Timing of breeding events of the Lesser Spotted ...

Timing of breeding events of the Lesser Spotted Eagle Clanga pomarina as revealed by remote cameras and GPS-tracking Ülo Väli Väli Ü. 2017. Timing of breeding events of the Lesser Spotted Eagle Clanga pomarina as revealed by remote cameras and GPS-tracking. Ardea 105: ###–###. doi:10.5253/arde.v105i2.a1 Proper timing of events in the annual cycle is essential for successful breeding in birds, especially in long-distance migrants. In this study, the phenology of the main breeding phases, including arrival at and departure from the nesting site, is reported for a Lesser Spotted Eagle population. Most data were gathered by 12 to 22 trail cameras annually installed at eagle nests. Additional information was obtained from two web cameras and up to nine GPS-tracked birds. Arrival and departure dates fluctuated between years, but no temporal trends were detected. There was no significant sex-dependent difference in mean arrival time; however, males departed significantly later than females and thus spent longer at the nesting site. Timing of egg-laying was determined solely by the arrival of the later partner. Breeding phenology of the Lesser Spotted Eagle is rather similar across its distribution range, although breeding tends to be earlier at the southern range limit. Key words: annual cycle, arrival, breeding, camera trap, departure, migration, phenology, raptor, telemetry Department of Zoology, Estonian University of Life Sciences, Kreutzwaldi 5, 51014 Tartu, Estonia; Eagle Club, Hauka, Valgjärve, Põlvamaa 63406, Estonia; ([email protected])

The regular change of the seasons shapes annual life cycles in nature. Animals must time their activities well in relation to these seasonal fluctuations, as this has direct consequences for their survival and reproduction (Perrins 1970, Thomas et al. 2001). For example, reproductive success usually declines during the breeding season and breeding too late decreases both the body condition and survival probabilities of offspring (Perrins 1970, Verboven & Visser 1998, Bêty et al. 2004, Verhulst & Nilsson 2008). Compared to sedentary species, which must respond only to local conditions, the breeding phenology of migratory species additionally depends on the timing of migration (Bêty et al. 2004, Drent 2006, Väli 2015a). This relationship is particularly influential in long-distance avian migrants (Sanderson et al. 2006, Both et al. 2010). The Lesser Spotted Eagle Clanga pomarina is a large migratory raptor breeding in Central and Eastern Europe and wintering in southern Africa. Its arrival and departure dates have been reported in observational studies (Abuladze 1996, Drobelis 1996, Ivanovsky 1996). In recent years satellite-tracking has revealed

more details on its migration (Meyburg et al. 1995, 2004, 2006, 2007, Meyburg & Meyburg 2009). Knowledge about timing of breeding phases, such as egg laying or the hatching and fledging of young, relies on occasional nest visits or observations made at only a few nests (Siewert 1932, Gentz 1965, Meyburg 1970, Scheller & Meyburg 1996). The link between migration and breeding phenology has also not yet been described. Previously, collecting phenological information at nests required a lot of effort (e.g. Meyburg 1970, Väli & Lõhmus 2002). Moreover, prolonged observation periods at nests of rare and sensitive species have been hampered by precautions for conservation reasons. Nowadays, the use of novel technologies, such as automated remote cameras (O’Brien & Kinnaird 2008, Rollack et al. 2013) and tracking devices (Meyburg & Fuller 2007), can diminish these obstacles. In the current study I used automated photo cameras and web cameras, as well as GPS-tracking, to characterise the breeding phenology of the Lesser Spotted Eagle. I focused on intra- and inter-annual

ARDEA 105(3), 2017

temporal variation of timing and tested how much could be attributed to sex-specific differences. I also analysed whether changes in phenology could be attributed to climate change, as has been found in many other species (Jenni & Kéry 2003). Finally, I discuss potential biases in phenological studies and assess spatial variation in the breeding phenology of the Lesser Spotted Eagle by comparing phenological records from different time periods and regions across its distribution range.

