North American Birds

VOLUME 68 NO4 2015

A Quarterly Journal of Ornithological Record Published by the American Birding Association. The mission of the journal is to provide a complete overview of the changing panorama of our continent’s birdlife.

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regression line between the record date (day and month) and the year. One advantage of this technique is that it is independent of ob- server effort. Sixteen of Indiana's 36 regularly occurring warblers (Table 1) were selected to evalu- ate changes in the spring arrival dates of mi- grant warblers. The chosen species were neither regular summer nor winter residents, which eliminated the problem of distinguishing mi- grant records from those of territo- rial birds. Black-and-white Warbler (Mniotilta varia) was also considered; it is a rare summer resident and was excluded from the analysis. A database of Indiana bird records has been constructed over the past 35 years. It initially contained the rou- tine feld reports and observations of hundreds of birders. In late 1998, the advent of IN-BIRD, a listserve of In- diana bird reports, greatly expanded the rate of data entry. More recently, eBird also provided copious amounts of data. Accordingly, the rate of data entry has accelerated in recent years. All records (n=67,942) of each species reported in Indiana between 1 March and 10 June were extracted from the database for the twenty-year interval 1996 to 2015, inclusive. These dates are generally accepted as the marginal limits for the spring passerine migration in the Midwest. The goal of this project was to an - swer a narrow question: Has the tim- ing of the spring passage of migrant warblers through Indiana changed in the last 20 years? A result easily expressed as a mathematical formula that quantifed the trend was also highly desirable. For those reasons, a simple linear regression was chosen as the most appropriate statistical procedure. The outputs of interest from the linear regressions are the slopes, which quantify the change in migration timing, and the tests of tire spring migration data set of routine bird- er feld reports to evaluate changes in the ar- rival dates for 16 warbler species in Indiana. Method The strategy of this approach is to employ routine birding observations to construct a n o r t h a m e r i c a n b i r d s 564 Earlier spring arrival of warblers in Indiana Earlier spring arrival of warblers in Indiana KEnnEth J. BrocK • 1265 Redbud dRive • ChesteRton, indiana 46304 • (Kj.bRoCK@ComCast.net) randall J. Pals • 1233 moRningside dRive • ChesteRton, indiana 46304 • (Rjpals@ComCast.net) abstract Spring feld observations of 16 migrant war- bler species, collected in Indiana over the past 20 years, were examined using linear regres- sion and revealed that all 16 species are in- deed arriving earlier. Regression of the data from all 16 species combined revealed that the entire group now arrives some 4.76 days earlier than they did 20 years ago. This result also under- scores the value of recording and maintaining routine feld data. Introduction Over the past dozen years, several studies dealing with changes in spring avian arrival dates have been published. Some of the earliest in- vestigations came from Europe: in a survey of the impact of climate change on various aspects of avian populations, Crick (2004) noted the existence of a substantial body of ev- idence indicating changes in migra- tion timing. Similarly, Cotton (2003) used 30 years of frst arrival dates to conclude that 20 migrant bird spe- cies were on average arriving 8.03 days earlier than they did in 1973. In North America, Strode (2003) found no signifcant change in Midwestern warbler arrival dates, and Mills (2005), investigating frst arrival dates at the banding Long Point Bird Observatory, Ontario, discovered that only two of 13 pas- serine species showed signifcantly earlier passage times. Miller-Rush- ing and others (2008) examined 33 years of capture data at Manomet, Massachusetts to evaluate varia- tions in the spring migration of 32 North American passerines. They concluded that frst arrival dates often differed dramatically from "the mean arrival date of the mi- gration cohort as a whole." With this conclusion, and these previous studies, in mind, we used the en- Table 1. statistical data for 16 spring warbler species. Warbler species n line slope (days/yr) 95% c.I. p-value golden-winged Warbler (Vermivora chrysoptera) 604 -0.2302 ±0.0861 < .0001 tennessee Warbler (Oreothlypis peregrina) 11,335 -0.2275 ±0.0234 < .0001 orange-crowned Warbler (Oreothlypis celata) 668 -0.2326 ±0.1251 0.0002 nashville Warbler (Oreothlypis rufcapilla) 7895 -0.2578 ±0.0267 < .0001 magnolia Warbler (Setophaga magnolia) 6253 -0.1117 ±0.0263 < .0001 Cape may Warbler (Setophaga tigrina) 1737 -0.2649 ±0.0546 < .0001 black-throated blue Warbler (Setophaga caerulescens) 713 -0.1358 ±0.0791 0.0004 black-throated green Warbler (Setophaga virens) 6046 -0.3883 ±0.0424 < .0001 blackburnian Warbler (Setophaga fusca) 2115 -0.2581 ±0.0552 < .0001 palm Warbler (Setophaga palmarum ) 16,672 -0.0889 ±0.0200 < .0001 bay-breasted Warbler (Setophaga castanea) 1714 -0.1925 ±0.0479 0.0001 blackpoll Warbler (Setophaga striata) 4121 -0.3546 ±0.0394 < .0001 northern Waterthrush (Parkesia noveboracensis) 4246 -0.2215 ±0.0375 < .0001 Connecticut Warbler (Oporornis agilis ) 382 -0.1846 ±0.0989 0.0002 mourning Warbler (Geothlypis philadelphia) 1038 -0.2714 ±0.0717 < .0001 Wilson's Warbler (Cardellina pusilla) 2403 -0.0606 ±0.0464 0.0053 16 species combined 67,942 -0.2380 ±0.0113 < .0001

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