The Impact of Food Availability on Gyrfalcon (Falco Rusticolus) Diet and Timing of Breeding
From the book Birds of Prey, in courtesy of Dr. Olafur K Nielsen and Des Thompson
The gyrfalcon Falco rusticolus is a big and a powerful falcon of arctic and boreal alpine areas. It has long, broad, tapered wings and long tail, and is capable of making very fast and prolonged aerobic flights catching prey (Pennycuick et al., 1994). The feet are strong, with thick toes and long sharp claws, and are used to knock down or grab prey, both in the air and on the ground. The gyrfalcon is a solitary hunter and searches for prey over open country, either in fast low flight or by high flying and soaring. It frequently uses perches for observations during hunting forays. It is thinly distributed within its breeding range and feeds mainly on birds, but also to some extent on mammals (Clum & Cade, 1994; Cade et al., 1998). This paper describes the falcon’s diet during the breeding season. Special attention is paid to how annual events within the population of its main prey, rock ptarmigan Lagopus mutus, affect prey selection and relate to the breeding phenology. The data come from a 20 year study in North-east Iceland (Nielsen, 1999a).
Previous studies have shown that the gyrfalcon has specialized feeding habits, relying mainly on rock ptarmigan and willow ptarmigan Lagopus lagopus (Bengtson, 1971; White & Cade, 1971; Mikkola & Sulkava, 1972; Pulliainen, 1975; Langvatn & Moksnes, 1979; Woodin, 1980; Bente, 1981; Lindberg, 1983; Poole & Boag, 1988; Huhtala et al., 1996). Other important prey groups include waterfowl (Anseriformes), auks (Alcidae), waders (Charadriiformes), hares (Lagomorpha), and microtine rodents (Microtinae). The importance of alternative prey depends on the abundance of grouse (Nielsen, 1999a). Among bird-eating raptors in temperate regions the general rule is for juvenile birds to dominate the diet during the brood rearing period (Newton, 1979; for Accipiters see Opdam et al., 1977; Opdam, 1978; Newton & Marquiss, 1982; Toyne, 1998). Studies have shown that the gyrfalcon is unusual in this respect by going through the breeding cycle by preying mainly on the adult segment of the ptarmigan (grouse) population during its annual low point in number (Hagen, 1952; Cade, 1960; this study). In this chapter I show that this pattern holds true during all phases of the ptarmigan cycle, and in all habitats, with predation directed mainly at ptarmigan males during the early part of the season and females later on.
The gyrfalcon study area, total 5,327 km2, was in North-east Iceland, and centred on Lake My ́vatn (65° 38’N, 17° 00’W). The countryside is wide and open, and dominated by heath and meadow vegetation, and sparsely or unvegetated land. Eighty-two traditional gyrfalcon territories were known, and 39–63 were occupied each year during 1981–2000. The falcons hunted in all available types of habitat. A total of 61 bird species bred in the area, and all, except for some of the larger species, were potential prey for the falcons. Rock ptarmigan was common and widespread. For more details see Nielsen (1999a).
Gyrfalcon food habits
Gyrfalcons carry prey back to the nesting cliff during the breeding season. This starts 2–3 weeks prior to egg laying, and continues well into the post-fledging period. The hunting bird plucks, beheads, and disembowls medium and large prey at the kill site, but small prey were carried to the nesting area more or less intact. During courtship, egg laying and incubation, prey remnants were gathered at plucking posts in the immediate area of the nesting cliff. During the nestling period most of prey remains were collected from the nest itself or on the ground beneath. After fledging, prey remains were again found on plucking posts, but as the young got older and strayed farther afield, these sites became increasingly difficult to find. Remains of medium-size to large prey consisted mostly of skeletal parts (the pectoral and pelvic girdles and the sternum). Frequently bones were disarticulated, and the keel and the distal part of the sternum and the lateral parts of the pelvis were bitten off. Remains of small prey consisted mostly of disarticulated legs and wings, but also some heads and loose feathers.
