Arrival and progression of the Swallow Hirundo rustica through Britain

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Arrival and progression of the Swallow Hirundo rustica through Britain

N. Huin & T.H. Sparks

To cite this article: N. Huin & T.H. Sparks (1998) Arrival and progression of the Swallow Hirundo rustica through Britain, Bird Study, 45:3, 361-370, DOI: 10.1080/00063659809461108

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Published online: 29 Mar 2010.

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A

fter wintering in southern Africa, the Swallow Hirundo rustica, reaches southern Europe by late February and arrives in Britain by the end of March to breed.¹ The spring migration is more rapid than the autumn migration and this is often explained by the urge of the spring migrants to breed. Males arrive before females to establish territories and build nests.^2,3 The timing of migration is influenced by photoperiod and is highly synchronized.^4,5 The progression rate is also affected by weather. Weather can affect migration directly, with birds being slowed down or stopped altogether during adverse conditions such as cold, rainy days with head wind.^6–8 It can also act indirectly by influencing the availability of insects⁹ on which Swallows

feed.^10,11Thus, it is likely that Swallow migration is affected by changes in food availability and that, in spring, migrants closely follow the progression of insects with breeding only occurring once insects become a reliable food resource.¹² Birds breeding further north arrive later and experience a shorter breeding season in which they produce smaller and fewer broods.¹³Thus any effect of temperature on the rate of migration may be a reflection of its effect on the insects on which migrants feed.¹⁴

Much research is currently concerned with global warming and its implications. Many studies aim to identify any effects of climatic changes on animals and plants in terms of their performance, abundance and distribution. One accepted feature of climatic change is a predicted increase in temperature. The approach used in this work, as in other research,^15–17concentrates on trying to establish how the Swallow has responded in the past to Bird Study (1998)45,361–370

Arrival and progression of the Swallow Hirundo rustica through Britain

NICOLAS HUIN*

and TIM H. SPARKS NERC Institute of Terrestrial Ecology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire, PE17 2LS, UK

Arrival dates of Swallows from 20 carefully chosen years of the phenological reports of the Royal Meteorological Society (1883–1947) were abstracted. These years represent observations from 1000 different localities in Britain and from between 24 and 325 localities per year. Arrival dates of the Swallow were processed in two different ways: (1) using a Geographical Information System, contour maps were produced for each year and compared subjectively;

(2) using climatic data from Britain and Europe, between-year variation in average arrival date and in speed of progression through Britain were analysed statistically. Swallows progressed through Britain at an average speed of about 50 km day

^–1

. In ‘early’ arrival years, Swallows progressed along a southwest–northeast axis, entering Britain by the southwest (the region receiving the most accumulated heat by April). They travelled faster across Britain in ‘late’ arrival years along a south–north axis. Arrival dates were earlier during warmer Aprils in Britain and western France, and following a warmer March in Iberia. Although the timing of spring migration of the Swallow is related to European temperatures, the possible future effects of climatic changes may not be predictable because environmental conditions in Africa and changes in population size must also be taken into consideration.

*Correspondence author. Present address: NERC British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK.

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weather changes by analysing spring arrival of birds throughout Britain.

MATERIAL AND METHODS

The Royal Meteorological Society published a series of annual phenological reports in its Quarterly Journal between 1875 and 1947.

These reports included data on trees, flowering plants, birds and insects recorded by a network of observers spread throughout Britain. Dates were recorded in Julian days, such that day 1 = 1 January. From these phenological records, we extracted the date on which Swallows were first seen in each of 20 years after 1883. The first criterion for selecting the years was that they should be representative of the range of weather conditions experienced during the period when Swallows were present in Britain (from March to November). This was achieved by a principal components analysis (PCA) conducted on monthly temperatures for central England¹⁸ and average monthly rainfall for England and Wales.¹⁹A plot of the 65 years on the first two PCA axes was then divided into nine different areas reflecting the different weather patterns. Years in each area were then selected randomly, but with the further restriction that each decade of the study period was represented by at least two years.

Several meteorological data sets were used.

For Great Britain, monthly temperatures for central England (CET) in degrees Celsius¹⁸and monthly average rainfall (in millimetres) for England and Wales¹⁹ (EWR) were used.

