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Abstract

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

Although there is very strong evidence for an advance in the spring arrival dates of migrating birds, there are few data with which to examine the changes, if any, in the dates of winter migration. In this paper we examine data on the first arrivals and last departures of winter visitors from two UK county bird reports. A small number of changes in phenology and some relationships with climatic variables were present. However, results were often contradictory or inconclusive. As data such as these are highly variable we recommend a more thorough examination for climatic impact where records exist.

Over the last decade much has been written about the arrival times of migrant birds to their breeding territories. In the UK, data for summer visitor arrival times are recorded in many county bird reports, at coastal bird observatories and by individual recorders either independently or as part of a network such as the UK Phenology Network (http://www.phenology.org.uk) or Migration Watch (http://www.bto.org/migwatch). In general, it has been shown that there is plenty of evidence that bird arrival times have been getting earlier in the UK in recent decades (e.g. Mason 1995, Loxton & Sparks 1999, Sparks 1999). This phenomenon has been witnessed in many other countries, including Russia (e.g. Sokolov et al. 1998, Gilyazov & Sparks 2002), Germany (Hüppop & Hüppop 2003), France (Sueur & Triplet 2001) and Poland (Tryjanowski et al. 2002). Recent meta-analyses have confirmed the changing phenology of bird populations (Parmesan & Yohe 2003, Root et al. 2003). However, these changes are not nearly as large as those recorded for certain plant and insect species (up to 1 month earlier) on which birds may rely (Cannell et al. 1999), but can still show large shifts, for example 2 weeks earlier for Sand Martin Riparia riparia in Essex (Sparks & Mason 2001).

For many species there is evidence of a temperature response, although we assume that this is an indirect response through the availability of invertebrate prey or an indication of more favourable conditions along migration routes (e.g. Huin & Sparks 1998). Changes to bird species as a consequence of a warming climate are also likely to affect nest timing, productivity and distributional range (e.g. review in Sparks et al. 2002). As with the magnitude of change experienced so far, the response of bird migration to temperature, at typically 2 days/°C, seems less than that of plants and invertebrates (typically 6 days/°C) (Sparks & Menzel 2002). Root et al. (2003) express the concerns of many conservationists when stating that differential responses ‘could result in a disruption of the connectedness among many species in current ecosystems (for example, a tearing apart of communities)’.

There are far fewer data with which to examine departure dates of summer visitors, although Sparks and Mason (2001) examined the data for Essex, which suggested more species showed a tendency to later departure over recent decades. Even fewer data exist to examine the arrival and departure dates of winter visitors, although such data are likely to provide the first clues to a changing winter distribution pattern as a consequence of a warming climate, and a change in status from migrant to resident, or from long-distance to short-distance migrant. In this paper we examine and report on winter migrant phenology from the only sources known to us at present. This is done in the hope that it will encourage others to examine such other data sets as exist for winter migrant phenology.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

The Essex Bird Report has reported the arrival and departure dates of winter visitors in two discrete time periods: from 1966 to 1974 and from 1990 to 2001, i.e. a total of 21 years of data spanning 36 years. Records consist of the first and last observations from those sent in by members of the bird club. The data used in this paper have been extracted from these publications and converted prior to analysis to day number after 31 December. The duration of stay has been calculated as the difference between arrival date in one year and the departure date in the following calendar year. Consequently, only 19 years of data exist for this variable as departure dates in 1975 and 2002 were not available at the time of writing. Fourteen species of bird were reported in both time periods (Table 1). Data on winter visitors were also abstracted from The Sheffield Bird Report/Birds in the Sheffield Area for the years 1973–2001. Data are available for the entire period for the four species listed in Table 1 and were manipulated as for the Essex data.

Table 1.  Arrival date, departure date and duration of stay of winter visitors.
Species/areaArrival dateDeparture dateDuration
MeansdTrendPMeansdTrendPMeansdTrendP
  • For each data series, mean dates, standard deviations and trend over time (slope ± se from regression) are given. Species are ordered by mean date of arrival.

  • *

    P < 0.05,

  • **

    P < 0.01,

  • ***

    P < 0.001.