All of my data come from collections made at nest sites that successfully fledged young and where I was able to gain access to collect remains. Two or three collecting trips were made to each nest during the season. The last visit was always made after the young had fledged. During each visit I removed all remains, including all bones and pellets, and also some feathers. Prey were identified in the field, and the remains discarded. For the analysis, remains were arranged according to species, age groups, and type. Young of the year were separated from adults on down, feather pattern or bone structure. The number of individuals in each group was estimated by counting the most frequent item representing one individual. For medium-size to large birds this was most often the sternum or wing, and for small birds and young of the year one of the legs. Pellets were only collected to look for legs and skeletal remains of small birds. Problematic specimens were brought back to the laboratory and identified with the help of a reference collection. Problems and possible pitfalls in using prey remains to study gyrfalcon diet have been discussed by Hagen (1952), Langvatn (1977), Poole & Boag (1988), Huhtala et al. (1996), and Nielsen (1999a).
The gyrfalcon territories were divided into three groups according to the distance of nest sites from the coast or a primary waterfowl area, e.g. Lake My ́vatn, Laxá River, Lake Vestmannsvatn, Lake Víkingavatn, and some other sites. These groups were termed ‘heath falcons’, ‘lakeland falcons’, and ‘coastal falcons’. Heath falcons were located on average 19.2 km from primary waterfowl areas (range 7.5–33.3 km), and 29.5 km from the coast (9.3–62.0 km). Lakeland falcons only had to travel on average 3.4 km to reach primary waterfowl areas (range 0.1-9.0 km), but 33.9 km (5.5–63.0 km) to the coast. Coastal falcons were on average 3.6 km from the coast (range 0.8–7.5 km), and were situated far from inland waterfowl areas, but had ready access to both adult ducks and eider colonies and brood rearing areas (see Appendix 24.1 for scientific names of prey species).
In 1982–1985, 1987, 1988, 1990, and 1991, samples of adult rock ptarmigan in the gyrfalcon’s catch were sexed. Sampling was done for three periods during the breeding season: April–May, June, and July. Sexing was done by measuring humeri, either left or right, from each sample. Measurements were done with callipers, to the nearest 0.05 mm.
In 1982–1985 only humeri length was measured (from caput humeri to condylus ventralis; terms according to Baumel, 1979). The mean length of male humeri was 59.68 mm (range 57.60–62.30 mm, n = 154, s = 0.9902) and the mean length of female humeri was 57.61 mm (range 54.60–59.90 mm, n = 193, s = 0.9846). There was considerable overlap between the sexes. To sex rock ptarmigan prey, I started by using 57.40 mm and 59.90 mm as dividing values for females and males respectively and calculating the normal cumulative distribution using the mean sex standard deviation. The mean values for humeri ≤57.40 mm was 41.4% females and 1.0% males, and for humeri ≥59.90 mm it was 41.2% males and 1.0% females. These values were used to sex birds from an independent sample of 149 humeri of known sex. Eighty (54%) could be identified to sex, of those two birds were in the wrong category (3%). The sex ratio (males/females) in the original data was 0.3925 and in sample 0.3559, the difference was not significant (χ2 = 0.151, df = 1, P = 0.65). Using these values on the total number of humeri measured (n = 2,556) allowed me to sex 46% compared with 42% of the expected proportion. In 1987 and later two measurements were taken from each humerus, total length as before, and breadth at the distal end. A discriminate function was derived to separate the sexes. The function is:
(L × -0.7529) + (B × -1.9121) + 64.6407 >0 = male; <0 = female
where L is length of humerus in mm, and B is breadth at the distal end in mm (from epicondylus ventralis to epicondylus dorsalis). This function was tested on an independent sample of 147 sexed humeri, of which 86% were correctly categorised. Because of the large number of wrongly categorised humeri, inclusion in the sample was set at 70% for the computed probability level classification. When applied to the sexed sample, 72% were included of which 5% were wrongly categorised. The sex ratio for this derived sample was 0.3718, compared with 0.3925 in the original sample (χ2 = 0.063, df = 1, P = 0.80). Nor was the difference significant in sex ratio derived using same independent sample and humerus length versus the discriminate function (χ2 = 0.017, df = 1, P = 0.89).