Average monthly temperatures for each 10-km grid square of Great Britain were extracted from Viner & Hulme.²⁰ Furthermore, monthly average temperatures of several European locations, for which fewer than nine years were missing from the period considered (1883–1947), were obtained from the National Center for Atmospheric Research, Boulder, Colorado, USA. Only data for March and April were considered, as they coincide with the period in which Swallows passed through Europe and Britain.¹ Ringing recoveries of Swallows, either ringed or recovered in the British Isles, suggest that British Swallows migrate along a narrow band along the west coast of France and the east coast of Spain.²¹ Thus Nantes (47.3°N, 1.6°W) was used to describe weather patterns of western France.

Madrid (40.4°N, 3.7°W), Palma (39.6°N, 2.7°E) and Gibraltar (36.1°N, 5.4°W) were averaged together to form a mean Iberian March temperature. However, values for March temperatures were missing in 1920 for Madrid and in 1937 for Palma. This problem was overcome by estimating the Iberian mean temperatures by a method of least squares; these values were then used in further analysis. Temperature and precipitation variables were related to each other as summa- rized in Table 1. During the study period, EWR was not correlated with any examined temperature variable (apart from, curiously, between March precipitation and Palma March temperature). CET was strongly correlated with Nantes April temperatures (P < 0.001) and less strongly with Madrid and Gibraltar March

Table 1.Correlations between the different meteorological variables used during the study period (1883–1947).

Significant coefficients (P < 0.05) are printed in bold. N= 65 for all variables except Madrid and Palma (N= 64).

Temperature

CET EWR EWR Nantes Madrid Gibraltar Palma Variable April March April April March March March Precipitation

EWR March –0.106

EWR April –0.185 0.147

Temperature

Nantes April 0.857 –0.056 –0.144

Madrid March 0.273 –0.033 –0.112 0.377

Gibraltar March 0.266 –0.130 0.049 0.451 0.312

Palma March 0.198 0.480 –0.138 0.283 0.453 0.122

Iberia March 0.353 0.166 –0.147 0.520 0.863 0.545 0.731

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Arrival of Swallows in Britain 363

temperatures (P < 0.01, Table 1).

For each of the selected years, phenological data from all the recording stations from Great Britain were abstracted with their grid references. From these data sets, maps were produced using the ARC/INFO Geographical Information System software. The method used is called triangulated irregular network, which links adjacent locations by triplets and interpo- lates intermediate points. The quality of the interpolation depends mainly on the accuracy of the values at each location and on the spatial regularity of the sampling (the interpolation was less accurate in years and/or areas where observations were few and far apart).

For easier manipulation, a linear conversion was then applied to create 1000 evenly spaced points (called a lattice coverage). These lattices were then smoothed and contour maps were produced for each year and for groups of similar years (in terms of their mean arrival dates). A contour interval of 10 days was used for this purpose; areas where no records existed were represented in white. The same methods were used for producing the contour map of spring cumulative heat, derived from the data published by Viner & Hulme.²⁰

Statistical analysis was conducted using

MINITAB v11 software. Following regression analysis, residuals were checked to ensure they met model assumptions. For each year, the mean arrival dates of all the locations were calculated. These were normally distributed and the interannual variation in the mean latitude of the recording stations had no influence on the mean arrival dates (weighted regression, R²= 7%, F_1,18= 1.35, P = 0.260). Mean arrival dates in Britain in each year were calculated from different sample sizes. Because precision increased with sample size, the latter was used to weight mean arrival dates in the regression analysis, thus giving more influence to the years with more records.

RESULTS

Progression through Britain

Spatial variation in the arrival dates of Swallows in Britain is presented in Fig. 1. Each of these maps is the average of the three most representative years of each category. Taking all years into consideration, Swallows first arrived

in southern Britain during late March/early April and progressed steadily northwards reaching the north of Scotland in May.

However, there is much variation between years when maps are examined in detail. The maps based on most records are complex as they possess small ‘islands’ with arrival dates different from the surrounding areas. There is a general trend for Swallows to arrive by the southwest of Britain and progress steadily in a northeast direction with the principal ‘ports of entry’ being Pembrokeshire, Devon and the Isle of Wight. East Anglia is often reached at the same time as the north of England. This southwest to northeast axis of spring migration is more pronounced when birds arrive early (Fig. 1a). Birds seem to migrate on a more south to north axis when they arrive later (Fig. 1c).

