population dynamics and partial migration of the european robin

Journal of Animal Ecology (1990), 59, 1077-1090

POPULATION DYNAMICS AND PARTIAL MIGRATION OF THE EUROPEAN ROBIN (ERITHACUS RUBECULA) IN DIFFERENT HABITATS By FRANK Department

ADRIAENSEN

AND

ANDRE

of Biology, University of Antwerp,

A. DHONDT

U.I.A., B-2610 Wilrijk, Belgium

SUMMARY

(1) We studied differences in habitat distribution, local survival and mating success • between the two morphs (resident/migrant) of the partially migratory European robin near Antwerp (Belgium). (2) Mean local survival of resident males (50%) was higher than local survival of migrant males (17%). During cold winters survival of residents decreased with about 50%. (3) Mating success decreased with settling date from 74% for early settling residents, over 44% for migrants to 19% for late-settling birds of unknown status. (4) The probability of breeding is two to four times higher in residents than in migrants. (5) Resident and migratory robins were habitat separated both in the breeding season and in winter: 70% of breeding males were migratory in the woodland, but in the park and gardens most males were resident. Almost all females were migratory. (6) Unbalanced reproductive success of the resident and migratory morphs and arguments for conditionality show that partial migration in the European robin is a conditional strategy in which the migrants are making the best of a bad job. (7) We explain why our conclusions do not contradict experimental evidence for a strong genetic influence on migratory tendencies in the European robin.

INTRODUCTION All studies on partial migratory bird species have shown a strong genetic basis for the • migratory habit (Berthold 1978, 1984; Berthold & Querner 1981, 1982; Schwabl 1983; Biebach 1983). This, however, does not exclude environmental or behavioural control mechanisms (Lundberg 1988), and many other authors pointed out the importance of non-genetic factors in controlling partial migration (e.g. Miller 1931; Nice 1937, 1943; Kalela 1954; Gauthreaux 1978; Hilden 1982). In ornithological studies migration is generally considered to be a seasonal, bidirectional movement between geographical areas (e.g. Baker 1978; Gauthreaux 1982; Taylor 1986: 'Homeostatic migration'). In a partial migratory population only some of the individuals in the population migrate. Generally, partial migration is considered to be an 'evolutionarily stable state' only if the pay-offs (life-time reproductive success) of both morphs in the population are balanced (Berthold 1984; Gauthreaux 1982; and references therein). In contrast to this hypothesis, most case studies show that resident species, populations and individuals do 1077

1078

Population dynamics and partial migration

better than migrants (Berthold 1984). In fact, it has been shown theoretically that the two morphs do not necessarily need to have equal pay-offs (Lundberg 1988). According to Lundberg (1988) only two hypotheses for the evolution of partial migration are theoretically possible: a mixed ESS at the population level (with equal pay-offs and a fixed behaviour for each individual) or a conditional strategy, a pure ESS, in which both morphs do not have to perform equally well and can change their behaviour. Apart from the control mechanisms and the evolutionary aspects in partial migration, little is known about the way in which the two morphs are distributed within a population. Some studies (Berthold 1978, 1984; Schwabl 1983) suggest habitat separation of migrants and residents. The European robin is a partial migrant showing a cline in migratory behaviour, the proportion of migrants declining from north to south (Lack 1965; Cramp & Simmons ~ 1988). In the British robin, Erithacus r. melophilus Hartert, almost all settled males as well as 30-50% of the females are resident (Mead 1984; Cramp & Simons 1988; Harper 1989). Only few British robins have been recovered abroad (Mead 1984), and where non- ~ residents actually winter remains unclear (Cramp & Simmons 1988; Harper 1989). In continental Europe, however, the long-distance movements of E. r. rubecula (L.) are well described (Erard 1966; Rendahl 1966; Lebreton 1968; Broyer 1982; Adriaensen 1987, 1988a,b). In Belgium about half of the local breeding population is migratory. Migratory robins winter mainly in the south-west of France and the Iberian Peninsula. About half of the Belgian winter population consists of wintering birds from more northerly breeding populations (see Adriaensen 1988a). We studied individually colour-marked European robins simultaneously in different habitats in Antwerp (Belgium). We compare the population dynamics and two components oflife-time reproductive success (local survival and mating success) between the two morphs (resident/migrant) and between habitats, followed by a further discussion on ecological and evolutionary aspects of partial migration.

MATERIALS

AND

METHODS

We observed individually colour-marked robins in three different habitats: parkland, woodland and gardens (see below). Observations were made all year round at least once a week (over 1400 hours in total), except during mid-summer when robins moult. Also, the immediate surroundings of the study plots were regularly checked for colour-marked ~ robins. Most observations were made before noon. Territories were mapped, and the owners' identity, arrival and departure data noted. In spring, the number of robins present was also counted several times a week during dawn chorus. No special efforts were I made to find nests and collect breeding data. Depending on the wintering area, four different types of robins will be distinguished: 'strictly resident' birds occupied their territory all year round, and 'locally wintering' birds wintered in the vicinity of, but not in, their breeding territory; 'long-distance migrants' wintered at considerable distance of their breeding area, and 'winter-guests' were longdistance migrants from more northerly populations which wintered in the study areas. Both strictly residents and locally wintering birds together will be called 'residents', in contrast to 'long-distance migrants'. This is in contrast with most British studies (e.g. Lack 1943; Harper 1985) in which only birds wintering in their breeding territory are

F.