Gyrfalcon breeding phenology
The date on which the first egg was laid in 1981–1998 was back-calculated using the estimated age of young and assuming the following: (1) 60 hours between eggs being laid; (2) incubation period of 35 days, starting with the penultimate egg; (3) synchronous hatching; (4) clutch size of three if three or fewer young were in the nest when first observed; and (5) nestling period of 47 days (Clum & Cade, 1994; Cade et al., 1998). Some broods were visited during hatching. The age of other broods, was estimated by comparing young with photographs of chicks of a known age or by measuring the length of the central tail feather (Ó.K. Nielsen, unpublished data), or the length of the seventh primary (cf. Poole, 1989). The formula for the age of young was:
D = 0.1894 × T + 13.624
where D is the age of the young in days, T is the length of central tail feather in mm, measured with a ruler from where the feather emerges from the papilla to the tip of the feather. The formula was calculated using measurements from young of known age (n = 38), r2 for the regression was 0.952.
Rock ptarmigan ecology
The breeding phenology for rock ptarmigan was estimated using data from Hrísey (66° 00’N, 18°22’W) 32 km west of the falcon study area. These data were collected by the late Finnur Gudmundsson in the 1960s and 1970s. Date on when the first egg was laid was back-calculated using information from observed date of hatch or from nests found during the laying period. The following variables were assumed: (1) 24 hours between eggs being laid; (2) incubation period of 21 day starting with the penultimate egg; and (3) synchronous hatching (Holder & Montgomerie, 1993).
Observations on the start of territorial behaviour among rock ptarmigan males in the falcon study area were made in spring 1984, 1985, 1987, 1988, 1990 and 1991 (cf. Nielsen, 1993). Territorial males were spaced, aggressive, had prominent combs and performed visual and vocal displays. Observations on the cessation of territorial behaviour were made using radio tagged males in 1995 (cf. below). The males were located once each week from late May until mid August.
A total of 22 rock ptarmigan males were trapped in late May and early June 1995 to study mortality. The birds were tagged with TW3 necklace type transmitters from BIOTRACK, and located once each week until mid-August. A survival curve was calculated using the staggered entry design (Pollock et al., 1989).
Territorial rock ptarmigan males were censused on six plots within the falcon study area in late May each year 1981–2000 (Nielsen, 1996; 1999b). The data used in this paper, as an annual index of rock ptarmigan numbers, are the total number of males on all plots.
Prey remains were collected at 359 nest sites during 1981–2000. A minimum of 31,813 individuals of at least 56 species were identified as prey from these collections, which included 53 bird species, two species of mammal, and one species of fish.
Composition of gyrfalcon diet
Birds formed more than 99% of the breeding season diet (see Appendix 24.1 which also gives Latin names of prey species). Seventy-nine per cent of the known breeding bird species within the study area were registered in the prey collections. All except five of the quarry species belonged to the local breeding community. These five included four vagrants, coot, woodcock, wood pigeon and fieldfare, and ‘carrier’ pigeon. Most birds taken were adults; 88% by number and 93% by biomass. Juveniles were preyed on mainly during July (Figure 24.1). Only one case of nest robbing was established when a ‘pipping’ rock ptarmigan egg was found in a gyrfalcon nest.
Figure 24.1. Importance (per cent frequency of all prey) of adults and juveniles in gyrfalcon breeding season diet, North-east Iceland 1987. Identified prey for April-May was 305 individuals, 600 individuals for June, and 1,405 individuals for July.
Adult rock ptarmigan was the single most important prey in all nests and in all years. Rock ptarmigan comprised 70% by frequency and 72% by biomass of the total prey sample. Three other species comprised more than 2% by frequency: wigeon (5.4%), puffin (4.1%), and whimbrel (3.7%). The mean geometric weight of prey was 471 g, and reflected the dominance of rock ptarmigan. The smallest prey were juvenile passerines that weighed less than 10 g and the largest was greylag goose at c. 3,400 g. The largest prey taken regularly was adult pink-footed goose (c. 2,500 g). The falcons were able to carry prey of this size, either whole or in pieces, as shown by complete disarticulated skeletons in nests. A few gyrfalcons were also recorded as prey, all were nestlings which had died.
