We're still in what garden people sometimes call the shoulder season- not quite the peak of fall but certainly not summer any more.

I've been noticing the little changes in my garden, and in particular the sudden-seeming emergence of showy berries on some of the shrubs and trees.

The dogwood that I love so much for its pink flowers at the beginning of May now has screaming red shiny berries at its branch tips, framed by dusky, deep wine foliage. And the viburnums I totally forgot about are decked out top to bottom like Christmas trees, spangled with red or yellow fruit that's clearly defined amidst the darkening purple-brown leaves.

I noticed that the brightest berries are often on plants whose leaves turn colors early, and I wondered if this was coincidence. It turns out that it's not, and it has to do with birds.

Migratory birds depend on berries to fuel their flight, and biologists think that early leaf-change creates a contrasting signal for them. Ripe berries appear in high relief against the lighter or darker leaves, and are easier to spot by birds just passing through. Scientifically this is known as fruit flagging, and is thought to make foraging more successful during birds' brief stopovers in unfamiliar locations.

Fun fact: if you make a list of plants with obvious fruit whose leaves change color early (besides dogwood and viburnum look at poison ivy, Boston ivy, sassafras, and spicebush) they tend to be the plants with the fattiest berries, and they're ripe right now. By eating them along their migratory route, birds can build up enough fat stores to successfully make the flight to their winter habitats. This helps the birds of course, but also helps the plants get their seeds airlifted throughout a wide area.

Wouldn't it be good for the birds if all berries were high in fat? No. The more fat in a berry, the faster it rots. The good keepers like crabapple and winterberry holly (both plants with little-to-no fall foliage color) are very low in fat. The birds won't turn to these until January or February, when they've finished up eating the more perishable high calorie berries.

Learning about this, I remember to be awestruck at the complex systems nature has developed to sustain such a diversity of species. This weekend I'm hoping to get to a lot of outdoor fall chores. When fading daylight forces me inside, I'll have a glass of something and toast Mother Nature. To life!