ADRIAENSEN

AND A. A. DHONDT

1079

considered as 'residents' and locally wintering birds together with long-distance migrants as 'migrants'. The 'Ynsen'-value (Y) (KNMI 1974) was used as a measure for winter cold, cold winters having a value of at least 16. It is calculated as

Y=0·000275 A2+2/3 B+ 10/9 C, where A is the number of days with a minimum temperature below 0° Celsius (in Antwerp), B is the number of days with a maximum temperature below 0° Celsius, and C the number of days with a minimum temperature below - 10° Celsius. Densities (per 10 ha) and numbers of robins will be expressed as the number of territories occupied, and not as the number of individuals present. Outside the breeding season robins are generally individually territorial and all robins present occupied a territory, so both values are equal at that time. However, during the breeding season territories can be occupied by paired or unpaired males, and the maximum number of individuals is twice the number of territories. This proportion of paired males varied considerably over years and habitats. Since territorial behaviour is minimal in summer (Lack 1965; Cramp & Simmons 1988; Harper 1989), and robins were very hard to observe, we considered the number of occupied territories during mid-summer to be zero. From the end of August onward, as they finished moult, robins gradually started to (re)claim territories. The gradual increase in numbers shown here therefore represents the number of occupied territories.

STUDY

PLOTS

Woodland The woodland study plot (plot W, 30 ha) was situated in the south-west of the Peerdsbos, a mixed woodland of about 150 ha to the north-east of Antwerp (for a detailed description see Van Styvendale 1963). Plot W was studied from September 1981 until November 1985. To the south of the study plot the woodland was continuous, to the north-east and west it was bordered by generally open areas, and to the north by a series of gardens (see plot G). The study plot consisted of several small subplots each with a more or less homogeneous vegetation. Oak (Quercus robur L.) was the most abundant tree species (46%), beech (Fagus silvatica L.), birch (Betula spp.) and pine (Pinus sylvestris L.) made up about 10% of the trees. In most subplots there was a dense scrub layer with mainly black cherry (Prunus serotina Ehrh.), rowan (Sorbus aucuparia L.) and alder (Alnus glutinosa (L.) Gaertn.). Large areas were covered with a dense layer of blackberries (Rubus spp.) and bracken (Pteridium aquilinum (L.) Kuhn). Gardens The garden study plot (plot G, 10 ha) consisted of the gardens bordering plot W to the north, plus some bushy, cultivated areas to the west and east. In 1981-82 the plot was studied making observation from the woodland, but from the autumn of 1982 onward it was possible to enter most of the gardens. The vegetation in this plot was highly variable with parts of it resembling the adjacent woodland, but also a lot of dense copses of exotic, mostly evergreen, trees and bushes (especially rhododendron), and large areas of lawn. Plots G and W together will sometimes be treated as one plot (GW, 40 ha).

1983 1982 season

1080

Population dynamics and partial migration Parkland

The parkland study plot (Plot P, 10 ha) was situated in the northern part of the 'Ter Rivierenhof', a park (130 ha) surrounded by urban areas. The park has a variable structure with lawns, sport fields and ponds, but also areas with high densities of a wide variety of trees and shrubs. The study plot itself was almost completely covered by mature beeches. The scrub layer consisted mainly of copses of rhododendron, separated by open areas and paths. Every day many people visit the park bringing large amounts of extra food for the birds. Plot P was studied from September 1982 until October 1986. RESULTS Density Breeding territory density (Table I) varied from 2·7 to 8·5 territories per 10 ha over all plots, autumn density from 4·1 to 13·5 territories per 10 ha in plots G and P. In plot W few robins were seen in autumn: 1·2-1·5 territories per 10 ha. In plots G and P, spring density was only about half that in autumn (Table 1). Although seasonal variation in territory density was in opposite directions in the adjacent plots G and W, and total density in both plots together (GW) was about the same in autumn and spring (Table 1), only on one occasion was a colour-marked bird observed to have moved from a breeding area in the wood to a non-overlapping autumn territory in a nearby garden. Population dynamics Figure I shows per plot and per half month the mean number of occupied territories averaged over the entire study period. Numbers are expressed as proportions of the mean breeding population in the plot (100%) to facilitate comparison. Autumn and \Vinter In autumn the populations changed in a qualitatively similar way in all habitats: after a period of reoccupation and settlement in August and early September, maximum numbers were reached from mid-September to mid-October. From mid-October to the end of November numbers declined rapidly. Between the end of November and the 3·7 4·5 4·8 4·[ 3·5 6·0 7·5 8·5 752·7 3·1 ·7 ·8 13·1 10·5 4·3 13·0 GW [3·5 G 10·0 13·6 4·4 4-4 9'1 1·2 W density in different 7·0 7·2 5·5 3·5 ·5 4·5 4·1 P 1·3 1·5 [·3 9·1 years, seasons and habitats TABLE 12·5 1. Robin 1982 1981 Year

(in territories

per 10 ha)

F.