Rock ptarmigan chicks were not an important prey for falcons in summer. Chicks only appeared in the diet in any numbers when they were about one month old. Smaller chicks were rarely taken.
Cross-correlation of rock ptarmigan numbers lagged by percent frequency of adult ptarmigan in falcon diet (arc-sine transformed and detrended) gave significant positive coefficients (P <0.05) for time-lags of 0 and 1 year, and a significant negative coefficient for a time lag of –6 years. The extent to which rock ptarmigan was the dominant prey item varied directly in relation to its population size. Other quarry species were alternative prey
for rock ptarmigan, as these species increased in gyrfalcon diet as ptarmigan numbers fell (cf. Nielsen, 1999a). The most important alternative prey groups were waterfowl (14% by number, 20% by biomass), and waders (7%, 3%; Figure 24.2). During years when rock ptarmigan numbers were high the importance of the grouse in the falcon’s diet reached a plateau at about 80% by frequency (Figure 24.3).
24.4.2 Local changes in gyrfalcon diet
There were local differences in diet composition (Appendix 24.1). Falcons living inland and away from the main waterfowl areas (‘heath falcons’) relied more on rock ptarmigan than either ‘lakeland falcons’ (G = 988, df = 1, P <0.001) or ‘coastal falcons’ (G = 75, df = 1, P <0.001). The difference was most pronounced between heath and lakeland falcons (Figure 24.4). Coastal falcons relied more on rock ptarmigan than did lakeland falcons (G = 302, df = 1, P <0.001). The main alternative prey groups were waterfowl for lakeland falcons, and auks for coastal falcons. Heath falcons used mainly waterfowl as alternative prey.
24.4.3 Seasonal changes in gyrfalcon diet
There was a seasonal pattern of prey utilisation, which was repeated on all territories and in all years. In April and May, during courtship through to hatching, rock ptarmigan was almost the only prey brought to nests (Figure 24.5). In June during the early nestling period, alternative prey started to appear, and this became most pronounced in July, during the late nestling and the post-fledging periods. The almost total dependence on rock ptarmigan during the first phase of the breeding cycle was not due to lack of other alternative prey, at least not at lakeland and coastal sites, as all these sites had an ample selection of alternative prey from mid-April.
24.4.4 Sex ratio of rock ptarmigan depredated by gyrfalcons
The sex composition of adult rock ptarmigan caught by gyrfalcons showed seasonal changes that were repeated each year. Sex ratios were mainly male-dominated in April-May, but the importance of males declined in June, and again in July (Table 24.1). Combining data from all years, the mean proportion of males in rock ptarmigan diet in April-May was 63% (±4.5%; 95% confidence limits), in June 59% (±3.5%), and in July 30% (±3.0%). Observations on the survival of radio tagged rock ptarmigan males during spring and summer 1995 supported this general picture (Figure 24.6). Mortality was heavy until mid-June and then non-existent.
All mortality of radio tagged birds (n = 6) was due to gyrfalcon predation. Two of those males were eaten at the kill site; the others were carried away. The leg rings of two males were subsequently found in a gyrfalcon nest 5 and 6 km from the kill sites.
24.4.5 Breeding in gyrfalcon in relation to events in the rock ptarmigan population
Gyrfalcons start their preparations for breeding early in the season, the first pairs before mid-March followed by the majority during the second half of March and the first half of April (Figure 24.7). Winter still reigns at this time of year, and rock ptarmigan are in the process of moving from winter habitat to breeding areas. Important mid-and late winter habitats for rock ptarmigan include birch Betula pubescens woods and scrub, and lava fields.
Most falcon pairs (c. 66%) had laid eggs before rock ptarmigan males became territorial. The onset of territorial behaviour was highly synchronous amongst males; the mean date was 23 April (range 20–28 April, n = 6, s = 2.81). Some movement of birds onto the heaths was noted in the first half of April in 1985, but not in 1984. The males advertised and defended territories vigorously throughout May and into June. Territorial activities waned in the first part of June. We used male rock ptarmigan decoys to trap territorial males (c. 450 since 1994) on the study area for banding. The latest date on which we have caught males this way was on 9 June. Radio telemetry in 1995 showed that although males had stopped advertising by mid-June, they remained on their territories throughout June and well into July, even though they were rarely seen. At the end of the territorial period, c. 10 June, more than 90% of falcons had young in the nest (Figure 24.7). Males predominated in falcon diet from start of courtship through to the end of June (Table 24.1).
