Social migratory connectivity: do birds that socialize in winter breed together?

Researching the complete life cycles of migratory animals is essential for understanding conservation and population dynamics. Many studies focus on the breeding season, but surviving winter is equally important. Living in groups during winter can play a vital role as social connections within groups can provide many benefits such as protection from predators and increased access to resources. However, it is often unknown how social connections change across seasons in migratory animals. We focus on social connections in a migratory bird and ask whether social connections in winter continue during breeding. Golden-crowned sparrows have distinct, stable winter communities which include both site and group fidelity across years: birds almost always rejoin the same social community each year. If these birds have social connectivity across migration, we would expect individuals that associate in winter would also associate together on their breeding grounds. Our small-scale GPS tagging study combined with intensive social behavior data revealed that sparrows in the same tightly-knit winter community migrated to highly disparate locations during summer, showing that social connections in winter do not continue in summer. This suggests that golden-crowned sparrows have entirely separate social structures across seasons and that long-term social memories allow them to reform stable groups each winter.

97 through vocal communication (e.g., [19]). Evidence from other species suggests that young 98 animals make social connections with neighbors early in life; for example, captive barnacle 99 geese (Branta leucopsis) formed connections early in life and carried these same preferences into 100 adulthood [20] and across seasons [21]. Currently, we know that golden-crowned sparrows have 101 broad-scale migratory connectivity, as birds wintering in coastal California tended to go to 102 coastal areas in Alaska while more inland birds went to more inland areas in the north of Alaska 103 to breed [22,23]. 104 Here, we ask if golden-crowned sparrows have social migratory connectivity, or, if they 105 maintain close social connections from the nonbreeding season during the breeding season. Two 106 main patterns are possible: 1) golden-crowned sparrows from the same winter social 107 communities could breed in close proximity, or 2) winter associations may be entirely 108 unconnected to breeding locations and associations. The first pattern would reveal that the 109 sparrows maintain social connections year-round and strongly suggest that associations during 110 the breeding season and first migration are critical for establishing and maintaining the close 111 social associations we observe in these birds during winter. The second pattern would be equally 112 interesting as it would reveal that these birds have a strong capacity to remember individuals and 113 have long social memories to reform stable communities each winter.

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All four of the tagged birds had summer locations far away from each other, with the 223 closest 698 km apart and the most distant 1837 km apart (Fig 1). The three birds in 2017, all 224 from the same tight-knit winter social group, had very distant breeding territories, so we found 225 no evidence that winter community affected the proximity of summer breeding territories for 226 these individuals. Note that the scale on which birds overlap during the winter is hundreds of 227 meters, while in the summer, all sparrows were many hundreds of kilometers apart. Bird 19388, 228 which we tracked in 2018, was also present on the wintering grounds at the Arboretum the 229 previous year with the three other birds (2017), and it was in a separate social group from those 230 three (Fig 2). The data from the GPS tags showed precise locations, with the ability to even see 231 approximate potential nesting locations from the density of points in one location (Fig 3). The two main clusters of points are sequential and could reflect a second nesting attempt, 263 perhaps after nest predation, or the adult's movement with fledglings away from the nest. We 264 found a similar pattern with bird 19388. 265

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The sparrows' spring migrations followed the general coastline, but the birds did not use 267 the same routes or stopover spots (Fig 5) How quickly birds migrated made the difference in how long they spent on their breeding 281 territory. Spring migration duration ranged from 32-62 days. The sparrows departed for spring 282 migration from the Arboretum within 12 days of each other, and fall migration departure was 283 within seven days of each other (Table 1). Due to such similar migration departure dates in both 284 spring and fall, time on the breeding territory appeared to be constrained by the duration of 285 spring migration (Fig 4). We might expect that a closer breeding location would allow the birds 286 to get there more quickly. However, spring migration distance appeared unrelated to how many 287 days it took the birds to migrate and even suggested a negative relationship (Fig 4).

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The sparrows had a wide range of within-community strength scores, but within-289 community strength showed no pattern when compared to migration duration or distance (Fig 4).
290 Nonetheless, the limited sample size warrants caution on this interpretation. following the curved coastline of Alaska (see Fig 1).

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Three of the GPS-tagged birds were in the same community (Fig 2), so if close winter 353 associations were linked to proximal breeding locations, we would have expected birds in the 354 same community to have close breeding associations. While this study is limited by a small 355 sample size, the fact that three of the sparrows were from the same social community indicates 356 that social relationships during winter do not carry over to the breeding season.