METHODS The study was conducted in eastern and south-eastern Estonia, in the counties of Jõgeva, Tartu and Valga (57.9–58.8°N, 26.2–26.9°E) in 2011–2017. Birds were followed at their nest sites from arrival until departure, using trail cameras, web cameras, or GPS-transmitters or -loggers. Automated motion- and heat-sensor trail cameras (Trophy Cam 119435 and 119436, Bushnell, or M80 and M880, Moultrie) were mounted at nests: 15 in 2011–2013, 18 in 2014, 12 in 2015, 22 in 2016 and 20 in 2017. Trail cameras were installed at distances of 0.5–5 m (usually 1.5–3 m) from the nest in the last days of March or first days of April, before arrival of the birds. Cameras had additional external batteries and operated until the end of the breeding season. If a nest remained unoccupied, the camera was removed and, if possible, moved to another nest, mostly during ringing of nestlings in July. C. 80,000 photos were taken at 10 nests in 2011, 68,000 photos at 13 nests in 2012, 69,000 photos at 13 nests in 2013, 45,000 photos at 8 nests in 2014, 60,000 photos at 8 nests in 2015, 136,000 photos at 17 nests in 2016, and 120,000 photos at 16 nests in 2017. Web cameras were used to observe activities at two breeding territories in 2011–2014. Cameras were installed c. 2 m from the nest, prior to arrival of the birds. Volunteers nearly continuously recorded activities of birds at the nest until the end of the breeding season, and a complete set of phenological information was collected up to the last nest visits. GPS-based satellite transmitters (45 g, Microwave Telemetry Inc.) and GPS-GSM loggers (25–30 g, Ecotone) were fitted on nine adult birds (two birds in 2011, one in 2012, three in 2013 and three in 2014; Kotkaklubi & 5DVision 2016). Life-span of birds and tracking devices varied, and therefore some birds gave information only for a few months while, on the other hand, a female equipped with a transmitter in 2011

provided data throughout the whole study period. As a result, two birds were followed in 2011, two birds in 2012, five in 2013, eight in 2014–2016 and five in 2017. All GPS-tagged birds were ringed with an aluminium ring and a white plastic colour-ring, and sexed by wing length or a molecular method using a feather or blood sample (Väli 2004, 2012). Additionally, 18 other adults have been trapped and colourringed. This facilitated identification of sexes and individuals at the nests. Arrival of birds at specific nests was recorded by cameras or GPS-tracking, but timing of departure could only be reliably estimated by GPS-tracking. In nine cases, estimated arrival date based on GPS-tracking and on trail cameras were the same. In three cases, the trail camera registered arrival one day later than GPStracking, and in one case two days later. This confirms that trail-cameras provide relatively reliable information about arrival time. However, I excluded three cases of extremely late arrival from the analysis (14, 17 and 26 May 2015), which were registered by cameras only. Although arrival in this year was very late indeed, I suspect that these birds had previously occupied another nest. Latest arrivals (in 2015 and in 2017), as verified by GPS-tracking, occurred on 5 May. Nesting territories of the Lesser Spotted Eagle often contain more than one suitable nest, built by the eagle or another large raptor. Cameras revealed that birds may visit several nests in their territories immediately after arrival, albeit they ultimately occupy only one. The Lesser Spotted Eagle starts incubation after laying the first egg (Cramp & Simmons 1980, Glutz von Blotzheim et al. 1989). Laying date of eggs was registered when the egg was seen on the photo or when an adult (presumably the female) stayed on the nest continuously while the presence of an egg was verified later from subsequent photographs. Similarly, hatching date was registered when the nestling was seen or when the female started taking care of the nestling, which was easily distinguished from the regular checking and rolling of eggs. Detecting the laying date of the second egg (and hatching date of the second nestling) was more difficult as behavioural changes were subtle. Hence, dates could be recorded for only eight second eggs and five second nestlings. Also, detection of fledging was difficult as this is a continuous process from the nestling’s first jumps to side branches (which in a few nests were out of camera range) until they fly away from the nest. In the current study fledging date was defined as the day when offspring left the nest for more than six hours, after that fledglings usually did not return to sleep at the nest.

Väli: BREEDING PHENOLOGY IN EAGLES

A

B

C

D

E

F

Photo 1. Pictures from the trail-cameras situated at nests of Lesser Spotted Eagles in Estonia, illustrating various phases of the breeding cycle: (A) arrival at the nest and reconstruction of the nest structure, (B) egg laying and incubation, (C, D) hatching and raising the offspring, (E) fledging and (F) the last visits of the season to the nest.