Essex
 Merlin (Falco columbarius)Sep. 428.2−1.87 ± 0.26***May 222.9 1.38 ± 0.26***24446.6 3.31 ± 0.37***
 Purple Sandpiper (Calidris maritima)Sep. 926.4−0.42 ± 0.44 Apr. 1820.2 0.66 ± 0.31 22436.4 0.69 ± 0.64 
 Hen Harrier (Circus cyaneus)Sep. 1133.5−1.85 ± 0.58**May 214.3 0.58 ± 0.29 23544.4 2.38 ± 0.89*
 Fieldfare (Turdus pilaris)Sep. 1820.7 0.20 ± 0.35 May 720.2−0.15 ± 0.35 23331.7−0.36 ± 0.57 
 Jack Snipe (Lymnocryptes minimus)Sep. 2314.0 0.62 ± 0.27*Apr. 2912.8−0.16 ± 0.29 21923.5−0.84 ± 0.54 
 Rock Pipit (Anthus spinoletta petrosus)Sep. 2310.3−0.22 ± 0.17 Apr. 413.8−0.35 ± 0.22 19417.6−0.08 ± 0.32 
 Redwing (Turdus iliacus)Sep. 25 7.8−0.15 ± 0.13 Apr. 2317.4 0.23 ± 0.29 21221.5 0.20 ± 0.39 
 Brambling (Fringilla montifringilla)Sep. 2815.6−0.29 ± 0.26 Apr. 2511.2 0.07 ± 0.19 21120.7 0.31 ± 0.37 
 Snow Bunting (Plectrophenax nivalis)Oct. 314.6 0.32 ± 0.24 Mar. 1517.6−0.10 ± 0.30 16821.1−0.95 ± 0.31**
 Twite (Carduelis flavirostris)Oct. 410.6−0.07 ± 0.18 Apr. 418.4−0.14 ± 0.31 18324.9−0.16 ± 0.45 
 Goosander (Mergus merganser)Oct. 1729.3 0.08 ± 0.51 Apr. 2021.6−0.18 ± 0.37 18634.8−0.46 ± 0.63 
 Long-tailed Duck (Clangula hyemalis)Oct. 2317.7−0.02 ± 0.42 Apr. 437.3−0.36 ± 0.88 16637.6−0.81 ± 0.99 
 Tundra Swan (Cygnus columbianus)Oct. 2416.5−0.74 ± 0.24**Mar. 1526.8−0.21 ± 0.48 14229.9 0.77 ± 0.57 
 Smew (Mergellus albellus)Nov. 1814.2−0.18 ± 0.24 Mar. 1615.8−0.32 ± 0.26 11922.9−0.17 ± 0.42 
Sheffield
 Fieldfare (Turdus pilaris)Sep. 1215.6 0.08 ± 0.35 May 813.5 0.26 ± 0.30 23820.8 0.09 ± 0.50 
 Redwing (Turdus iliacus)Sep. 22 8.7−0.03 ± 0.20 Apr. 2912.8 0.29 ± 0.28 22017.8 0.31 ± 0.42 
 Brambling (Fringilla montifringilla)Sep. 30 9.6−0.37 ± 0.21 Apr. 29 7.6 0.12 ± 0.17 21313.8 0.58 ± 0.31 
 Goldeneye (Bucephala clangula)Oct. 422.6−0.02 ± 0.51 May 7 9.7−0.24 ± 0.21 21723.2−0.17 ± 0.55 

Trends in arrival date, departure date and duration of stay have been assessed using linear regression. Correlation and regression were used to compare arrival and departure dates with monthly temperature data from the Central England series (Parker et al. 1992), North Atlantic Oscillation (NAO) monthly indices (Hurrell 1995) and monthly wind direction derived from Jenkinson's weather types (Jenkinson & Collison 1977). A positive NAO reflects strong westerly airflow over the UK, i.e. warming in winter and cooling in summer. For the weather types, the daily wind direction was summarized into the proportion of days in each month with northerly (NW–NE) or southerly (SW–SE) winds. Arrival dates were compared with monthly data for July to October and departure dates compared with monthly data from January to April.

RESULTS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

Arrival dates

Thirteen of the 18 series had a negative regression coefficient, i.e. towards earlier arrival (Table 1), which fails to reach statistical significance (P = 0.096 from a binomial test). Of these, three species were significantly earlier: Merlin Falco columbarius, Hen Harrier Circus cyaneus and Tundra Swan Cygnus columbianus. The magnitude of these changes, 7–18 days earlier per decade, is large. Jack Snipe Lymnocryptes minimus was significantly later by 6 days per decade.