1081

ADRIAENSEN AND A. A. DHONDT

160

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6., "

120

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Nov

Dee

Jan

Feb

Mar

!~

Apr

May

Jun

Jul

Month

FIG. I. Mean number of robins present, per plot and per half a month, over 4 years. Numbers are expressed as proportions of the mean breeding population in the plot. Breeding population (100%): woodland (D) 7·0 individuals, gardens (L» 10·0 individuals, parkland (e) 6·8 individuals.

beginning of spring numbers declined more slowly. In plot W only small numbers of robins settled in autumn territories and all of them disappeared before the end of November, most of them even before mid-November. Spring From the first warm spells in spring onward, sometimes as early as the end of January but in most years not before the second half of February, breeding territories were occupied. In the park this coincided with only a slight increase in the number of territories occupied. The few birds that did enter the plot in spring probably all did so because of the enlargement of their winter area to breeding territory size. In plots Wand G large numbers of robins entered the study plot in spring. Numbers started to increase slightly earlier in plot G than in plot W. By mid-March on average over 80% of the territories were occupied in plot G, while in plot W less than 30% was occupied. By the end of March all territories were occupied in plot G, but in plot W numbers kept increasing until early June. Territory settlement in spring was not a continuous process. Before 10 March (see Fig. 2) a number of individuals started occupying a spring territory. Part of them were strictly resident birds that had also been present during the previous winter, the rest were immigrants. Most of these settlements occurred in plot G. After this followed a period without new settlements, which was rather variable in length. In a1l4 years no new arrivals were found during a period which coincides with the second 10-day period of March. After this period new birds arrived again, but in some years this was not before the end of the month. By mid-April most territories were occupied. However, every year unoccupied areas in plot W were claimed still later, in May and June. Spring territory occupation thus appears to occur in three waves: before 10 March; late March and early April; May and June.

12 ~~ 8 12 z2'.DE.. ~10 ~ 1082 488 '0"E 4 "2 r CD I), as yearling (I)

GW Plot Year

>1 1

82

P

83

84

I 2 554

2

83

I 3

84

85 2 2

1084

Population dynamics and partial migration

P (0·30) and plot GW (0·32) a low proportion of the birds that settled in autumn also stayed until the next breeding season, including one out of four adults in plot G and two out of six in plot P. Forty per cent of the autumn immigrants disappeared again before the end of the migratory season (i.e. mid-November).

DISCUSSION Population dynamics Several British studies showed marked differences in population dynamics between habitats, with higher numbers in suburban areas in winter and in woodland areas in spring (e.g. Lack 1948, 1965; Jackson 1958). Our results show that numbers in the park were highest in early autumn and about equal in winter and spring. In the woodland plot no robins stayed during winter, and in the adjacent gardens numbers where highest in autumn. Winter population size in the gardens was about half that in the breeding season. Mean density of both areas together was about the same in autumn and spring. However, very few movements from the woodland to the gardens were observed. So, the locally wintering birds in these areas cannot have moved far, but further than the adjacent gardens. A similar result was found by East (1982), who found only some of her emigrating colour-marked robins in the immediate vicinity of the study plot. The timing of the observed changes in numbers coincides well with data on the timing of robin migration according to ringing recoveries from Belgium (Adriaensen 1987) as well as from elsewhere in Europe (see Glutz von Blotzheim & Bauer 1988 for a review). In the study plots, peak numbers in autumn territories were noted during the main period of migration (September-October). In autumn most individuals settled in the study plots before mid-September, and no northerly birds were found in Belgium before 10 September (Adriaensen 1987), so most of these settlers must have been members of the local breeding population. From mid-October until the end of November numbers declined at about the same rate in all three plots. Thus, it appears that a considerable proportion of the individuals of the local population settle in a temporary territory in early autumn, and leave again in the course of the migration period. Few individuals disappeared from the plots between December and February, a period in which no longdistance movements could be found in the ringing data (Adriaensen 1987; Glutz von Blotzheim & Bauer 1988). In spring, three waves of territory settlement were observed. According to the ringing recoveries all individuals observed in the first period (well before mid-March) must have been locally wintering birds, as no long-distance movements were observed before mid-March (Adriaensen 1987). Most territories in the second period became occupied between late-March and mid-April, i.e. after the beginning of spring migration and well before the end of it. In plot W some birds still arrived in May and June. The status of these birds is hard to assess. They might have been late migrants but, in two cases, birds arrived as a pair and in one other case the individual involved certainly wintered locally; these may have been birds that failed to breed or to attract a partner elsewhere, or birds that had bred already and then moved to another territory. Survival and reproduction Because of the clear differences in settling time in spring, we were able to assign every individual in the breeding population (especially the males) to one of the two morphs

0 Month