Methods
Study system : The Massachusetts Butterfly Club (MBC) is a group
of citizen scientists which documents sightings of butterflies and
maintains a database of observation records. The MBC compiles records of
butterfly sightings from organized group trips and as reported
opportunistically by individuals. The current club database of records
from 1992-2018 includes 30,707 species lists from these reports
(hereafter referred to as trips) encompassing 162,670 sightings of 99
different butterfly species. We thinned this to 10,884 trips
encompassing 126,087 sightings of 89 species suitable for analysis using
multiple criteria. First, although the database includes historic
records used to build the Atlas of Massachusetts Butterflies, we
excluded records prior to 1992, the year in which the club started to
record large numbers of lists (i.e., > 1200
sightings/year). Next, following Breed et al. (2013), trips with
< 5 species seen were excluded, to minimize the effect of
anecdotal sightings (see Appendix S1 in Supporting Information ).
Unlike Breed et al. (2013), we combined and retained records for two
species groups. Canadian Tiger Swallowtail Paplio canadensis and
Eastern Tiger Swallowtail Paplio glaucus records were combined
(rather than excluded) because they are known to hybridize and can
sometimes be difficult to visually distinguish. Spring Azure Celastrina ladon and Summer Azure Celastrina neglecta were
also combined because of a change in species reporting within the MBC
during the study period. With these two exceptions, species were
excluded if during the study period they underwent a taxonomic shift or
realignment, or if there was an outbreak of the population
uncharacteristic of conventional population dynamics (following Breed et
al., 2013; see Appendix S2 ). Species were also excluded if there
were fewer than 15 observations before the year 2000, to prevent
outliers or a lack of available data from driving trends (See Appendix S4 ). MBC records were processed into a database,
generating 89 species-specific lists of all trips. Each species file
included presence/absence of the species on each trip, the date of each
trip, the number of individuals of that species seen on each trip, and
the total number of species seen on each trip (see Appendix S1 ).
To interpret patterns of phenology and abundance change we compiled data
on two life history traits: voltinism and range type. Following Breed et
al. (2013), voltinism was categorized as either obligately univoltine (a
single generation per year) or multivoltine (two or more generations per
year). Categorization agreed with Breed et al. (2013), with a few
exceptions in which scatter plots of observation day of year supported
an update (see Appendix S4 ). Following Breed et al. (2013), we
categorized species’ range type based on the center of their geographic
range distribution, relative to Boston, Massachusetts (the approximate
mid-latitude for the state of Massachusetts). Species were ordinally
identified as having ranges centered north, central, or south of the
latitude of Boston (species classified as northern would therefore be
closer to the southern limit of their range).