Dates of phenology were described by the median, 25% and 75% quartiles, and minimum and maximum. In most cases the difference between mean and median values was limited to just one day, and therefore means are usually not presented. Inter-annual variation could be analysed for arrival and departure dates, whereas data of breeding events had to be pooled due to small

annual sample sizes. Linear mixed models (function lmer in the package lme4; Bates et al. 2015) were used in the statistical environment R v. 3.2.3 (R Development Core Team 2016) to test for differences in timing of breeding events between various groups. I included date as a dependent variable and sex or age (adult, juvenile) as a categorical predictor. Identity of breeding

ARDEA 105(3), 2017

territory and year were included as random factors, to account for differences in territory quality. To study inter-annual variation among arrival and departure dates, year was included as a fixed factor and only breeding territory remained as a random factor in the model. A likelihood-ratio test with chi-square approximation was used to test the significance of the models and multiple Tukey post hoc comparison was used subsequently using the function glht in the package multcomp (Hothorn et al. 2008). Fisher's exact test was used to test for bias from equal ratio of dates of firstarriving males and females. 12

The long-term trend in arrival of the species was estimated by a linear regression model based on published observations from all over Estonia made by the ornitho-phenological network of the Estonian Naturalists Society and Estonian Ornithological Society in 1953–1996. This data was obtained from previous publications compiled by Rootsmäe & Rootsmäe (1974, 1976, 1978, 1981) and Rootsmäe (1991a, 1991b, 1998). Differences between means and medians from the long-term and recent data sets were assessed using t-tests and Mann-Whitney-Wilcoxon tests, respectively.

arrival

laying eggs

hatching

fledging

last nest-visit

departure

10

no. of observations

8 6 4 2 0 1

8

15 22 April

29

6

13 20 May

27

3

10

17 24 June

1

date

8

15 22 July

29

5

12 19 26 August

2

9 16 23 September

30

Figure 1. Frequency distributions of main events in the annual cycle of the Lesser Spotted Eagle in Estonia in 2011–2017 (years pooled). Table 1. Phenology of main breeding season events for the Lesser Spotted Eagle in Estonia in 2011–2017 (data from different years pooled). Activity Arrival to the nest Laying of the 1st egg Laying of the 2nd egg Hatching of the 1st offspring Hatching of the 2nd offspring Fledging of offspring Last visit to the nest All birds Males Females Juveniles Start of autumn migration

Median

25%–75% quartiles

Min–Max

n

18/04 31/04 05/05 10/06 15/06 08/08

12/04 – 21/04 27/04 – 04/05 31/04 – 07/05 05/06 – 12/06 09/06 – 15/06 04/08 – 09/08

07/04 – 05/05 24/04 – 09/05 30/04 – 12/05 02/06 – 19/06 08/06 – 15/06 31/07 – 15/08

103 21 8 18 5 17

19/08 18/08 19/08 18/08 19/09

12/08 – 11/09 09/08 – 07/09 12/08 – 31/08 10/08 – 29/08 17/09 – 21/09

31/07 – 17/09 06/08 – 17/09 04/08 – 12/09 31/07 – 18/09 7/09 – 27/09

18 11 10 18 36


RESULTS 40

date (1 = 1 April)

30 20

12

16

10

40 30

date (1 = 1 September)

19

2

2

5

21

16

2012** 2014** 2016** 2017***

2012* 2014* 2016* 2017**

1

20

10

14

8

A

7 7

5

10 1

2011

2012

2013

2014

year

2012** 2016*

2015

2016

2017

B

Figure 2. Inter-annual variation in arrival (A) and departure (B) times of the Lesser Spotted Eagle in Estonia. Thick line indicates median, box quartiles and whiskers minimum and maximum values. Sample sizes are indicated above and significant differences between specific years below the boxes (*Padj < 0.05, ** Padj < 0.01, ***Padj < 0.001).

Arrival dates of Lesser Spotted Eagles in Estonia ranged from early April until early May, but most of the studied birds arrived at their nest sites during the third week of April (Table 1, Figure 1). There were no differences in arrival date between nests which were later occupied and those which were not (c21 = 1.6, n = 54, P = 0.45). Therefore all nests were included in the analysis of arrival date. Arrival dates differed significantly between years (c26 = 31.6, P < 0.001; Figure 2A): birds arrived relatively early in 2012, 2014, 2016 and 2017, and relatively late in 2013 and 2015. In only five cases partners from the same nest arrived on the same day, on average there was a four-day difference (Table 2). In 13 cases the female was the first to arrive, and in nine cases the male arrived first, but this difference was not significant (P < 0.76). Also, there was no significant difference between mean arrival dates of males and females (c21 = 0.35, P = 0.86; Figure 3). The first egg was laid in late April or within the first days of May, the second one during the first week of May (Table 1, Figure 1). On average, the first egg was laid about two weeks after the arrival of birds (Table 2). There was no difference between this interval in males and females (c21 = 0.28, P = 0.59). There was a positive linear relationship between laying date and the arrival date of the second bird (c21 = 4.9, P = 0.03), but not with that of the first bird (c21 < 0.1, P = 0.93; Figure 4). The correlation between laying date and