Figure 1d presents for comparison a map of the cumulative heat experienced throughout Britain between February and April (accumulated day degrees above 0°C). The map produced shows a gradient in heat from warmer in the south to colder in the north, with mountains colder than lowlands and coastal areas. The areas that received the most heat were concentrated along the coastline of southwest England, most of the south coast and along the river Thames.

In years classified as early arrival, most of west Wales and parts of the southwest of England were reached by 9 April, the rest of England by 19 April and Scotland by late April/early May (Fig. 1a). In years classified as average arrival, the areas where birds arrived early were smaller (10-day periods up to 30 March and 9 April) with most of England reached by 19 or 29 April and with Scotland still reached by the end of April or beginning of May (Fig. 1b). In years classified as late arrival, the areas corresponding to first arrival by the end of March and beginning of April were much smaller, with most of England reached by 29 April and Scotland still reached by May. It thus appears that Swallow progression through Britain is slower when arriving early and faster when arriving later.

Arrival dates, weather and speed

This was further investigated in the following manner. For each year, the date by which 5% of the localities were reached (5th percentile) was

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Figure 1. Contour maps of arrival dates of the Swallow in Britain in years when they arrive (a) early (1893, 1937 and 1944), (b) normally (1912, 1929 and 1933) and (c) late (1908, 1916 and 1936) and (d) contour map of spring cumulative heat (day degrees above 0°C). The white areas correspond to the absence of data in at least one of the years used in each map. In maps (a) to (c), contour bands represent ten-day periods ending on the date shown.

In map (d), contour lines represent 3°C intervals within shading bands up to the values shown.

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calculated and used as an estimate of the date by which Swallows first entered Britain. This eliminates exceptional outlying records (Table 2). Arrival dates at each location were regressed against their position north (in 100-km units).

The coefficients of regression for the northing values (Table 2) represent the number of days to progress 100 km northwards (e.g. on average Swallows took 1.93 days to progress 100 km north, i.e. a speed of 51.5 km per day). Thus the higher coefficients represent slower progression northward. The rate of progression northward was then compared with the date at which Swallows first entered Britain (expressed by the 5th percentile values), by a regression analysis (Fig. 2). This regression was significant (P < 0.02). A 10-day delay in the date of Swallows entering Britain is associated with an increase in the rate of progression, decreasing, by nearly one day, the time needed to move 100 kilometres northward. Thus the progression through Britain (approximately 1200 km) would be predicted to take 12 days less when Swallows enter 10 days later. The tendency for Swallows to follow a southwest to northeast axis was also investigated further by estimating north and east coefficients by multiple regression. However, easting coefficients were significant in only eight of the 20 years analysed and no trend was detected in associa- Table 2.Mean arrival dates, number of localities, date

at which 5% of the localities were reached and rate of progression northwards (see Methods for calculation of the latter two variables) of the Swallow in Britain.

5th North Year Mean Number percentile coefficient

1888 110.2 24 104 1.55

1893 103.2 86 87 2.08

1896 108.8 81 100 1.01

1900 111.3 86 102 2.23

1903 111.3 81 95 2.48

1908 116.3 81 99 2.21

1912 110.0 77 96 1.76

1913 108.4 86 93 1.77

1916 113.6 88 101 1.33

1920 109.0 146 98 1.07

1921 110.4 136 94 2.31

1922 108.4 172 98 1.23

1929 108.9 290 96 1.90

1933 109.5 325 98 1.43

1936 112.2 319 100 2.02

1937 107.4 294 95 2.51

1938 110.4 277 92 2.81

1943 107.5 157 94 2.85

1944 105.5 160 92 2.30

1947 110.8 155 95 1.69

Average 109.7 – 96.5 1.93

sd 2.78 – 4.02 0.55

Range 13.1 – 17 1.84

Figure 2. Speed of progression northward plotted against the 5th percentile of arrival dates of Swallow in Britain.

The equation for the regression line is: speed = 10.8 – 0.0916 ×date (R²= 30.9%).

