Field Notes Entry
Interpreting Seasonal Curves
How simple monthly summaries can reflect different underlying processes — from behaviour and flowering to migration and social structure
Up to this point, the “Year in the Life” posts have taken a simple approach: grouping everyday observations by month, and looking at how both total sightings and encounter frequency vary through the year.
Individually, each example reveals something recognisable. Taken together, however, they suggest something more general.
The charts may look similar — rises, peaks, declines — but they are not all showing the same thing.
The Same Method, Different Meanings
Each chart in the series is produced in the same way:
- Total sightings by month
- Number of days on which the species was recorded
Yet the patterns that emerge reflect different underlying processes, depending on the species.
In some cases, a dip in the curve reflects a change in behaviour. In others, it reflects flowering, migration, or even how individuals are grouped.
The challenge, then, is not in producing the chart, but in interpreting it.
Four Types of Seasonal Pattern
From the examples so far, four broad patterns begin to emerge.
| Curve type | What drives it | Example species |
|---|---|---|
| Detectability | Behaviour (moult, concealment) | Blackbird, Robin |
| Phenology | Flowering and visible growth | Daisy, Dandelion |
| Presence | Arrival and departure (migration) | Chiffchaff |
| Aggregation | Changes in group size and distribution | Starling |
These categories are not rigid, and any given species may be influenced by more than one factor. They are better thought of as tendencies, suggested by the data rather than imposed upon it.
Detectability
In resident birds such as Blackbird and Robin, the species is present throughout the year, but becomes less visible in mid-summer.
A decline in both sightings and encounter frequency suggests not absence, but a reduction in detectability — likely linked to moulting, quieter behaviour, and increased use of cover.
Phenology
In plants such as Daisy and Dandelion, the curves are more directly tied to visible growth and flowering.
Here, the close match between total sightings and presence suggests that the data is capturing when the plant is conspicuous, rather than when it is physically present.
Different species show different strategies:
- Daisy: a concentrated spring peak
- Dandelion: a longer, more persistent flowering period
Presence
For migratory species such as the Chiffchaff, the interpretation is more direct.
The curve reflects the species’ presence in the local area: a clear arrival in spring, a peak during the breeding season, and a disappearance by mid to late summer.
In this case, the absence of records is likely to correspond closely to true absence.
Aggregation
The Starling introduces a different kind of pattern.
Here, the species is present throughout the year, but the number of individuals seen at any one time varies considerably.
Winter flocks produce high counts, while summer dispersal leads to lower numbers, even though the species remains frequently encountered.
The divergence between total sightings and presence is particularly informative in this case.
A Note on Simplicity
What is striking is that all of these patterns emerge from very simple transformations of informal data.
There is no attempt here to standardise effort or produce precise estimates. Instead, the emphasis is on consistency over time.
That consistency allows seasonal signals to become visible, even in relatively modest datasets.
Provisional Patterns
These categories are not intended as a formal classification, and there will be many species that do not fit neatly into any one type.
Rather, they provide a way of thinking about what these curves might represent.
A similar shape in two charts does not necessarily imply a similar cause.
Closing Thoughts
What began as a way of summarising observations has become, to some extent, a way of asking different questions of them.
Not just:
What was seen, and when?
But:
Why does it appear in that pattern?
Even in simple monthly summaries, there are traces of behaviour, life cycle, and movement — provided the patterns are read with a little care.
As more species are added, it will be interesting to see how these provisional categories hold up, and where they begin to blur or overlap.