Table 2. Phenology of main breeding season events for the Lesser Spotted Eagle in Estonia in 2011–2017 (data from different years pooled). Activity Between arrival of the 1st and 2nd adult Between arrival of male and female Between arrival of the 1st adult and laying the 1st egg Between arrival of the 2nd adult and laying the 1st egg Between arrival of the male and laying the 1st egg Between arrival of the female and laying the 1st egg Between laying the 1st and 2nd egg Between hatching of 1st and 2nd nestling Incubation time of the 1st egg Incubation time of the 2nd egg Nestling time of the 1st offspring Time at the breeding territory All birds Males Females

Median

25%–75% quartiles

Min–Max

n

2 0 16 12 12 14 4 4 39 39 58

1–5 -2–2 13–19 8–15 10–18 12–16 3–4 2–4 39–40 38–39 56–59

0–19 -10–21 7–27 6–23 6–27 7–26 3–5 2–4 38–43 38–39 56–63

43 29 19 18 14 16 8 5 14 5 10

154 155 145

148–156 154–159 140–149

131–167 144–167 131–155

28 18 10

ARDEA 105(3), 2017

25

date (1 = 1 April)

30

10

20

25 20

15

15

10

10 5

females

A

5

males

males

females

B

Figure 3. Arrival (A) and departure (B) times of female and male Lesser Spotted Eagles in Estonia. Thick line indicates median, box quartiles and whiskers minimum and maximum values.

laying date (1 = 1 May)

10 5 0 1st bird linear

–5

–10

5

10

15

20

arrival date (1 = 1 April)

25

2nd bird linear

30

Figure 4. Relationships between arrival of Lesser Spotted Eagles at nests and laying dates of the first egg in Estonia.

arrival was significant for males (c21 = 5.4, P = 0.02), but not for females (c21 = 2.3, P = 0.12). Chicks hatched in the first half of June (Table 1, Figure 1). In all cases where two chicks hatched, the second chick died due to siblicide. Offspring fledged within the first two weeks of August. Fledging dates of young from two-chick broods (4, 8, 9 and 11 August) were similar to fledging dates of young from one-chick broods. Eaglets continued visiting nests after fledging, but the variation among dates of the last visit was notably large (Table 1, Figure 1). There was no significant difference between dates of last visits by adults and juveniles after fledging (c21 = 0.9, P = 0.36), nor between males and females (c21 = 0.1, P = 0.74), although the variance was larger among males (Table 1). Adult birds started their migration usually in the third week of September (Table 1, Figure 1), but departure times differed significantly between years (c26 =

16.7, P = 0.010; Figure 2B). Birds left the breeding grounds relatively early in 2012 and 2016 and relatively late in 2015. Males departed later than females (c21 = 5.9, P = 0.014; Table 1, Figure 3), thus they spent longer time at the nest site than females (c21 = 11.7, P < 0.001; Table 2). Based on occasional observations in 1957–1996, the mean and median arrival date of Lesser Spotted Eagles in Estonia was 14 April (quartile range = 6–22 April, n = 97), which was not significantly earlier than the arrival of first birds recorded by cameras and GPStracking in the current study in 2011–2017 (mean = 17 April, median = 16 April; t151 = 1.6, P = 0.11). There was no long-term trend towards earlier or later arrival during that period (r = –0.02, n = 97, P = 0.88; Figure 5A). The mean departure date in 1953– 1986 (18 September, quartile range = 13–20 September, n = 16) was similar to that in recent years (19 September; t17 = 0.15, P = 0.88). Also, the median departure date (15 September) was not significantly different from the median from recent years (W = 377, P = 0.08). Long-term data on timing of departure did not suggest any temporal trend (r = 0.08, n = 16, P = 0.77; Figure 5B). 50

A

40 30

days (1 = 1 April)

26

20

10

0

–10

–20 50

B

40

days (1 = 1 September)

41

31

35

30

date (1 = 1 September)

40

30 20

10

0

–10

1950

1960

1970

year

1980

1990

2000

Figure 5. (A) Arrival dates of Lesser Spotted Eagles in Estonia in 1957–1996 and (B) departure dates in 1953–1986. The line indicates linear trend.