86 88 90 92 94 96 98 100 102 104

3 2.8 2.6 2.4 2.2 2 1.8 1.6 1.4 1.2 1

Speed northward (days/100 km)

Arrival date (Julian day)

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tion with the date at which Swallows first entered Britain.

In regressions involving a single weather variable, mean annual arrival date in Britain

was only correlated with April temperatures in England and in Nantes, and with March temperature in Iberia (Fig. 3). Each of these three weighted regression analyses was signifi-

5 6 7 8 9 10 11

a

Mean monthly temperature (°C) 118

116 114 112 110 108 106 104 102

8 9 10 11 12 13 14 15

b

116 114 112 110 108 106 104 102

11 11.5 12 12.5 13 13.5 14 14.5 15

c

116 114 112 110 108 106 104 102

Figure 3. Weighted linear regression of Swallow arrival dates against English and European mean monthly temperatures. (a) April in central England; (b) April in Nantes; (c) March in Iberia (average of Madrid, Palma and Gibraltar). The equations for the regression lines are: (a) arrival date = 118 – 1.02 ×(CET April temperature), R²= 37.3%; (b) arrival date = 121 – 1.10 ×(Nantes April temperature), R²= 39.9%; (c) arrival date = 128 – 1.50 × (Iberian March temperature), R²= 21.7%.

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cant (P < 0.005, P < 0.005 and P < 0.05 respectively). In each case, the higher the temperature, the earlier Swallows arrived in Britain. A 1°C increase in average temperature for central England (April), Nantes (April) and Iberia (March) was respectively associated with Swallows arriving 1.0, 1.1 and 1.5 days earlier in Britain.

Weighted multiple regression analysis was used to try to explain more of the variation in arrival dates. The best result was produced with a model in which Iberian March temperature and the gradient between CET in April and Iberian March temperature (i.e. Iberian March minus CET April) were included. This model explained 43.1% of the variation in Swallow arrival dates and both independent variables were significant. In this model, a 1°C increase in Iberian March temperature was associated with arrival 1.7 days earlier and a 1°C increase between Iberia and central

England (i.e. England temperatures low rela- tive to those in Iberia) was associated with arrival 0.84 days later (Fig. 4).

DISCUSSION Data reliability

The phenological records of the Royal Meteor- ological Society provided the dates on which Swallows were first seen at each locality. The advantage of recording first events is that they are easy to collect, especially for such a common, popular and easily identifiable species as the Swallow. Detailed study of bird migration using first observations has been criticized⁶ because it was thought that they might report atypical behaviour of individual birds before the main flow of the population under study. However, as the current study is involved with broad-scale movements of

Figure 4. Arrival dates of Swallow in Britain plotted against Iberian March temperature and the temperature gradient from Iberia in March to England in April. The equation for the regression line is: arrival date = 126.6 – 1.69

×(Iberian March temperature) + 0.843 ×(Iberian March temperature – CET April).

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Swallows throughout Britain over a 65-year period, records from single localities will not have undue influence.

Spring migration

The arrival dates of the Swallow presented here agree fully with the results provided by Riddiford and Findley²²and with the pioneer- ing paper of Southern.²³Although Southern did not note a southwest to northeast migratory axis in Britain, it is clearly visible in his data from continental western Europe. Furthermore, he estimated an average speed of progression northwards of about 25 miles per day (roughly 40 km per day), which is slower than the present results suggest, but of the same order.

The speed of migration must be faster in Africa to achieve the crossing from southern Africa to Britain in around three months (the average speed will be around 100 km per day, consider- ing a distance of 10 000 km between wintering and breeding grounds). Southern²³ explained this ‘leisurely’ final pace by mentioning that the birds seemed to be restricted by temperature in Europe.

Temperature is likely to have an indirect effect on the Swallow by affecting the abundance of its insect prey and the timing of their emergence. The increase in insect abundance starts at the beginning of April,²⁴but temperature also will have affected their larval development. The map of spring cumulative heat (Fig. 1d) and the maps of arrival dates (Fig. 1a, 1b and 1c) show that the southwest part of Britain by which Swallows arrive corre- sponds to the areas which received the most heat prior to and during their arrival and thus are likely to have the greatest insect abundance.

