spacing is primarily food supply. Where food is scarce, territories are large, and where there is much food, along seacoasts and in areas of intensive sheep pasturage, territories become smaller. In the Shetland Islands, for example, twelve active nests were found within 5 kilometers of a large garbage dump (Ewins, Dymond, and Marquiss, 1986). Ratcliffe concluded that under conditions of copious food supply, ravens become extremely tolerant of the close proximity of neighbors, and territoriality virtually breaks down.
Many corvids are mutually attracted to one another, and clumped nest spacing and even dense colon
ies is typical in species such as rooks, jackdaws, and pinion jays. Ravens are almost unique among corvids, and more like raptors in their generally wide nest dispersal and strong territoriality. With many birds, territoriality is considered the result of an inflexible and innate antagonism to neighbors that functions in monopolizing an adequate food supply, but is not directly related to it, so that territory size remains constant despite food supply. Is the raven different? Is the raven’s intolerance a function of its stomach contents? We don’t know. We’ve seen the result—the black dots on the map, but we’ve seldom seen the dramas that transpire to achieve the specific nest spacings.
I suspect that neither Grünkorn’s nor Ratcliffe’s results can be explained by one cause alone. Low food supply probably reduces the level of tolerance and causes nest dispersion, while increasing time of residency (as neighbors get to know one another) may increase tolerance, instead tending to reduce nest dispersion.
We know that the ravens’ adult occupancy in their territory is not all-or-nothing. For several years, I have seen a pair spend weeks near a cliff less than a mile from my house in Vermont. They roost there at night, and in the spring start to bring sticks for a nest foundation, then leave for a month or more. Goliath and his eventual mate Whitefeather, a wild raven, who was a sometime resident of my aviary, were absent from their territory for a two-week stretch after breeding in the summer of 1996, and for months during the following years. After finding a mate, ravens apparently try to settle down, but whether they are successful in becoming permanent residents and consistently raising young is another matter. It may take a long time to get established, and even once a site is chosen, the birds do not necessarily stay there all the time. “Vagrant” and “resident” are not absolute categories. Individual differences, or the birds’ finely attuned responses to conditions, could determine just how resident or vagrant they are.
Mind of the Raven: Investigations and Adventures With Wolf-Birds Page 11