Rock ptarmigan females started egg laying on 18 May (mean 29 May; Figure 24.7). The first females were incubating full clutches on 28 May and 90% by 13 June. The first chicks hatched on 17 June; 75% of hens hatched their eggs by 1 July, and 90% by 4 July. Females dominate in falcon diet in July (Table 24.1). The chicks were about one month old when they started appearing in the falcon’s diet. Most chicks had reached this age by the last week
of July and the first week of August. An important event in the annual cycle of gyrfalcons is the dispersal of young. The first falcons dispersed by mid-July and 70% were independent by 8 August, coinciding with the period when rock ptarmigan chicks become huntable (Figure 24.8).
The gyrfalcon population in North-east Iceland had specialized feeding habits. Rock ptarmigan were the main food during spring and summer in all years. Changes in prey selection occurred over the course of each breeding season. Rock ptarmigan were most important in the first half of the season, and somewhat less so at the end when more alternative prey were taken. The falcons killed mainly adult rock ptarmigan, predominantly males in April through to June, and females in July. Rock ptarmigan chicks became acceptable as prey when about four weeks old. The gyrfalcon’s affinity to Lagopus grouse seems to be deeply rooted in its habits, and alternative prey species are taken in accordance with grouse numbers (this study; Nielsen, 1999a). This also applies to falcons that have an ample supply of alternative prey throughout the breeding cycle (Nielsen, 1986, 1999a).
24.5.1 Comparison with food habits of gyrfalcons in other areas
It can be difficult to compare results of other studies on gyrfalcon diet because of differences in methodology. The data must relate to the same phase of the nesting cycle, preferably the whole cycle, to be comparable. Collections of prey remnants must be made at all sites where remains gather, including the nest itself, because of seasonal changes in prey selection and in the locations where remains accumulate (perches versus nest site).
Dement’ev & Gladkov (1966) divided gyrfalcons into two distinct groups on trophic habits; namely coastal nesting falcons and inland nesting falcons. Coastal nesting falcons fed largely on aquatic birds and the interior falcons predominantly on grouse. This distinction was not supported by my data, which showed that all coastal nesting falcons were predominantly rock ptarmigan hunters and used aquatic birds only as alternative prey during the latter part of the breeding season. These findings concur with other studies. Most gyrfalcon populations base their reproduction on grouse (Figure 24.9).
Depending on location important alternative prey groups were, arctic ground squirrel Spermophilus parryii, waterfowl, waders, auks, microtine rodents, and hares. Seasonal changes in prey selection have followed the same general pattern as described in this chapter, namely near complete dominance of grouse in spring, alternative prey species taken during the latter part of the breeding cycle and with most avian prey being adults (Kishchinskiy, 1957; Poole & Boag, 1988; Huhtala et al., 1996; Hansen, 1999). To my knowledge, the only gyrfalcon population that does not have a grouse-based diet was from the High Arctic on Ellesmere Island (latitude 78oN). Those falcons fed mainly on arctic hare Lepus arcticus, collared lemming Dicrostonyx groenlandicus and waders (Muir & Bird, 1984). Data for gyrfalcons in North-east Greenland indicate that this might also be the case there, with arctic hare, collared lemming and snow bunting as the main prey species (Fletcher & Webby, 1973; Summers & Green, 1974; Cabot et al., 1988).
24.5.2 Food and breeding
The gyrfalcon is able to breed by feeding on the adult sector of grouse populations during its annual trough in numbers (Hagen, 1952; Cade, 1960; Figure 24.7). Further, in Iceland the gyrfalcon is able to breed successfully during all phases of the 10 year population cycle of the rock ptarmigan, using this species as its main prey (Nielsen, 1999a). How the gyrfalcon is able to do this is not so much a question of grouse numbers as prey vulnerability.