That Swallows enter Britain by the southwest rather than the southeast, where they would have experienced a shorter crossing over water, is further explained by looking at the route taken by Swallows in Spain and France before reaching Britain. Maps plotting the distribution of recoveries of ringed Swallows show that they follow a narrow corridor along the eastern coast of Spain and then along the western coast of France.²⁴Temperature maps of Europe^25,26 show that, in February and March, temperatures are higher along the eastern side of Spain and along the western fringe of France than in central parts of these countries. Swallows trav-

elling along this corridor are led to the tip of Brittany in France and thence to southwest England. The band of warmer weather along the coast of France expands inland during April. Swallows then have the possibility of moving along a broader front in France and to enter England along a broader front. This is consistent with the data published by Hudson.²⁷ He listed the 16 earliest records of Swallow in the UK between 1862 and 1972 during February. Out of these, nine were from southwest England, four from southern England and three from further north in Britain, but none was from southeast England.

Swallows progressed faster when arriving later. This suggests that in ‘early’ years Swallows are limited in their progression northward by environmental conditions, but that in ‘late’ years, weather conditions become suitable over the full extent of Britain allowing them to move northwards at a quicker pace.

The mean arrival dates in Britain were strongly negatively related to temperatures in central England and Nantes in April, and the average temperature in March in Iberia, each taken separately. Out of these variables, a single model was built to explain arrival dates in Britain. In this model, arrival date was influenced simultaneously by Iberian March temperature (arriving earlier when warmer) and the gradient between Iberian March temperature and England April temperature (the smaller the difference, the earlier they arrived).

Trends in spring migration

Spring migration is strongly influenced by the temperature patterns in England and Europe along the migratory route. Thus, Swallow spring migration could be a potential indicator of climate change. Mason¹⁵ showed that for Swallow between 1942 and 1991, although no trend over the years occurred, arrival dates in Leicestershire varied significantly between decades. Over an earlier but longer period of study, Sparks and Carey¹⁶ also found that different species of plants and birds had different trends over two centuries, with Swallows arriving significantly later between 1736 and 1947. Similarly, Crick et al.²⁸ found that most species of birds in the UK showed trends towards earlier egg-laying over the last 25 years.

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Thus, even if one can expect Swallows to arrive earlier if spring temperatures become higher, other factors will have an effect. It has been shown that, in long distance migrants, the onset of migration is controlled by photoperiod; this ensures that they arrive at their breeding grounds when environmental conditions are suitable.⁵If photoperiod were the only factor influencing Swallow migration, changes in the timing of migration would occur only after some time lag through natural selection.

However, as shown in this study, local weather conditions can influence arrival dates on a finer scale; Swallows can adjust their advance in accordance with local conditions encountered along the migration path.

Furthermore, European climate will only affect Swallows when they are present in Europe. Other factors in Africa, where they spend the remainder of the year, will also affect them. Mead showed that British Swallows moved their wintering quarters in southern Africa towards the west in the 1960s as a consequence of drought occurring in southeast Africa.²⁹ Møller also found that rainfall in Africa affected arrival dates of Swallows in Denmark.³ Marchant found that declines in European populations of trans-Saharan migrants were related to rainfall in Africa.³⁰ Finally, population size has an indirect effect on arrival dates. Birds are less likely to be detected very early in a smaller population. Thus population size should also be taken into account when studying changes in arrival dates. This is especially true for the Swallow which was expanding its range during the early part of the current century towards the northwest of Europe and Britain,^31,32 but has subsequently decreased in number.³³Therefore, even with a rise in spring temperatures in Europe, the Swallow tendency to earliness may be masked by population effects. Monitoring phenological events during the years to come might shed further light on this problem.

ACKNOWLEDGEMENTS

This study was part of a three-month project of the MSc course in Biological Computation of the Department of Biology of the University of York, conducted at Monks Wood research station of ITE. The GIS personnel helped us with the creation of maps. Chris Wernham at

the British Trust for Ornithology provided an updated map of Swallow ringing recoveries and also helpful comments. Humphrey Crick provided helpful comments on an earlier version of the manuscript and Chris Mason and Will Peach supplied constructive criticism.

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