Autumn departure Distance travelled Spring arrivalSource earliestlatest nearestfurthest earliestlatest

AF, AM, J Whitfield, in Wernham et al.

Moore (1976), Kilpi & Saurola (1984) Robertson (1969) Wernham et al. (2002)

Bregnballe et al. (1997) Phillips et al. (2005)

For migration dates of other North American species see Benson & Winker (2001) and Carlisle et al. (2005), and for sex-related differences in migration distances of Nearctic species wintering in the tropics see Komar et al. (2005).

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Day of arrival (standardised)

Figure 15.1 Spring arrival schedules of White-crowned Sparrows Zonotrichia leucophrys at Toiga Meadow Pass, California. The records for four years are standardised so that the date the first bird was captured each year was called Day 1 of arrival for that year. Older males (N = 82) arrived on average 5.4 days earlier than first-year males (N = 62, t = 4.20, P < 0.001), and older females (N = 43) arrived, on average, 5.0 days earlier than first-year females (N = 51, t = 2.51, P = 0.014). Overall, however, the arrival dates of different sex and age groups overlapped considerably. From Morton (2002).

- 1st year male

--1st year female

10 20 30 40 50

Day of arrival (standardised)

Figure 15.1 Spring arrival schedules of White-crowned Sparrows Zonotrichia leucophrys at Toiga Meadow Pass, California. The records for four years are standardised so that the date the first bird was captured each year was called Day 1 of arrival for that year. Older males (N = 82) arrived on average 5.4 days earlier than first-year males (N = 62, t = 4.20, P < 0.001), and older females (N = 43) arrived, on average, 5.0 days earlier than first-year females (N = 51, t = 2.51, P = 0.014). Overall, however, the arrival dates of different sex and age groups overlapped considerably. From Morton (2002).

in winter or returning there earlier in spring (the so-called arrival time or territorial defense hypothesis, Myers 1981). By remaining nearer the breeding areas, the argument goes, males should be better able to monitor and respond to variations in weather and thus return there as soon in any year as conditions permit, and before the females which winter further away (Alerstam & Hogstedt 1982). Because the females are not involved in territory establishment, they are under less pressure, and can arrive later, closer to the time when they can begin nesting. This does not mean that females do not also compete among themselves -for example, for the most desirable territory-holding males. The idea is that there are both costs and benefits associated with arrival time, and the males gain most from early arrival and the females from somewhat later arrival (Morbey & Ydenberg 2001). Adults that arrive after their former territory has already been occupied by another individual of the same sex are sometimes able to oust the newcomer, the outcome depending partly on how long the new bird has had to establish itself (Newton 1998b). In general, however, birds that arrive late relative to others of their sex have less choice and are relegated to less favoured territories, and might thereby produce fewer young (Chapter 14).

That this sex difference in arrival time has something to do with reproduction is consistent with the finding that in many species in which males migrate earlier than females in spring, when reproduction is about to begin, the two sexes migrate at the same dates as one another in autumn when reproduction has finished (for Bluethroat Luscinia luscinia see Ellegren 1990a, for Blackcap Sylvia atricapilla see Izhaki & Maitav 1998b, for Red-backed Shrike Lanius collurio see Tryjanowski & Yosef 2002). It is supported also by the fact that in species in which females rather than males establish territories, it is the females that arrive first. For example, in some shorebird species, females are bigger than males, and take on some aspects of breeding that in most bird species are undertaken by males. In these species, the females compete with one another for territories and for the later arriving males, as observed in Eurasian Dotterel Eudromias morinellus, the three phalarope species Phalaropus spp., Spotted Sandpiper Tringa macularia and others (Myers 1981, Oring & Lank 1982). In addition, in five European songbird species, the degree of difference between the spring passage dates of males and females was found to correlate with the levels of extra-pair paternity in these species (Coppack et al. 2006). It seemed that males arrived relatively more in advance of females in species in which sexual selection through female choice was likely to be intense.

Another possible explanation of the arrival difference is that it results from competition for food at stopping sites, in which the larger sex is less affected by aggressive interactions, so can refuel and complete the journey more rapidly (see later). Because food is scarcer in spring than in autumn, competition is more intense in spring, producing a bigger difference between the sexes then. Yet other explanations of the sex difference in arrival dates have been proposed, but are mainly applicable to animals other than birds (Morbey & Ydenberg 2001).

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