DISCUSSION Tracking individual Lesser Spotted Eagles gave objective information about arrival and departure dates, whereas automated cameras revealed the activities at their nests. By combining the two methods the whole breeding period, from arrival to the nest in spring to departure in autumn, was covered. Previously published data suggested that the first Lesser Spotted Eagles would arrive in Estonia in late- or even mid-March and that the majority of birds would arrive in April (Randla 1976, Tammur 1994). The first birds arrived by the end of the first week of April, and half of the population during the third week of April. I did not record any arrival in March, but this may have been due to the limited duration of the study or restricted sample size. The small, non-significant difference between (visual) observations and the results from the current study may also reflect methodological differences, as casual observations tend to be biased towards earlier records (Lehikoinen et al. 2004). For example, tracked birds in Germany arrived later than estimated from observations (Meyburg et al. 2006, Meyburg & Meyburg 2009). However, it has been suspected that German Lesser Spotted Eagles do indeed now arrive later on the breeding grounds than they did in the past (Meyburg et al. 2007). No significant trend in arrival dates was found in Estonia during the second half of the 20th century (Figure 5A). Another drawback of visual observations is that they may suffer from

misidentifications of Greater Spotted Eagles Clanga clanga (Lõhmus 1996), a species that was more common (but still rare) in the past than nowadays in Estonia (Väli 2015b), and arrives relatively often in late March or early April (Ü. Väli & U. Sellis, unpubl. data). The sparse information that was hitherto available on clutch initiation of Estonian Lesser Spotted Eagles (Randla 1976) indicated that clutches were laid in early or mid-May, but our data showed first egg dates between late April and early May. Although the earlier conclusion may have been based on a limited dataset, the advancement of laying date may also be real as the same has been noticed in many other bird species, least so, however, in long-distance migrants (Lehikoinen et al. 2004). Departure of Lesser Spotted Eagles occurs within a shorter time frame than arrival (Table 1, Figure 2). This leads to the synchronized autumn migration of the population, which has been noticed in migration counts. For instance, 90% of the Lesser Spotted Eagles pass Georgia between 16 September and 9 October (Verhelst et al. 2011) and Israel between 21 September and 5 October (Leshem & Yom-Tov 1996, Alon et al. 2004, Krumenacker 2012). These dates fit well with the results of the current study as the eagles fly on average in two weeks from Estonia to Israel (Kotkaklubi & 5DVision 2016). I did not detect any advancement of the autumn departure over time, which has been recorded in many long-distance migrants (Jenny & Kéry 2003).

Table 3. Information on the breeding phenology of Lesser Spotted Eagles from countries other than Estonia. Minimum, maximum and mean (if available) values are presented, except for Lithuania from where only 5%- and 95%-quantiles were available. Country

Study period

Egg-laying

Hatching

Germany

1930s, 1950s, 1965, 1992–93

30/04 – 10/05

12/06 – 20/06

Kaliningrad 1928–36 (Russia) / Poland

5/05 – 12/05

11/06

Lithuania

1978–91

21/04 – 15/05

Belarus

1981–91

21/04 – 7/05 (mean = 27/04)

Slovakia

1968–69

26/04 – 01/05

05/06 – 13/06 (mean = 9/06)

Georgia

1973–91

12/04 – 28/04

23/05 – 12/06

Greece

1884–88, 1985–87

10/04 – 30/04

Early June

6/06 – 24/06

Fledging

Method

Reference

Observations or video-camera at nest

Siewert 1932, Wendland 1959, Gentz 1965, Scheller & Meyburg 1996

Not described

Wendland 1959

Not described

Drobelis 1990, 1996

Not described

Ivanovsky 1996, Ivanovsky et al. 1999

11/08

Observations at nest

Meyburg 1970

7/07 – 01/08

Not described

Abuladze 1996

Not described

Makatsch 1950 in Glutz von Blotzheim et al. 1989, Vlachos & Papageorgiou 1996

30/07 – 18/08

28/05 – 14/06 23/07 – 16/08 (mean = 06 – 08/06) (mean = 06/08)

