There are many factors which influence garden bird populations throughout the year from changes in the weather, the amount of daylight, habitat loss and availability of food.
In the cold of winter many birds simply starve as finding food becomes more difficult due to freezing weather and shorter days. This is particularly true of small birds such as Robins, Wrens and Blue Tits who need to eat approximately 40% of their body weight per day to survive. During the breeding months of May and June the availability of food can be critical to how many chicks survive to fledge.
Blue Tits breeding success is at its best when spring weather is warm and sunny coupled with low rainfall which results in a healthy population of caterpillars and thus healthy broods. Unfortunately this same weather is not so beneficial to Blackbirds who feed mainly on earthworms which are driven out of reach, deeper into the soil as the surface layers dry and harden.
Conversely if spring is characterised by wet weather caterpillar numbers are reduced and many are washed away, leading to a shortage for Blue Tits and their young, whilst moist soil brings earthworms nearer to the surface within easy reach of Blackbirds.
As a result, depending on the weather, the following season may see a decline in the populations of certain birds as surviving young from the previous season are lessened by a shortage of food. Some songbird species can take several seasons to recover their populations from these poor breeding seasons.
Modern farming and gardening practices often destroy or severely limit birds’ ranges of habitat. Hedgerows and wild meadows are becoming more rare, the use of pesticides affects insect populations which in turn affect insect loving birds.
Pigeons do not predate on other birds but may scare them away due, mainly, to their size and numbers. Magpies are omnivorous and, as such, will take eggs and small nestlings but only as a small proportion, less than 10%, of their diet. Unless you have a particularly high population of Magpies, it is more likely that this is simply a coincidence caused by other factors such as weather, disease or limited food supply.
There has been much research done into the effect of Magpies on breeding successes amongst other smaller birds and evidence suggests no link between the two. For example finches, which nest in relatively vulnerable open nests, are showing signs of increasing populations whilst birds such as the House Sparrow, which nest in holes, cavities or nest boxes safe from Magpie attack, are showing dramatic declines in population numbers. Clearly there are other forces at work, other than Magpies, which are affecting population numbers.
Populations of House Sparrows are in serious decline in many parts of the UK which is thought to be mainly the result of a change in farming methods. Lack of stubble fields and less waste grain around silos are thought to be, at least partly, to blame for this reduction in numbers. House Sparrow populations have seen an approximate 50-60% drop in numbers since 1970 especially in large city centres. House Sparrows are also communal nesting birds, they like to nest in groups close together, probably feeling safety in numbers. Many of their preferred nesting places, holes and crevices in buildings or in dense foliage, are not as common as they used to be. House Sparrows will readily take up residence in nest boxes, especially multiple occupancy ones where small colonies can feel safe and secure. Multi occupancy boxes are like small terraced housing units, usually one box in the middle with a front entrance and one box either side with outward facing entrances. Several of these multi-home boxes either hidden in foliage of placed up in the eaves may encourage a regular small colony in your garden, especially when coupled with a ready source of food and water.
Although there is no hard evidence of a reduction in numbers there are a few factors which are thought to influence this apparent decline in numbers.
The popularity for converting farm buildings into houses or simply modernised them may be reducing the numbers of suitable nesting sites. Swallows feed on flying insects, which thrive in wetland areas and pastures, and as more grazing land is lost to arable land, the amount of insects for them and their young to feed on is falling.
Nesting and feeding territories for Swallows need to be close together and as they require mud to build their nests, if the weather is dry they may not be able to locate building materials near nesting sites and so move to another location. Some farmers have had success in encouraging Swallows by simply keeping an area of ground wet near outbuildings and have thus benefitted from their natural pest control.
Birds do sometimes change long established habits regarding nesting. This may be because:
Birds may find an alternate, more preferable nest site in the same territory possibly due to the removal of an important tree or shrub which they used as food either in your garden or a neighbours, or a neighbour may have installed a new nest box in which they have set up nest. The removal of a key food source will make a previous good territory of no use.
Due to competition, even from their own young, birds may need to change the extent of their territory, redraw the boundaries so to speak, with the result that your garden may no longer gain as much attention as it once did.
Autumn is the most plentiful time, in terms of food sources, for birds especially if the weather is mild and the natural sources of food are good. The result of this being that birds may not need to supplement their diet with the food you have on offer, but it is still important to continue offering it as some birds will get used to it and when shortages do occur will remember where they can get food. Simply reduce the amount on offer during these plentiful times to minimise wastage and remember to ensure it remains fresh, and keep fresh water available.
Birds have strong instincts regarding feeding and prefer natural foods if they can get them and it may be quite late in Autumn, when food sources are diminishing, that birds will begin to re-visit your feeders.
Singing is used by most birds to establish and defend a territory as well as attract a mate. Because of this most birds only need to sing during spring and early summer.
As most birds raise only one or two broods they don’t need to sing long after the end of June especially if the young have fledged and left the nest as they no longer need to defend their territories with as strong a voice.
This change in the dawn chorus can be quite sudden and it is only natural to think something may be wrong. It is, however, a natural change in bird behaviour and should not be viewed as a cause for concern.
In August many birds seem to disappear but this is, again, a natural behaviour cycle and is usually for one of two reasons.
At the end of the breeding season birds begin the process of moulting, losing old feathers which have become damaged or are no longer needed in favour of fresh new ones. This process can take several weeks during which time birds are vulnerable to predators so they tend to conceal themselves giving rise to the false impression that they have vanished.
Toward the end of summer natural food sources such as grain, fruit and berries become more plentiful. This may lead to birds abandoning their territories in favour of this natural bounty which means your food offerings may go uneaten further adding to the illusion that populations have declined. This can be particularly marked in areas with farmland nearby when sparrows, starlings and finches may move en mass to new and plentiful food sources. Even a patch of waste ground within a city overgrown with seed providing weeds may cause a sudden shift in bird numbers. Some species, particularly Tits, move into the tree canopy where they can easily be overlooked.
With the onset of colder weather towards the end of autumn birds begin to return to garden feeders as natural food sources become harder and harder to find.
It is highly unlikely that normal winter weather will have such a drastic effect on birds.
Although it is true that winter is probably the hardest time for birds and many do die of starvation it is only when freezing temperatures continue with no thaw during the daytime for several days in a row that large numbers of birds will die.
Birds may simply have found a slightly better habitat nearby, maybe an area protected from the worst of the cold where water and food are easier to find, maybe somewhere where they feel slightly safer from predators. It may not be immediately apparent or seem to make sense but with so many factors influencing bird behaviour it does not necessarily indicate a problem. However it would be advisable to continue to offer food and un-frozen water as birds could just as easily and inexplicably return.
During autumn and winter many birds spend time moving around in flocks and, as creatures of habit, tend to feed in familiar places on their regular routes.
As a result, the numbers of birds visiting your garden may vary from large numbers to very few.
Unless the food sources at their established feeding areas become unavailable they may well ignore your feeding stations at least until some of them discover it for themselves and the flock then becomes aware of it. Even if your garden is on a regular flight route it may take several days for birds to become comfortable with and accustomed to a new feeding station.