Studies of gyrfalcon diet during the non-breeding season in North-east Iceland have shown that ‘heath falcons’ did not change their take of rock ptarmigan compared with the breeding season. ‘Lakeland falcons’ and ‘coastal falcons’ on the other hand ate less rock ptarmigan during the non-breeding season, the two groups increased respectively their take of waterfowl, wood mouse and snow bunting, and waterfowl and auks. The importance of rock ptarmigan increased again in March and April (Nielsen & Cade, 1990). This change in diet also marked the beginning of the falcons’ courtship period. Increased predation on rock ptarmigan at this time could be due either to changes in their vulnerability associated with local movements from winter (shrublands) to breeding habitats (heaths) or by an influx of grouse wintering in other parts of Iceland. Grouse movements have been associated with the start of courtship in gyrfalcon populations in the Canadian Arctic (Poole & Boag, 1988) and Siberia (Voronin, 1987). The vulnerability of the grouse in spring culminates when males establish their territories, but it is of interest to note that by this time most gyrfalcons in Iceland have already laid (Figure 24.7). Behaviour and plumage determine differences in the vulnerability of the sexes, making the males more susceptible to predation than females at this season. Sex ratios of rock ptarmigan in Iceland are equal in spring (Gardarsson, 1988).
Faced with a diminishing number of rock ptarmigan as the season progresses, and a sharp decrease in male vulnerability as they stop defending territories and females commence incubation, the falcons turn to alternative prey. The importance of alternative prey is determined by the spring density of rock ptarmigan (Nielsen, 1999a). In mid- summer the falcons switch from hunting males to females. Again, it is behaviour of rock ptarmigan that determines their vulnerability. Most females have broods, while males have moulted to a brown summer plumage and are secretive.
These changes in movements, habitat selection and behaviour of rock ptarmigan from late winter to mid-summer are predictable and affect their vulnerability in such a way that gyrfalcons are able to use them as the predominat prey for breeding in all years.
The breeding cycle of the gyrfalcon is long and, from the start of courtship to the dispersal of young, takes around 140 days to complete. This is almost five months, and is a longer period than the growing season of plants at these latitudes. How do they fit such a long breeding cycle into the calendar; what phase of the breeding cycle is critical and sensu Lack (1954) are they able to match this critical phase with the peak of prey abundance?
I propose that the post-dispersal period of the young falcons is critical for the success of the breeding season, hence the weeks succeeding dispersal are when the fate of the cohort is probably determined. The falcons should strive to time their breeding in such a way as to match the post-dispersal period of juveniles with peak numbers of vulnerable young grouse. In Iceland, the peak dispersal of young falcons occurs during last week of July. Therefore, the falcons must prepare for breeding as soon as possible in late winter to be able to achieve this.
The length of the window when dispersing falcons have access to abundant young rock ptarmigan is about 50 days. It begins when rock ptarmigan chicks are large enough to be hunted in any numbers (c. 30 days old) and ends when chicks have completed their wing moult and gained full flight capabilities (c. 80 days old) by mid-September (Figure 24.8). The early part of this period should be the optimum time for the young gyrfalcons to become independent. At this time the grouse population is at its annual peak, naive young form more than 80% of the population and their flight capabilities are impaired. Thus, young falcons dispersing in early August have some weeks to master their hunting skills before rock ptarmigan become harder to catch.
These studies were funded in 1981-1993 by the National Geographic Society, the Peregrine Fund Inc., the Icelandic Science Fund, the Andrew Mellon Foundation, the Alexander Bergstrom Memorial Research Fund, and the Arctic Institute of North America. For 1986 and 1994-2000 this research was funded by the Icelandic Institute of Natural History. Many people have provided field assistance through the years including J.Ó. Hilmarsson, G. Thráinsson, I. Petersen, Ó. Einarsson, H. Bárdarson, E. Thorleifsson, Ó.H. Nielsen and A. Snæthórsson. G. Pétursson assisted with data analysis and L. Ásbjörnsdóttir produced the figures. T.J. Cade, S. Petty and an anonymous referee greatly improved the chapter with their comments.
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