ARDEA 105(3), 2017

Earlier publications suggested that female Lesser Spotted Eagles arrive before males, although it always was indicated that this pattern was not absolute (Meyburg 1970 and references therein). The current study draws the same conclusion: although earlier arrival of females was observed more frequently, the difference was not significant, and arrival date did not differ between sexes for the population (Figure 3A) nor within the pair (Table 2). Instead, timing of departure differed clearly between sexes, with males leaving later than females. This resulted in longer residence times for males at the breeding territories compared to females. One should note, however, that in the current study departure times of females were based on data from two GPS-tagged birds only; more birds should be studied to be sure about sex differences in the timing of departure. Nevertheless, these estimates are similar to the previous estimations from Germany (Scheller & Meyburg 1996, Meyburg et al. 2004, 2006, Meyburg & Meyburg 2009). In the northern hemisphere, length of the breeding period tends to increase from north to south in raptors (Newton 1979). Table 3 provides an overview of the breeding phenology of Lesser Spotted Eagles across their range. Although the general north-south trend of timing is noticeable, differences are not large and only in the most southern parts of the breeding range (Georgia, Greece) timing of breeding seems to be about 10 days earlier. This similarity across most of the climatically varying range suggests that breeding phenology is not determined by local factors at the breeding grounds only, but that factors during the nonbreeding period also play a role. Indeed, late arrival was registered in several European countries in 2015 (personal communication with European Lesser Spotted Eagle experts). Reasons for that were hidden in Africa, since all tracked Estonian birds had a long stopover in Uganda during spring migration in 2015 (Kotkaklubi & 5DVision 2016). Moreover, six out of eight birds were still in Africa on 9 April 2015, which normally is the time when the first birds arrive to Estonia. Notably, in 2015, the eagles also departed the breeding grounds earlier than in other years. It has never been easier to share observations than in the current era of internet and online databases, thus observational archives are growing fast. However, it is often not possible to extract enough relevant phenological information from these datasets, especially on the timing of breeding phases. At the same time, novel technologies facilitate the exploration of timing of life history events as demonstrated here for the Lesser Spotted Eagle. Quantitative studies of phenological

responses of various species remain necessary, especially in the context of climate change, which may affect the timing and duration of each phase of their annual cycle, breeding performance and ultimately their survival (Møller et al. 2008, Dunn & Winkler 2010, Lehikoinen & Sparks 2010).

ACKNOWLEDGEMENTS Urmas Abel, Tarmo Evestus, Jaan Grosberg, Pelle Mellov, Ain Nurmla and Joosep Tuvi assisted in setting up trail cameras, Urmas Sellis and Joosep Tuvi installed and maintained the web cameras. Urmas Sellis also helped to equip birds with GPStransmitters and -loggers and to compile the raw GPS-data. Numerous people were following web cameras and participated in data gathering in a special web-based forum. Rob G. Bijlsma, Raymond Klaassen and an anonymous reviewer commented on the first drafts of the manuscript. The study has been supported by the Estonian Environmental Board and by the institutional research funding grant IUT21-1 from the Estonian Ministry of Education and Research.

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SAMENVATTING Voor een succesvol broedseizoen is het erg belangrijk dat vogels een optimale timing van de verschillende fasen binnen de jaarcyclus aanhouden. Dit geldt in het bijzonder voor langeafstandstrekkers. In dit onderzoek is bij een populatie van Schreeuwarenden Clanga pomarina in Estland de fenologie van de belangrijkste broedfases gemeten, inclusief de timing van de aankomst op en het vertrek van de nestplaats door elk van de ouders. De meeste gegevens werden verzameld door in de broedseizoenen van 2011–2017 12 tot 22 wildcamera’s te installeren bij nesten. Aanvullende informatie werd verkregen door middel van twee webcamera’s en door maximaal negen vogels per jaar uit te rusten met GPS-satellietzenders. Aankomst- en vertrekdata konden behoorlijk variëren tussen jaren, maar, gebaseerd op deze en gepubliceerde gegevens, was er geen trend te ontdekken in de periode 1953–2017. Ook was er geen significant verschil in de gemiddelde aankomstdatum van mannetjes en vrouwtjes. Wel vertrokken mannetjes significant later dan vrouwtjes naar de overwinteringsgebieden, waardoor mannetjes langer aanwezig waren op de nestplaats. De datum waarop de eieren werden gelegd, hing af van de aankomstdatum van de laatste vogel van een paar bij het nest, niet door de aankomst van de eerste vogel. De broedfenologie van de Schreeuwarend varieert weinig over het verspreidingsgebied, hoewel er een tendens is naar eerder broeden in de meest zuidelijk gelegen broedgebieden. Corresponding editor: Raymond Klaassen Received 18 October 2016; accepted 7 October 2017