Departure dates

Changes in departure dates were more evenly balanced: ten species earlier and eight species later. Only Merlin was significantly later by almost 14 days per decade. The trend in Hen Harrier departure just failed to reach significance (P = 0.06).

Duration of stay

Equal numbers of species had longer and shorter duration of stay. Two species, Hen Harrier and Merlin, had a significantly longer duration of stay. Only Snow Bunting Plectrophenax nivalis had a significantly shorter residence.

Correlation with temperature, NAO and wind direction

Relative to published results on summer migrants, there were relatively few correlations with mean monthly temperatures. Three significant correlations existed with arrival date (two positive and one negative) and four significant correlations with departure date (three positive, one negative). Six significant correlations with monthly NAO (five positive, one negative) existed for arrival and one (negative) significant correlation with departure. Wind direction produced two significant correlations (one negative with southerlies, one positive with northerlies) for arrival dates and three significant correlations with departure (one negative with northerlies and two negatives with southerlies). An example of the latter is given in Figure 1.

image

Figure 1. The relationship between last departure date of Redwing in Essex and frequency of southerly winds (r = −0.60, P < 0.01).

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Relationship between arrival and departure dates

As for summer migrants in Essex (Sparks & Mason 2001), there is a significant correlation between mean arrival date and mean departure date (r = −0.72, P < 0.01). This concurs with the findings of Preston (1966), who suggested that arrival and departure dates were symmetrically arranged around the warmest (or coldest) day of the year.

DISCUSSION

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

Winter migration strategies are likely to be species specific and ideally we would have the full migration distributions rather than first and last birds. First arrivals in the UK may include non-breeding individuals, failed breeders or even those whose breeding was completed first. Whereas first departures in spring may be those birds attempting the earliest possible establishment of breeding territory, last departures in spring may represent non-breeding individuals.

Traditionally there has been less interest in recording the arrival of winter visitors than there has been of summer visitors. The only exceptions to this are probably the arrival dates in the UK of Fieldfare Turdus pilaris and Redwing Turdus iliacus. As with summer visitors, it is more difficult to record departures than arrivals because this implies the need for continual monitoring. For Essex summer visitors the interyear standard deviation for species averages 8.6 for arrival and 16.0 for departure. By contrast, the figures for Essex winter visitors are 18.5 and 19.3, respectively. These figures suggest that recording for winter visitor phenology is less accurate, that their phenologies are more variable or a combination of both. In dealing with inherently more variable series it is not surprising that we detected a much lower frequency of significant results. This implies that we need either to use longer series or to bulk data from many locations in the UK to provide more reliable data. Logically we might expect few correlations between arrival and UK temperatures if migration is triggered by conditions in breeding territories. The low frequency of effects on departure date may reflect the difficulty with which such data are recorded.

Most change has been apparent in two birds of prey, the Hen Harrier and Merlin, which migrate relatively short distances within the UK. These have both arrived earlier and departed later, extending their period of residency by 2–3 months. This is in keeping with the findings of Tryjanowski et al. (2002), who reported greater response in short-distance (European) summer migrants compared with long-distance (African) summer migrants. It is intuitive that short-distance migrants are most likely to respond faster if climate or environment conditions change in either the breeding or the wintering areas; however, the reasons for such dramatic change in these two species are unknown but may just reflect increasing populations and hence greater visibility. We need to be cautious in interpreting change. Sparks (1999), Tryjanowski and Sparks (2001) and Sparks et al. (2001) have all identified situations in which apparent changes in migration phenology may result from changes in population size or in recorder effort. It is difficult to assess the recorder effort (a function of the numbers of observers and time spent) for a particular species in a particular season. Bird reports tend to list observers contributing to the whole annual report, and these numbers have increased. How this may affect reported bird timing is beyond the scope of this short paper. However, changes in population size and recorder effort will be a more serious problem for species with low population size such as the Hen Harrier and Merlin reported above. In order to understand better the full implications of climate change on migrant birds, it is important to consider changes throughout their annual cycle.

The hints and suggestions in this dataset suggest that the identification and analysis of arrival and departure dates of winter visitors would justify the effort expended and we would be interested to know of the existence of additional data.

Acknowledgments

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

We are grateful to all those whose observations contributed to the data reported here, to the BTO and RSPB libraries for access to their collections, and to Mark Rehfisch and an anonymous referee for comments on earlier versions of this paper.

REFERENCES

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES
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