LIt’s quite a difficult concept, and there is no real answer. Every species has its crosses to bear, and every pair everywhere has had challenges to face. Many individuals, of all kinds and for all manner of reasons, have failed completely. They might have failed even to acquire a territory, or died heroically trying to save their offspring. A combination of conditions might have rendered breeding harder than usual. But what I’m asking is actually a deeper question. Are there some birds for which breeding is, routinely, unusually difficult? And is there any scientific evidence that might show such a thing? Well, it might stretch a few arguments a little, but I would argue that there is. Step forward, the Jackdaw. You’re probably surprised. The Jackdaw is a jaunty creature, with a carefree mien if not a carefree nature. Our human impressions, irrelevant though they are, would place the Jackdaw in the happy camp. We sometimes see them near their roost-sites in an evening playing – yes, surely playing – in the skies, circling, tumbling, calling in excitement. Jackdaws going to bed are like children on a sleepover, full of exuberance. These corvids nest in colonies, too, where they are equally good value to watch, messing about by tree holes or chimney pots. So are the Jackdaws suffering up in the rooftops? It does not seem right. But the Jackdaw’s breeding lot is a tough one to bear, and there are two sound biological reasons to assume that this is so.
The assumptions stem from the fact that Jackdaws find it hard to acquire enough food to feed their youngsters. In contrast to the more omnivorous nature of the larger corvids, Jackdaws feed their offspring on the summer insect bloom. They bring in such items as grasshoppers, flies, ants, beetles and caterpillars, which they predominantly find on the ground, in long grass. The insect bloom is short, and sometimes catches Jackdaws out, if their breeding is delayed. So what biological quirks tell us that the Jackdaw has a hard time bringing in food? Well, I hope you are ready for this, because it is quite upsetting stuff. Jackdaws practise a method of chick-rearing called brood reduction, which is every bit as chilling as it sounds. Brood reduction is an overproduction strategy designed to cater for good years and bad when a food supply is unpredictable. It is actually quite a simple alteration of normal practice, but the consequences are profound. What Jackdaws do is to space out their egg-laying and to begin incubation before the clutch is complete. Sometimes they lay each egg in their clutch of three to six a couple of days apart, or even three, and then the female begins sitting after her second egg comes along. This is quite different from what most birds in your garden and beyond do. Tits, for example, lay First-born Jackdaws get a head start over their siblings An adult pauses before resuming the grind of feeding the brood