The authors provide an alternate explanation for the difference between contemporary and historic data

> the span of queen flight periods in the two datasets do not match: the flight period in the national-citizen collected data ranges from 110 to 130 (day of the year) (Fig. 2b) while the regional museum dataset ranges between 155 and 180 (day of the year) (Fig. 4b). This is likely due to the fact that the citizen dataset is more recent, while museum records span periods where species were on average observed later in the season.

> Datasets based on opportunistic reporting can indeed provide a complementary source of information compared to systematic sampling, especially when deployed at large spatial scales (Henckel et al. 2020; Zattara and Aizen 2021). The most abundant species are usually the easiest to identify and report and they will therefore also drive community trends. In contrast, there are many uncertainties surrounding rare species trends, since these are less abundant and perhaps not detected by observers. This is one of the downsides of citizen collected data and may affect the capacity to capture robust trends for less common species. 

> We acknowledge the uncertainties involved with both opportunistic and targeted sampling from our museum data, and non-systematic citizen science observations, as these may not consistently reflect the occurrence or abundance of specific species or species groups.

Maria Blasi (2022) Historical and citizen‑reported data show shifts in bumblebee phenology over the last century in Sweden

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